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Investigation of the microwave effect

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Capsule-Based Microwave Digestion
Jean-Guy Joseph Legere
A Thesis subm itted to th e Faculty of G raduate Studies and Research in p artial
fulfillment of the requirem ents of the degree of Doctor of Philosophy
Septem ber 1995
D epartm ent o f Chem istry
McGill U niversity
M ontreal, Quebec
CANADA
© Jean-G uy Joseph Legere 1995
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Abstract
A large tube microwave digestion system with capsule sample introduction has been
developed. This is the first autom atic pressurised microwave digestion system to
use: 1) capsule sam ple introduction, 2) reagent addition, and 3) controlled venting,
during a digestion. The digestion tube has built-in cooling, an infrared tem perature
sensor, an in-line pressure sensor, autom atic venting, and a new type of valve,
called the "Flange Valve". The flange valve was designed for loading capsules into
the digestion tube and for easy cleaning of all valve and tube parts w etted by the
sample.
The digestion tube is made of Teflon PFA*, which is capable of operating a t 200 psi
and 200°C. W ater, salt solutions, and concentrated nitric acid were used to
characterize the system.
A process was developed to m ake capsules from ultra-clean polyacrylamide gel; it
was used to m ake capsules for the analysis of soils, botanicals, and biological
sam ples A "Squeegee", a device equipped w ith a soft, gas-tight Teflon* end pushed
through the digestion tube w ith a flexible rod, was used to insert capsules into and
remove digestate from th e digestion tube.
Micro2, an interpretive language, w ritten in-house, uses English-like instruction
files to control the digestion. Micro2 uses pressure, tem perature and tim e d a ta to
control venting, cooling, and heating during the digestion. Triggers and feedback
loops in the instruction file allow Micro2 to adapt to changing conditions in the
digestion tube to complete a digestion w ithout loss of analyte.
Analysis of the digested sam ples revealed th a t, for the sam e digestion tem perature,
dissolution is identical to th a t performed in a conventional microwave bomb.
ii
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Resume
U n digesteur micro-ondes a tube a grand din mot re pouvant acceptor des
echantillons encapsules a ete developpe. II s'agit du prem ier digesteur micro-ondo
qui perm et sim ultanem ent: 1) Tintroduction de capsules, 2) 1'addition de reactifs, et
3) le degazage, le tout en cours de digestion. Le tube de digestion comprend un
system e de refroidissem ent, un capteur infra-rouge, un capteur de pression et un
nouveau type de valve a bourrelets. L'ensemble autorise le degazage autom atise.
La conception de la valve a bourrelets perm et le chargem ent de capsules dans le
tube de digestion e t le nettoyage aise des parties de la valve et du tube qui ont ete
m ouillees p ar le digestat. Le tube de digestion cst fait de Teflon PFA1M. capable
d’operer a tine pression de 200 psi et une tem perature de 200 "C. De l'eau deionisee,
des solutions salines et des solutions concentrees d'acide nitrique ont etc utilisees
pour caracteriser le systeme.
U n precede de fabrication de capsule en gel de polyacrylamide ultra-pur a ete mis au
point e t utilise pour l’encapsulation d’echantillons botaniques, biologiques e t de sols.
U n tu b e flexible equipe d’un embout de teflon etanche perm et d’inserer les capsules
e t d’expulser le digestat du tube.
U n language in terp rets developpe en cours d’etude, Micro2, utilise des instructions
en language courant pour controler la digestion. Micro2 integre les donnces de
pression, de tem perature e t de tem ps ecoule pour controler le degazage, le chauffage
e t le refroidissem ent en cours de digestion. Des param etres seuils et des boucles de
retroaction inclus dans le program m e Micro2 rendent le system e apte a suivre tou t
changem ents dans les condition de digestion, ce qui evite les pertes d’echantillon.
L’analyse d’echantillons digeres a revele que la dissolution est identique a celle
obtenue dans une bombe a micro-onde conventionnelle, pour une tem perature de
digestion semblable.
nt
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Contributions to Original Knowledge
1. A capsule-based sample introduction system for on-line microwave digestion has
been developed. The capsule-based approach provides quantitative sample
transfer into the digestion vessel and eliminates abrasion of the inlet valve
2. A large diam eter on-line digestion tube and valve system allows capsule sample
addition and venting of the digestion vessel.
3. An in-line pressure sensor was developed th a t m easures the pressure in a
flowing stream w ithout creating a dead volume. No pressure transfer line is
required.
4. A configuration for m easuring the tem perature of the digestion tube using
infrared sensing was developed. A scheme for calibrating insures th a t the
tem perature inside the tube is obtained.
5. An interpretive language for control and data acquisition w as developed. This
language, “M ICR02”, can be used for other system s as well a s microwave
digestion.
6. A m anufacturing process was developed for m aking polyacrylamide capsules.
iv
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Acknowledgments
I would first like to thank my wife Helene, who. from the very beginning, supported
my decision to pursue a doctorate and then sustained me in my effort.
One of my m ain reasons for choosing McGill University was my thesis advisor Dr.
E.D. Salin. He was always there to help me. generous with his time, and patient
w ith my w riting skills. Looking back now, the choice is the best one th a t I could
have made.
I have always believed th a t working w ithin a group is better than working alone.
My experience in ‘‘Eric’s” group has reinforced this view, and I would like to thank
everybody in th e group for always being ready to help me out.
I would like to give special thanks to Tanya Tadey, whose help was invaluable in
finding th e ideal capsule m aterial and developing the capsule m aking process.
T here are m any people th a t I relied on for th eir expertise. Wayne Branagh, Doug
Webb, and C hristine Sartoros m ade M ICR02 work smoothly. H uinan Yu built and
tested pressure sensor and narrow tube configurations th a t helped resolve problems
in pressure m easurem ent. Anne Morinville assembled th e valve control box, th a t
worked flawlessly for th e last four years. B rian Spencer gets th an k s for moral
support and insightful discussions. Cameron Skinner, who tends to see things my
way, h a d useful suggestions for tem perature m easurem ent. Robin R attray, JeanFrangois A laiy, an d W ayne B ranagh all helped me analyze th e m any solutions th a t
I generated. K athy Singfield ra n the capsules on the DSC and clarified those
“polymer” questions th a t I had.
C onrad Gregoire and Doug Goltz saved me from the E lan 250, and gave me
experience on th e PE/Sciex 5000, th a t worked so well (I’ll rem em ber you a t Sciex).
R alph S turgeon gets a big thanks, not only for always having his door open, but also
for te stin g th e CEM system; saving m e the ta sk of guessing and being wrong.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Much of the thesis work has required the building of instrum entation. This would
not have been possible without the expertise and patience of Fred Kluck and his
assistant Bill Bastien.
vi
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L ist o f T ables
---------------------------------
•
T a b ic 1:
M ain Problem s w ith N arrow I ube D ig estio n S y s t e m s ................................................28
Table 2: Reasons for Inclining Digestion Tube...............................................................41
Table 3: Blank Intensities in 15^1 ConcentratedNitric Acid...................................... 120
Table 4: Blank Study for 1CP-MS.................................................................................121
Table 5: Multi-Element Standard Repeat Runs............................................................. 122
Table b: Commercial Capsule Digestion and Analysis :i:................................................123
Table 7: Polyacrylamide Capsule Digestion and Analysis *.......................................... 123
Table 8: Lobster Hcpatopancrcas TORT-1 Individual Digestion Run Values...............127
Table 9: Lobster Hcpatopancrcas TORT -1 Staiities........................................................128
Table 10: Bovine Liver 1577 Digestion and Analysis.....................................................129
Table 11: Capsule Made Using Stainless Steel Grips............................................... . 130
Tabic 12: Orchard Leaves 1571 Digestion and Analysis.................................................131
Table 13: SO-2 Digestion and Analysis.......................................................................... 133
Table 14: MESS-1 Digestion and Analysis..................................................................... 134
List of Photographs
•
Photograph 1: Squeegee Molding A pparatus and Assembly
Photograph 2: Capsule Dipping Rack w ith Pins
Photograph 3: Capsule Drying A pparatus
Photograph 4: IR T C A ttached to Digestion Tube
P h o to g rap h s: F ront of Microwave Oven
Photograph 6: Overall View of Microwave Digestion System
Photograph 7: Back View of Microwave Oven
Photograph 8: Digestion Tube Flanging Setup
Photograph 9: Close-up of Flange Mold
37
60
61
71
79
80
SI
158
161
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List of Drawings
page
Flange Valve N ut
191
1/8“ I.D. Flange Bolt
192
Pressure A dapter
193
Digestion Tube Holder
194
Pressure Transducer A dapter and Components
195
Pressure Transducer A dapter and Exploded
View of Components
196
Flanged PFA Tube -1 /2 “ O.D.
197
Removable Back Panel for Microwave
19S
Flange Valve Housing
199
Stop Down Ring
200
Double Flange Valve Mount
201
Body and Cap Pins and Rubber R etainer
202
Capsule Pin Wheel
203
Capsule Pin Rack
204
Flange Mold
205
viii
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L ist o f F ig u res
»
Figure 1: Microwave Bomb with Vent............................................................................... b
Figure 2: McGill Slurry Tube Digestion........................................................................... 2.'
Figure 3: Needle Valve......................................................................................................24
Figure 4: Narrow Diameter Tubiiig.................................................................................. 2S
Figure 5: Ball Valve...........................................................................................................2l>
Figure 6: Swagelok Type Fitting for Attachment ofTulv To Valve...............................30
Figure 7: Flange Valve Showing Main Features...............................................................'2
Figure S: Initial Digestion Vessel Design, using Conventional Bomb and Tube Features with a
Flange Valve.............................. *...........' .................................................................>3
Figure 9: PFA Digestion Tube with Stainless Steel Holders............................................35
Figure 10: Squeegee......................................................................................................... 3b
Figure 11: Side View of Digestion Tube with Flange Valves Installedin Microwave Oven
..................................................................................................................................3K
Figure 12: Glass-Sheathed Tube with Ferrule System to Mold Glass Sheath................ 40
Figure 13: Flange Valve with Pressure Adapter.............................................................. 42
Figure 14: Cooling Tube Wrapped Around the Digestion Tube.....................................4b
Figure 15: Capsule.............................................................................................................53
Figure 16: Automatic Capsule Loading using a Modified ThreeWav Valve..................54
Figure 17: Capsule Manufacture Processes......................................................................5b
Figure IS: Stainless Steel Sheathed, Grounded Thermocouple....................................... (>4
Figure 19: Thermocouple Trials....................................................................................... b7
Figure 20: Thermopile Temperature Measurement......................................................... 70
Figure 2 1: IR/TC Calibration with TC Inside Digestion Tube........................................ 72
Figure 22: Quadratic Least Squares Fit of Signal Response of Tube Signal from IR/TC73
Figure 23: Pressure Line with Liquid Transfer Line and Membrane...............................74
Figure 24: Front Surface Pressure Sensor In-Line............................................................ 7b
Figure 25: Total System Sensor Configuration.................................................... ...........S3
Figure 26: Heating of Water in Digestion Tube................................................................S4
Figure 27: Comparison: Measured Temperature vs Calculated Temperature............... 89
Figure 2S: Salt Water Run..................................................................................................90
Figure 29: Integration of Time Microwave Percent Time-On..........................................91
Figure 30: Blank Nitric Acid R un..................................................................................... 92
Figure 31: Nitric Acid Power Integral............................................................................... 94
Figure 32: Cooling Configurations A. and B.....................................................................95
Figure 33: Cooling Configurations D, and C.....................................................................96
Figure 34: Pressure Cooling Comparison.........................................................................100
Figure 35: Newton Cooling Curves of the Digestion Tube.............................................101
Figure 36: Polyacrylamide Capsule Digestion................................................................ 103
Figure 37: DSC Analysis of Polyacrylamide Capsule Fragment................................... 105
Figure 38: Capsule Pushed in by the Squeegee..................................................................106
Figure 39: The System After Reagent Addition...............................................................107
Figure 40: Microwave Energy is Applied-.......................................................................108
Figure 41: The Tube is Cooled and then Vented............................................................. 109
ix
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cure 4 2 S g u e e e e e used to R e m o v e D in e s !..................................................................................... 1 10
Soil D ig e s tio n .......................................................................................................................... 1 11
gurc 43
cure 44
gurc 45
pure 4 6
gure 47
gure 4S
sjurc 4 0
S u crose in C'upsule D ig e s tio n ........................................................................................... 113
Orchard L e a v e s D ig e s t io n .................................................................................................. 114
B o v in e Liver D ig e s t io n ...................................................................................................... 1 15
Plan ck 's B lackbodv Radiation C u r v e s ........................................................................ 140
Integration o f B la ck b od v C urve throunh b -1 4 um W i n d o w ............................. 151
T h e r m o p ile S c h e m a t i c .............. ........................................................................................ |5">
jrii re 51
IR /TC R e s p o n s e V ie w in n Graphite.............................................................................. 153
E x p lod e d V ie w o f F la n s e V a lv e with Pressure Adapter..................................... 155
gure 52
F lange V a lv e and D ig estio n T u b e M ounted on Back o f M ic r o w a v e O v e n 157
"urc 50
X
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Table of Contents
»
Abstract
Resume
Contributions to Knowledge
Acknowledgements
List of Tables
List of Photographs
List of Drawings
List of Figures
ii
iii
iv
v
vii
vii
vi i i
ix
1. Design and Construction of a Capsule-Based Microwave Digestion System
1
•
I. I Introduction...........................................................................................................I
1.2 The Need for Digestion......................................................................................... I
1.3 Sample Type Classification................................................................................... 2
1.4 Methods of Sample Digestion............................................................................... 3
1.4.1 Atmospheric Digestion................................................................................. 3
1.4.2 Pressurized Digestion................................................................................... 4
1.4.3 Fusion-Based Digestion................................................................................ 4
1.4.4 Ashing Techniques....................................................................................... 4
1.5 Microwave Digestion: Open Digestion Vessel.....................................................5
I .6 Microwave Digestion: Closed Bomb Vessel........................................................5
1.6.1 Venting.......................................................................................................7
1.6.2 Discstate Removal........................................................................................7
1.6.3 Digestion Vessel Cleaning............................................................................ 7
1.7 Microwave Digestion: Tube Digestion Systems................................................. 7
1.5 Limitations of Present Digestion Technologies......................................................8
1.S. 1 Maximize Throughput..................................................................................8
1.5.2 Reagent Disposal..........................................................................
1.8.3 Handling Errors........................................................................................... 9
1.8.4 Safety Considerations.................................................................................. 9
1.9 Literature Review.................................................................................................11
1.9.1 Literature Review: Focus................................ .........................................11
1.9.2 Microwave Bomb Digestion: Miscellaneous..............................................12
1.9.3 Microwave Bomb Digestion: Automation.................................................13
1.9.4 On-Line Microwave Digestion Systems...................................................... 14
1.9.5 Literature Review: Conclusions................................................................20
1.10 Objectives.......................................................................................................... 21
1.11 Summary of Thesis Contents............................................................................. 21
1.12 Contributions to Thesis......................................................................................22
2. Capsule /T ube-B ased Microwave Digestion-------------------------------------- 23
2.1 Introduction to Microwave Tube Digestion.......................................................23
2.1.1 The Advantages of Narrow Tubc Digestion..............................................23
xi
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9
2.1.2 Narrow Digestion Tube Characteristics......................................................... 24
2.1.2.1 Valve Selection: Needle Valve........................................................... 24
2.1.2.2 Valve Selection: Rotary Valve........................................................... 25
2.1.2.3 Sample Transfer....................................................................................25
2 .1.2.4 Volume Dilution and Internal Standards............................................. 25
2.1.2.5 Deposit Formation.................................................................................26
2.1.2.6 Localized Healing..................................................................................26
2.1.2.7 Tube Temperature Measurement................................... ...................... 27
2.1.2.S Tube Diameter Considerations..............................................................27
2.1.3 The Disadvantages of Narrow Tube Digestion.............................................. 2S
2.2 Valve Selection and Design..................................................................................... 29
2.2.1.1 Ball Valve 7........................................................................................ 29
2.2.1.2 Custom Valve.................................................
29
2.2.2 Design: Pressure and Temperature.......................
30
2.2.2.1 Tube Attachment to Valve..................................................................... 30
2.2.2.2 Closing Diameter................................................................................... 31
2.2.2.3 Valve Material....................................................................................... 31
2.2.3 Valve Access...................................................................................................31
2.2.4 Flange Valve....................................................................................................32
2.3 Digestion Vessel Design.......................................................................................... 33
2.3.1 Large Bore Tubing...........................................................................................35
2.3.2 Squeegee is the Answer.................................................................................. 36
2.3.3 Orientation.......................................................................................................3S
2.3.4 Materia! Selection of Digestion Tube.............................................................39
2.3.4.1 Memory...................................................................................................39
2.3.4.2 Temperature and Pressure Considerations............................................ 40
2.3.4.3 Final Selection of Digestion Tube Material.......................................... 40
2.3.4.4 Glass-Sheathed PFA Tube.....................................................................40
2.3.4.5 Pyrex®Tube........................................................................................... 41
2.3.5 Benefits of Inclining the Digestion Tube........................................................41
2.3.5.1 Venting/Pressure.................................................................................... 42
2.3.5.2 Cool Gas Phase...................................................................................... 43
2.3.5.3 Effective Temperature Measurement................................................... 43
2.3.5.4 Boiling and Refluxing...........................................................................43
23.5.5 Keeping Sample from Reaching Valve................................................ 44
2.4 Cleaning.................................................................................................................... 44
2.5 Automatic Control....................................................................................................44
2.5.1 Microwave Oven Interlocks........................................................................... 44
2.5.2 Manual Control of Valves.............................................................................. 45
2.5.3 Digestion Tube Cooling and Venting.............................................................45
2.6 Conclusions - Chapter 2 ......................................................................................... 47
3. Control Software_________________________________________________48
3.1 Software Control Requirements............................................................................ 48
3.1.1 MICR02: Language................................................................................... 48
xii
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3.1.2 MICR02: Input.............................................................................................44
3.1.3 MICR02: Output..........................................................................................4l>
3.1.4 MICR02: Conditional andAsynchronous Control...................................... 30
3.2 Conclusions - Chapter 3 :...................................................................................... 51
4. C a p su le In tro d u c tio n ................................................................................................52
4.1 Traditional Sample Transfer Methods.................................................................... 32
4.2 Early Development Efforts..................................................................................... 32
4.3 A New Approach Needed...............................................................
32
4.4 Capsule Benefits...................................................................................................... 33
4.5 Capsule Concept...................................................................................................... 53
4.6 Sample Transfer....................................................................................................... 54
4.7 Capsule Selection.................................................................................................... 54
4.S Clean Capsule Material........................................................................................ 55
4.9 Methods of Manufacture: Molding........................................................................56
4.10 Methods of Manufacture: Dipping..................................................................... 57
4.11 Polyacrylamide Formulation..................................................................................5S
4.12 Polyacrylamide Capsule Manufacture.................................................................. 5S
4.13 Conclusions - Chapter 4 : .......................................................................................62
5. S e n s o r s ..........................................................................................................................63
5.1 Temperature Sensors............................................................................................... 63
5.1.1 Introduction.................................................................................................... 63
5.1.2 Fiberoptic Temperature Sensor...................................................................... 63
5.1.3 Thermocouple Temperature Probe................................................................ 64
5.1.4 Tube Temperature........................................................................................... 65
5.1.5 Thermocouple Approach To Temperature Determination............................65
5.1.5.1 Thermocouple Calibration.................................................................... 66
5.1.5.2 Thermocouple Installation.................................................................... 66
5.1.5.3 Thermocouple Self-Heating and Arcing............................................... 66
5.1.5.4 Methods to Eliminate Sclf-hcating and Arcing....................................68
5.1.5.5 Thermocouple Tube Temperature Abandoned..................................... 68
5.1.6 Infrared Detection for Temperature............................................................... 69
5.1.6.1 Infrared Viewing of Temperature:Photoconductive Detectors.............69
5.1.6.2 Infrared Viewing of Temperature:Thermopile Detectors.................... 70
5.1.6.3 Infrared Tube Temperature Considerations..........................................71
5.1.6.4 ER/TC Mounting....................................................................................71
5.1.6.5 IRyTC Calibration..................................................................................72
5.1.7 Magnetron Temperature.................................................................................73
5.1.8 Cooling Water Temperature.......................................................................... 74
5.2 Pressure Sensors......................................................................................................75
5.2.1 In-line Pressure Sensor.................................................................................75
5.2.1.1 Pressure Calibration.............................................................................77
5.3 Capsule-Based Microwave Digestion System:Hardware Overview......................78
5.4 Conclusions - Chapter 5 :......................................................................................... 82
xiu
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6. O peration an d C haracterization of a C apsule-B ased M icrowave D igestion
S y s te m ............................................................................................................................... 83
6.1 Introduction............................................................................................................. S3
6.2 Data Acquisition...................................................................................................... 84
6.2.1 Tube Temperature........................................................................................... 85
6.2.2 Tube Pressure................................................ ................................................85
6.2.3 Percent Microwave Time On......................................................................... S6
6.2.4 Cooling Water Temperature.......................................................................... S6
6.2.5 Magnetron Temperature................................................................................. 86
6.3 Microwave Heating Characteristics: Digestion Tube...........................................S7
6.3.1 Microwave Heating of Water in Digestion Tube.......................................... 87
6.3.1.1 Tube Temperature Determination Using Water Vapor Pressure
SS
6.3.2 Salt Solution Heating in Digestion Tube........................................................89
6.3.2.1 Integration of Microwave Percent Timc-On......................................... 91
6.3.3 Acids Heating in Digestion Tube: Nitric Acid............................................ 93
6.3.4 Digestion Tube Cooling.................................................................................. 97
6.3.4.1 Tube Cooling During Digestion............................................................97
6.3.4.2 Cooling After Digestion Completion..................................................101
6.3.4.3 Newton Cooling of Digestion Vessels.................................................102
6.3.5 Polyacrylamide Capsule Digestion............................................................... 103
6.3.6 Polyacrylamide Digestion..............................................................................105
6.3.7 CapsuIc/Samplc Digestion Sequence........................................................... 107
6.3.8 Capsule Digestion Observations................................................................... 109
6.3.9 Soil Digestion................................................................................................ 111
6.3.10 Vent cycle.................................................................................................... 112
6.3.11 Sucrose Digestion........................................................................................ 113
6.3.12 Temperature Fluctuation Sources............................................................... 114
6.3.13 Digestion of a Botanical Sample................................................................ 115
6.3.14 Digestion of a Biological Sample............................................................... 116
6.3.15 System Cleaning.......................................................................................... 116
6.4 Conclusions - Chapter 6 :........................................................................................ 118
7. A nalysis....______________________________________________________ 119
7.1 Introduction.............................................................................................................119
7.2 Detection Limit Determination...............................................................................119
7.3 Precision Determination......................................................................................... 123
7.4 Commercial Capsule Digestion and Analysis....................................................... 124
7.5 Polyacrylamide Capsule Digestion and Analysis..................................................125
7.6 Tort-1 Digestion and Analysis................................................................................126
7.7 Bovine Liver Digestion and Analysis.................................................................... 129
7.8 Orchard Leaves Digestion and Analysis................................................................ 131
7.9 Soil SO-2 Digestion and Analysis..........................................................................132
7.10 Marine Sediment MESS-1 Digestion and Analysis.............................................133
7.11 Conclusions - Chapter7:.......................................................................................135
xiv
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8. C onclusion...........................................................................................................136
9. Future Work and Limitations...........................................................................138
9 .1 Future Work............................................................................................................138
9.2 System Limitations................................................................................................ 139
10. Bibliography...................................................................................................... 140
11. APPENDICES..................................................................................................... 148
11. 1 APPENDIX A: IR/TC Calculations.................................................................. 14S
11.1.1 Blackbody Radiation: Thermopiles..........................................................148
..........................................................150
11. 1.2 Choice of Infrared Thermopile
11. 1.3 Characterization of IR/TC.........................................................................154
11.2 APPENDIX B: Flange Valve and Digestion Tube Flanging..........................155
11.2.1 Flange Welding Instructions................
160
11.3 APPENDIX C: Computer Control.................................................................... 162
11.3.1 Micro2 Instructions..................................................................................... 162
11.3.2 Implementation of Macros.......................................................................... 1S3
11.3.3 Hotkey Function......................................................................................... 184
11.3.4 Example Program: "Dll .SEQ
..................................................... IS5
11.4 APPENDIX D Drawings.................................................................................. 190
11.5 APPENDIX E: Microwave Control Hardware................................................. 206
11.6 APPENDIX F Suppliers....................................................................................208
xv
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1. Design and Construction of a Capsule-Based
Microwave Digestion System
1.1 Introduction
1.2 The Need for Digestion
Knowledge of the elemental content of m aterials is required in decisions we face
every day concerning the food we eat, m aterials used in m anufacturing, or disposal
of waste products. Direct elemental analysis of solids has the undesirable
requirem ent th a t every sam ple type needs a separate set of standards. However,
when a sample is dissolved, the history of the sam ple is lost and any effect th a t the
sam ple type would have had on the determ ination is elim inated. The inorganic
elem ental analysis of the wide variety of sam ple types is best performed by
dissolving the sam ple. The argum ent in favor of sam ples in a liquid form is
reinforced by th e fact th a t alm ost all analytical instrum ents today a re equipped to
handle samples in a liquid form. Therefore, m ost sam ples to be analyzed are
dissolved in a suitable solvent. Since m any sam ple types do not readily dissolve, the
sam ple is treated to break the m atrix down into soluble components. This
treatm en t is called a digestion.
The digested sam ple is not only dissolved: it is also homogenized. Exceedingly sm all
and representative portions of the sample can be obtained simply by tak in g a sm all
volume of the digested sam ple solution. G raphite furnace atomic absorption
spectroscopy (GFAAS) is a technique th a t uses sm all volumes of solution (typically
50 pi, which represents a sam ple weight o f250 pg for a 0.5 g sam ple dissolved in
100 ml). A lternately, a slurry of th e dried pulverized sam ple can be used to select a
sm all sam ple size. However, if too sm all a sam ple volume is used, segregation can
occur.
A nother reason to use digestion is one of sam pling statistics. The d istribution of an
elem ent of in terest in a sam ple and the precision required in th e d eter m in a tion
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dictate the size of sample needed: a larger particle size requires a larger sample
size. For example, in gold analysis of geological samples, gold is present at the parts
per billion level as discreet particles of native gold. Pulverizing the sample does not
reduce the particle size of gold. Digesting a 20 g portion and then selecting a 250 pg
portion from the digestate gives a much better chance of obtaining an accurate value
th an sim ply taking a 250 pg sam ple for solid analysis.
One of th e m ain analytical requirem ents today is total m ulti-elem ent determ ination,
which requires th e sim ultaneous dissolution of many elements. This is not a simple
or straightforw ard problem, since each type of sample m aterial has special
requirem ents. The selection of a digestion method depends not only on the final
analytical procedure, but also on the sample type.
1.3 Sample Type Classification
Sam ples can be classed into different types by the form and concentration of the
m ajor elem ents, and the origin of the sample. For example, granite (rock) is
inorganic and is composed m ainly of silicon oxide. In contrast, iron ore sam ple is
also in orga n ic b u t is composed m ainly of iron oxide. The granite requires a
digestion to break down th e silica, while the iron ore requires a digestion for iron
oxide. For the m ost part, sam ples are identified by th eir origin, e.g. botanical,
biological, soil, and geological, to nam e but a few.
For th e purpose of analysis, all sa m p les can be divided into two categories: inorganic
and organic. Inorganic sam ples can be further separated into oxide or reduced
forms. Inorganic sam ples, such as rock, soil, and sam ples th a t have been ashed in
th e presence of oxygen, are, for th e m ost part, in th e oxide form. However, a n acid
a ttack is not alw ays capable of total dissolution and a fusion m ay be required.
M etals an d alloys a re examples of inorganic reduced sam ple types. Ore sam ples
have a high content of reduced m aterial and m ay require a separation, followed by
two sep arate digestions. On th e other hand, alloy sam ples, which a re alm ost entirely
reduced m aterial, can evolve hydrogen gas during digestion, as well as leaving a
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small but undissolved residue. The hydrogen m ust be properly vented, and the
residue m ust be dissolved with a fusion, or else oxidized and then dissolved in acid.
Organic sam ples are completely different. They are, for the most part, composed of
water, carbon, hydrogen, and oxygen, which is generally of little interest. Therefore,
much of the sam ple m ust be elim inated. This is done by drying the sam ple and then
performing a digestion, which releases a considerable volume of decomposition
products (made up of CO., NOxamong others). The evolved gas is eventually lost to
the atm osphere, and the rem aining residue becomes the sample. An acid digestion
of organics m ust a ttain tem peratures exceeding 250”C 1to remove all organic
m atter.
1.4 Methods of Sample Digestion
1.4.1 Atmospheric Digestion
The m ost common way of dissolving a sam ple is to h eat it w ith m ineral acids in a
beaker on a hotplate open to th e atm osphere. The digestion is norm ally done in
several steps, cycling th e tem perature, and adding selected acids a t different phases
of th e digestion. The choice of acid a t any step in the digestion is dictated by the
function of the acid. For example, perchloric acid is used in th e destruction of
organic m atte r because of its oxidizing power, while sulfuric acid m ight be used
because of its relatively high boiling point. The boiling point of th e acid used
determ ines th e maximum tem perature of th e digestion. However, a t atm ospheric
pressure, th is tem perature is often below th a t required. For example, the
destruction of an organic sam ple w ith nitric acid1( whose boiling point is 122"C)
actually requires a tem perature of250°C. I t is th u s necessary to use sulfuric and
perchloric acids to complete th e digestion.
O th er disadvantages also exist in a n open system: the a d d boils off and needs to be
replenished over the usually long digestion tim es, resulting in contam ination from
th e surrounding a ir and loss of volatile analyte.
’Nitric acid refers to HNOj as its azcotropc mixture (685- WAV with water, unless otherwise stated).
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1.4.2 Pressurized Digestion
The use of a pressurized system allows higher tem peratures to be obtained for any
particular acid. This, in turn, increases the acid’s dissolving power. Some
commercially available systems are capable of very high pressures using only nitric
acid; they can dissolve organic m aterials th a t could previously only be dissolved in a
m ulti-step digestion with a combination of nitric/perchloric/sulfuric acids. The
closed system also contains any volatile analytes or acid th a t would otherwise boil
off. This reduces the quantity of acid used, which is not only more economical but
reduces the blank level, since a significant portion of the contam ination comes from
the acids used
The other obvious benefit of a closed system is th a t it prevents
contam ination from the atm osphere.
1.43 Fusion-Based Digestion
Fusion is another method of digestion. Sample is mixed with a flux (salts) th a t
m elts a t a high, tem perature to form a molten salt which can dissolve difficult
sam ples. The sam ple which has been converted to a salt form dissolves easily in
dilute acid. A sodium peroxide fusion (oxidative fusion) is used in the classical wet
chem istry assay of alloys to dissolve the last rem nants of a sam ple which could not
be dissolved by liquid acid digestion. Lithium m etaborate fusion, a reducing fusion,
will dissolve m ost oxides, making it the method of choice for geological sam ples. The
disadvantages w ith fusions are the resulting high salt solution, and dilution. The
high sa lt content m akes certain analytical techniques difficult, if not impossible, and
th e dilution of th e sam ple may reduce the analyte concentration below detectable
levels.
1.4.4 Ashing Techniques
I f th e sam ple is organic, one of the sim plest ways to break down th e sam ple m atrix
is to h e a t it in th e presence of oxygen to form a n ash, which th en easily dissolves in
acid. This ran ta k e place a t room tem perature if the oxygen atm osphere
surrounding th e sam ple is ionized. This is a slow process and useful for sam ples
composed prim arily of organic m atter.
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1.5 Microwave Digestion: Open Digestion Vessel
Open vessel microwave-aided digestion was first demonstrated by Abu-Sam ra in
1975 \ He placed beakers containing sam ple and reagent in a domestic microwave
oven. The inside of the microwave was lined and vented to protect the microwave
from fumes generated during the digestion. The heating was very effective, and
digestion progressed rapidly. A more recent approach to open vessel microwave
digestion uses a focused microwave oven ’4. The unit focuses microwaves onto a
removable glass vessel containing sample and reagent. The digester is equipped
with a reagent dispenser and a carousel th a t can hold 12 vessels. The gases
generated are vented to atmosphere; acid vapors can also be recondensed by a
refluxer positioned ju s t above the digestion vessel. Reagent addition, cooling, and
power are all under computer control, thus th e digestions are essentially autom ated.
Because of the glass digestion vessel, the digester can operate a t high tem peratures
with sulfuric and perchloric acids. However, the glass vessel cannot be used w ith
HF. Teflon vessels are currently available w ith which to undertake H F degistions.
1.6 Microwave Digestion: Closed Bomb Vessel
High pressure, closed digestion vessels have been used for m any years ' '". These
bombs are steel jacketed for strength and lined inside w ith plastic to isolate th e acid
from th e steel. The bombs are heated in resistively heated ovens. These bombs
develop high pressures and tem peratures. Microwave bombs are m ade entirely from
microwave tran sp aren t m aterials and heated in a microwave oven. The microwave
bomb technique is m uch faster th a n th a t of th e sieel-jacketed bomb since it is not
necessary to h e a t th e bomb itself. The reagent is heated directly in a microwave
bomb.
5
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In th e microwave bomb digestion, the operating procedure involves starting with a
set of cleaned vessels, using a precise am ount of reagent, weighing a sam ple into the
digestion vessel, capping the vessel, and placing it in the microwave oven. The
vessels are then heated using a preselected power program. The program may be
simple, using a fixed power level for a period of tim e, or a tem perature/pressure
feedback program m ay be used to a ttain and hold the tem perature a t various levels
during th e digestion process.
W**oHot«
ftu p im t M c n to u n e
Aoaptot BOOy
V w stt Bofly
Figure 1: Microwave Bomb with Vent
(US Patent #5,230,865 - Fig 7)
6
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1.6.1 Venting
As the digestion progresses, gaseous decomposition products may need to be vented.
This can be accomplished using a mechanism th a t will vent at a certain pressure,
regardless of the tem perature; alternatively, the system can be cooled and vented
manually. If the venting is done m anually, the bombs m ust be removed, cooled,
vented by opening the bomb or opening a vent valve (Figure 1) on the top of the
bomb, closed, and put back into the oven. Releasing the pressurized decomposition
gases a t high tem perature causes sudden boiling of the liquid in the bomb and the
formation of sample aerosol, which can escape through the vent, resulting in loss of
analyte. It is also extremely dangerous to handle a hot bomb, or to try to vent it
while it is still hot.
1.6.2 Digestate Removal
When the digestion program is completed, the vessel is cooled, and the digestate is
rinsed from the digestion vessel and brought up to volume. If th e digestate is not
made up to a known volume, an internal standard is needed since the volume of
digestate retrieved from the bomb after digestion can vary due to venting of
decomposition products. V enting not only removes decomposition products b u t also
any acid or w ater vapor present a t th e tim e of venting.
1.63 Digestion Vessel Cleaning
It is necessary to remove all traces of th e last sam ple before another digestion can
be performed in the vessel. This is norm ally done by ru n n in g a blank solution in th e
vessel through a digestion cycle, and/or soaking it in a solution to leach out previous
analyte. In some cases, th e inner lin in g of th e digestion vessel is replaced w ith a
new or previously cleaned one.
1.7 Microwave Digestion: Tube Digestion Systems
C urrently, two m anufacturers supplying microwave digestion system s have a tube
as th e ir digestion vessel
Both system s are open to th e atm osphere a t the exit,
an d have a backpressure arrangem ent to obtain m oderately high pressures. The
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CEM system (S p e ctro P re p ™ )u se s 1/16” OD tubing. Using a w ater carrier an acid
slurry of the sam ple is pumped in and digested. At the end of the digestion period,
the sample is pumped out. A detector a t the output detects the acid front, and
continues collecting th e contents of the tube until the detector reaches a lower limit.
A Rhyeodynewvalve is used to select the flows in the digestion tube. The system is
limited to slurries of less than 1c/c solids and particles of less than ISOpm. The
Rhyeodyne* valves used are also prone to plugging. Stringy biological sam ples have
been known to clog th e inlet valve, requiring a predigestion of the sam ple". At the
tim e of publication, no information was available about the Questron Corporation
digestion system (AutoPrep-Q5000™).
1.8 Limitations of Present Digestion Technologies
1.S.1 Maximize Throughput
One of th e goals in autom ating a digestion is to provide a system th a t can run
essentially unattended 24 hours a day, and be cost effective while sim ultaneously
doing a n equivalent or b etter digestion th an th a t obtained using the traditional
microwave bomb digestion.
The bomb system can have two sets of bombs. Thus, while one batch is digesting,
th e next batch can be readied. If two sets of twelve bombs a re used, and the
digestion tim e in th e oven is 30 m inutes, one could argue th a t th e system is capable
of ru n n in g 24 sam ples an hour. However, the tim e needed for a digestion is not
limited to th e digestion tim e itself; it also includes the tim e needed for cooling,
rem oving th e sam ple from th e bomb, cleaning and weighing of sam ple and reagent.
If th e digestions a re to tak e place in the microwave continually, one batch of vessels
m u st be prepared before another is removed from the oven. W hen the concentration
ranges of elem ents vary greatly between sam ples, th e vessels m ay need to soak
overnight (12hrs) to decontam inate them . Typically, th e sam ples do not vary
greatly; if they do, certain vessels can be designated for this type of sam ple. A good
estim ate would be a turnaround tim e of 2 hours for a batch of twelve sam ples.
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1.8.2 Reagent Disposal
Fumes generated in open beaker digestions are scrubbed out of exhaust a ir
generating a toxic waste product. When the system is closed, the acid th a t would
otherwise evaporate from an open beaker on a hot plate is retained, reducing the
am ount of make-up acid used, disposal of which m ust be managed.
1.8.3 Handling Errors
Many of the problems in sample handling occur after the sam ple has been dried and
pulverized. If the sam ple is weighed into a weighing boat, and then transferred to
the bomb, sam ple can be lost or contam inated. To make m atters worse, it is possible
for the dried m aterial to have a very fine portion, which can become statically
charged. The statically charged sam ple sticks to everything with which it comes in
contact, requiring the operator to m anually scrape, clean, and rinse all item s th a t
come in contact with the sample. Converting the sam ple to a slu rry for
transportation to the digester can lead to errors if the sam ple sticks to tubing and
valves.
1.8.4 Safety Considerations
The pressure in a closed digestion vessel can easily exceed 200 psi during the
digestion of a n organic sam ple. Some commercially available bombs have a safety
vent which releases w hen a m axim um pressure is exceeded. I f th e sam ple vents
during a digestion, th e digestion m ust be repeated w ith a sm aller sam ple weight.
After th e digestion cycle is completed, th e bombs m ust be cooled inside th e oven for
a period before th e operator is allowed to remove them to continue cooling outside
th e oven. The cool-down period perm its vapors to condense, thereby lowering the
pressure. I f th ere a re decomposition products such as CO„ or NO,, these gases will
not condense, and the pressure inside th e vessel will rem ain high. To release these
gases, th e bomb is placed in a fumehood, and th e excess gas is released by slowly
unscrew ing th e vessel cap, or by using a vent valve on th e bomb.
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The handling of digests th a t contain concentrated acid, or dangerous acids such as
HF and HCIO.,, demand the same care and attention to detail as open beaker work.
The sam ples not only give off potentially noxious fumes, but they can also be
contam inated when the digestion vessel is open.
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1.9 Literature Review
The benefits of microwave energy as a replacement for hotplate heating in acid
digestion was first dem onstrated by Abu-Samra et al in 1975'. The microwaves
were absorbed by the reagent and sample but not by the vessel. The technique,
known as microwave digestion, rapidly evolved over the next ten years to become
th e most common method of digesting samples. Open beakers were replaced with
pressurized microwave transparent bombs, resulting in higher tem peratures and,
consequently, more efficient and rapid digestions. The digestion procedure, as w ith
open beaker methods, w as still very labor intensive. In 19SS Kingston w, compiled
extensive information on everything from fundam ental concepts to the m any ways of
performing microwave digestions. The coverage is w ritten entirely from the
perspective of traditional microwave bomb digestion. A chapter w ritten by
Labrecque11discusses th e autom ation of microwave bomb digestion, which is
traditional microwave bomb digestion performed by robots. A year later, a review
by Matusiewicz and Sturgeonu describes microwave digestion’s future as very
promising. The review states “U nfortunately, work reported to date is, w ith few
exceptions,:M\ experim ental and does not advance th e sta te of the a rt theoretically.”
These authors a re also concerned about the complete destruction of organic sam ples.
They see pressurized digestions as the method of choice w ith the introduction of
tem perature and pressure monitoring equipm ent. They suggest th a t th e use of
robotics has th e potential for solving m any of th e rem aining problems in microwave
digestion. In concluding, they describe a microwave digestion vessel which h a s a
steel jacket for high pressures and a microwave waveguide into th e bomb.
1.9.1 Literature Review: Focus
This literatu re review will focus on im portant literatu re related to th e design of
microwave digestion system s. The literature includes observations on existing
system s, and discussion of new polymers, methods o f pressure and tem p eratu re
m easurem ent, dissolution chem istries, and ways of improving throughput.
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1.9.2 Microwave Bomb Digestion: Miscellaneous
To a large extent, the efficiency of digestion depends on the reagents used. Now
th a t microwave digestion can attain high pressures and tem peratures with low
boiling point reagents, the chem istry can be reevaluated. Sulfuric and perchloric
acids are no longer needed to achieve high digestion tem peratures. Matusiewicz ct
al'" and Xu et a lu obtained improved digestion efficiency using moderate am ounts of
hydrogen peroxide added to nitric acid Zehr ”*'1vhas shown th a t bases can be used to
dissolve refractory inorganics and details the use of many reagents, their properties,
and uses for digestion of samples.
Since th e days of th e C arius tube'1’, it has been known th a t digestions a t very high
pressures are efficient. Studies by Wurfels ct a/*' and Krushevska ct a!" showed
th at, even i f the sam ple was dissolved, organic m atter remained. This organic
m a tte r is not a serious problem for such techniques as inductively coupled atomic
emission spectroscopy (ICP-AES), or atomic absorption spectroscopy (AAS); it is,
however, a problem for differential pulse polarography" and absorptive stripping
voltam m etry23. Because organic species adsorb on active surfaces and alter the
response for th e elem ents being determined. High pressure/tem perature vessels
already e x iste d 21 and were adapted for the microwave by replacing the bomb with
microwave tran sp aren t polymers2*. However, the steel casings of previous
generation high pressure bombs lined w ith polytetrafluoroethylene (PTFE) or
tetrafluorom ethoxil-PTFE (TFM-PTFE) are much stronger th a n th eir polymer
counterparts. Accordingly, M atusiewicz ef als’ designed a stainless steel/TFM-PTFE
cased bomb, w ith microwaves fed to the interior of the bomb through a waveguide.
This bomb could m ake use of nitric acid alone to completely decompose the organics
o f botanical and biological sam ples.
In a n effort to reach higher tem peratures a t lower pressures, Reid et al* cooled th e
en tire digestion vessel in liquid nitrogen during th e digestion. (Liquid nitrogen is
microwave tran sp aren t; the microwave energy supplied th u s h eats only the sam ple
and reagent.) The microwave energy was sufficient to su stain th e reagent a t
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digestion tem perature feven with intense cooling). However, the vapor formed did
not absorb microwave energy, and froze. Nitrogen dioxide, which plays a key role in
digestion*', w as effectively removed, resulting in an incomplete digestion. A sim ilar
effort to reduce gas pressure inside the bomb*- during digestion used a cooling tube
fed into th e digestion vessel through the cap of the bomb. A nother approach*" w ith a
sim ilar design used a two step digestion. The cooling tube w as used to reflux the
acid back into the vessel during the first p art of the digestion; th e cap w as loosely
screwed on to let decomposition gases escape. The second stage w as sim ilar to
regular microwave bomb digestion.
A tem perature of260°C can be obtained if sulfuric acid is used in a digestion. A
system th a t w as open to the atm osphere and used quartz vessels*-' relied on sulfuric
acid to reach th e high tem peratures needed to completely break down organic
m aterial. Decomposition gases formed during digestion were easily taken away as
well a s lower boiling point acids such as nitric acid used as oxidizers during initial
stages of th e digestion.
However certain labs still find it cost-efficient to use a microwave w ith open
beakers3"*2, even in light of all th a t h as been done in th e la st 10 years.
1.93 Microwave Bomb Digestion: Automation
In 1986 K napp33 described th e autom ation of batch, flow, and high pressure
digestion m ethods; however, he does not discuss microwaves a s a h e a t source.
Labrecque13describes a complete robotically-operated microwave digestion system
used to digest sam ples from a copper sm elting process. The digestion vessels were
identical to those used for m anual microwave digestion. Although th ere were some
problems w ith equipm ent corrosion, th e system improved throughput and precision
of th e analysis. However, a class 100 fumehood was required w here th e digestion
vessels w ere opened. The vessels w ere still cleaned m anually. L ater, using a
system sim ilar to Labrecque’s, S aati34 performed quality control analysis of
tan talu m powder. The system reduced th e exposure of laboratory operators to
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hazardous acids, while m aintaining the precision of analysis. W alter ct al describes
a system geared to two methods (3015, 30511 '
which was used by the
Environm ental Protection Agency (EPA) for waste management. The hardw are was
sim ilar to th a t used by Labrecque11; however, the user interface was more
sophisticated. Ball™ refined the same setup as Labrecque to dissolve sam ples from a
steel mill. Norris et al*1*' takes a fresh look at the robotics and m akes some
interesting changes. This system uses high pressure microwave digestion vessels41
and everything th a t comes in contact with the sam ple is disposable.
In essence, th e robotic microwave digestion system duplicates the operations of a
laboratory w orker while reducing the hazards and improving precision.
1.9.4 On-Line Microwave Digestion Systems
Flow Injection Analysis (FIA) dem onstrated how open vessel reactions could be
replaced w ith a flowing system to create a technique th a t was less labour intensive
yet still precise. In 19S6, following th e FIA lead, B urguera ct a f ‘\ m ineralized the
Fe, Pb and Zn in whole blood w ith the first on-line microwave digest system and
m ade th e tube digestor p a rt of an FIA system. Whole blood mixed w ith acid and
Triton X-100™ w as pum ped through a Pyrex" tube coiled inside a domestic
microwave oven. The m ineralized blood w as pum ped directly to the FIA injection
loop of a flow injection manifold which supplied a n atomic absorption nebulizer.
Triton X-100™ w as used to prevent clogging in th e 1 mm ID digestion tube. For
complete m ineralization, a 100 pi of sam ple mixed w ith 150 pi of acid required 15
seconds residence tim e in th e digestion tube a t full power.
Four years later, H inkam p et at* improved B urguera’s system for determ ination of
P in w astew aters. The digestion tube w as m uch longer (10 m), m ade of .5 mm ID
PTFE tubing, an d w as continually cooled during digestion. After digestion, the
sam ple w as pum ped through a gas diffusion u n it m ade of gas-perm eable PTFE tape.
The gas liquid sep arato r on th e exit of th e digestion tube removed m ost of th e gas
bubbles from th e flow system . The bubbles w ere a source of noise in th e flow
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system. Before injection into the flow system, undissolved organic m atter was
filtered from the digestate. This prevented clogging of the narrow orifices of the
valves of the FIA system used for analysis of the digestate.
Carbonell ct a t improved H inkam p’s system by adding a 100 ml w ater load to the
microwave, and introducing an ice bath to condense the hot vapors exiting the
digestion tube. They claimed th a t the w ater load reduced fluctuations caused by
reflected microwave energy reaching the magnetron.
None of the previous on-line system s was designed for, or capable of, complete
dissolution of the sam ples introduced. For the types of analysis being done, w here
the elements of in terest were not bound in the m atrix, this approach was adequate.
However, if complete dissolution was required, an elevated tem perature w as needed.
In an effort to reach higher tem peratures, Salin ct a t" pressurized th e digestion tube
by placing valves on th e inlet and outlet of the digestion tube; th is was called a
“stopped-flow” system . The tem perature w as read a t the exit of the digestion once
th e digestion w as complete. The 420 cm 4 mm inside diam eter (ID) Teflon
perfluoroalkoxy (PFA)™ digestion tube w as capable of much larger sam ple loads
th an previous in-line system s, with no danger of plugging; it did, however, have a
lower pressure limit.
A therm ally heated on-line system open to th e atm osphere4' w as unable to control
th e formation of gas. This resulted in ejection of sam ple from the digestion tube,
and the production of incomplete digestions. The problem w as rem edied by
converting the system to a stopped-flow arrangem ent.
T he above on-line system s all acquire sam ples in a slu rry form; they a re e ith er
pumped directly to the digestion tube, or sam pled w ith a n injection loop. Gluodenis
et a t ’ noted sam ple segregation problems w hen sam pling th e mixed slurry; a n
addition of Triton X-100™ to the sam ple slurry did not improve the recovery. The
other problems noted w ere th e relatively sm all sam ple size and the large rin se
volume, which resulted in a large dilution volume.
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Using a flowing system th a t operated a t higher pressure and tem perature, Haswell
ct al** employed a back-pressure (75 psi) regulator on the exit of the digestion tube.
Again, overheating of the system generated volumes of gas th a t prem aturely ejected
sam ple and acid from the digestion tube. In an effort to reduce the pressure while
m aintaining the same tube tem perature. Peltier coolers immersed in an antifreeze
bath were used to cool the exiting gases. The 20 m 0.S mm ID PTFE tube could
handle sam ple particle sizes sm aller th an ISO pm, and had a maximum sam ple load
of 25 mg. However, the slurry concentration used could not exceed lfr w ithout
blocking the Rhyeodyne™ valve. Furtherm ore, high concentrations of acid created
m ajor pressure problems; if low pressures were used, incomplete digestions were
obtained. The slurry segregation problem resulted in relative standard deviation
(RSD)’s of 4-5%. Dispersion problems which prevented reproducible slurry' sam pling
were noted w ith th e use of dried botanical National In stitu te of S tandards and
Technology (NIST) S tandard Reference M aterial (SRM's); SRM 1577 Bovine Liver.
In order to prevent m aterial from reaching the back-pressure regulator, a filter was
added to collect undigested sam ple particles from the digestion liquid.
For cold vapor m ercury analysis, all the different forms of mercury m ust be
dissolved and converted to th e Hg(II) form. Welz ct at** dem onstrated th a t only a
m ild sam ple digestion w as necessary to convert all th e m ercury before analysis of
w aters and urine employing a hydride generation unit. The sam ple w as pumped
through a coiled PTFE tube placed in a focused microwave cavity. Gas separation
and filtering of sam ples was needed before hydride generation. Mercury absorption
to tu b e w alls w as noted for certain sam ples and chem istries.
U sing a previously described system*1, Carbonell et
used HNO, and H ,0 , to
dem onstrate quantitative extraction of Mn and Cu from raw sewage sludge and
vegetation. However, particle sizes g reater th an 200 pm caused blockage of th e
system .
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Using Welz et cifs system 4’, Tsalev ct al ,l'r,‘ demonstrated th a t high organic content
sam ples could leave a deposit on the digestion walls. When the power was increased
in order to obtain a better digestion, gas was created, disrupting the flow system.
T em peratures were m easured a t the exit of the digestion tube using a thermocouple
fitted through a “T” in the tube. Tem peratures m easured were always lower than
100°C.
Gluodenis et at"' refitted a previous system 4, with microwave heating, a pressure
sensor, and a PTFE digestion tube. The pressure sensor was connected to the
digestion tube using liquid in a tube and an interfacing m em brane m ade of PTFE.
However, gases evolving during digestion pushed sam ple out of th e digestion tube.
Therefore, the PTFE digestion tube was replaced with a vertical digestion vessel
fitted with valves (inside the microwave oven) on both ends. The vertical tube
design allowed gases to rise into th e headspace provided and exit, w ithout removing
analyte. Microwave heating w as found to be superior to therm al heating because it
was easier to control th e energy supplied to the digestion tube. Sam ples were
slurried w ith nitric acid and injected into the digestion vessel through a 200 pi
sam ple loop. However, Gluodenis ct al found th a t th e system w as lim ited to 1.4 - 2.S
m g o f carbon p er m illiliter of vessel size, resulting in a m axim um load of 45 m g of
sam ple w ith 0.5 m l of nitric acid. The relatively large elution volume (10 ml)
yielded a large dilution factor. C ertain sam ple types, such a s pine needles, bovine
liver, etc., could not be m ade into a slurry. A sulfuric/nitric acid combination did not
provide a b e tte r digestion th a n nitric a d d used alone; hydrochloric a d d w as
elim inated from consideration because it generated rapid pressure increases and
digested th e stainless steel pressure sensor body. Digestions needed to be followed
dosely to avoid over-pressurization of th e system . Cooling of th e digestion vessel
during h eating did not significantly reduce th e operating pressure. The system did
yield a lower residual carbon content th an conventional microwave digestion bombs.
T he vessel w as able to operate a t higher pressures and tem p eratu res t h a n
traditional microwave bombs; however, trace elem ents bound to th e undigested
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silicate m aterial in the digested sample, giving a large spread in accuracy.
Therefore, the borosilicate digestion vessel was found to be unacceptable for use in
trace elem ent analysis.
Using Haswell ct al's system4*, Williams ct a t repeated the work of H inkam p ct at*.
However, unlike previous experiments, no particles were found in the digested
solution.
In a n on-line system , de La Guardia ct a t fed the output stream of th e digestion
tube into a n ice-cooled constant head vessel. Digested sam ple w as pumped to a
nebulizer from th e bottom of the constant head vessel. The constant head device
reduced th e noise in the nebulizer. Cu and Mn were determ ined in different types of
solids w ithout using a stopped-flow arrangem ent.
W ith a previously dem onstrated system*', Welz ct at "' digested whole blood using
perchloric acid and nitric acid. A flow restrictor on the output of the digestion tube
gave a back p ressure of 6 Bar, minimizing the formation of bubbles in the liquid
digestate. The organic m aterial was not digested, but w as sufficient to obtain
inorganic As p resent in th e blood. Spike recoveries were poor, and excessive
foaming occurred when the digested sam ple w as mixed with the reductant in the
hydride analysis o f th e blood.
To obtain direct m easurem ents inside th e digestion vessel, Benson ct a t *used a
fiberoptic probe to m easure pressure and tem perature through a “T ” in th e digestion
tube placed inside th e microwave oven. The m anufacturer of th e probe and p a rt
num ber w ere provided58, b u t no explanation w as given a s to how i t operated. The
tem p eratu re m easured w as always lower th a n 100°C. Tiles w ere placed in the
bottom of th e microwave oven to prevent reflected microwave energy from reaching
th e m agnetron, resulting in smoothed tem perature and pressure profiles.
In w ork sim ilar to th a t of Welz et at*, Guo e£ a t 0 successfully achieved th e
determ ination o f H g in whole blood. The type of arid used for m ineralization in th e
incom plete digestion of th e blood sam ples w as found to be unim portant a s long a s all
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mercury was converted to the Hg(II) form. They used a knitted coil, instead of a coil
wrapped around a shaft as Welz et al" had done. When acid was used to carry a 500
pi injection of blood to the digestion tube, deposits were observed; therefore, w ater
was used instead.
A part from a few changes, such as liquid-gas separation, current in-line system s had
not evolved much since the first in-line system was dem onstrated by B urguera et
al*\ This changed in 1992 when Stew art et a t ' elucidated the problems w ith on-line
digestion, and showed how they could be solved. They designed several digestion
tube and vessel system s, used different acid and sample types, and obtained
complete sam ple dissolution for some difficult sam ple types. All digestions were
done using a focused microwave cavity. The digestion vessel designs fell into two
categories: tube and vessel. Various tube diam eter and coiling arrangem ents were
evaluated. One design had bulges in the tube to allow gas to escape w ithout ejecting
the sample. However, all b u t one of the coiled Teflon tube designs still pushed
sam ple out w hen heated too strongly. W hen larger diam eter tubes allowed gas to
escape, sample particles clung to the cooler p arts of th e tube and were not removed
by the refluxing acid. According to Stew art e t al, an ID of 7 mm w as necessary for
gas-liquid separation; if a peristaltic pum p w as used to remove th e digestate, a
maximum 2 mm ID w as required.
The m ost successful digestion vessel w as an 8 ml glass ball designed to fit into the
focused microwave cavity. A tube coming from the bottom of the glass digestion
vessel w as used for adding and removing sam ple and reagent. A water-cooled vent
exited from the top. Gases th a t formed during the digestion could escape through
th e top vent, m aking evaporation possible; th e vent could also be cooled in order to
reflux acid back into th e vessel. A tube coming from the bottom tube of th e
digestion vessel allowed reagents and sam ple to be added during th e digestion. Air
pumped into th e vessel through the bottom m ixed th e contents an d prevented
partially digested m aterial from getting into th e tube. The high tem p eratu re of
sulfuric a d d in th e glass digestion vessel allowed total sam ple decomposition.
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In a recent publication1, Matusiewicz ct a! compared a high-pressure/tem perature
focused-microwave heated system to an on-line microwave heated system. The
high-pressure system had no tem perature or pressure sensors, while the on-line
system had tem perature and pressure probes a t the exit. Using only nitric acid, the
high-pressure bomb successfully digested botanical and vegetation samples. The
digested sam ple solutions were colorless, and had nearly negligible residual carbon
content. The on-line system consisted of a Teflon PFA tube wrapped around a
plastic tu b e and placed in a microwave-focused bomb. The tube system was open on
th e exit end; th u s, pressures and tem peratures were lower th an in the closed focused
microwave bomb. Residual carbon content w as sim ilar to th a t obtained with
trad itio n al microwave digestion bombs. Hydrogen peroxide w as not effective
because tem peratures were not high enough; the use of peroxodisulphate (Oxisolv"),
one of th e strongest oxidizers in aqueous media, gave promising results, w ith a 50%
reduction of residual carbon in the digestion solution.
In 1994 CEM Corporation became the first to commercialize an on-line microwave
digestion system . The system, a continuous flow system based on the design of
Hasw ell et o f 8, w as evaluated by Sturgeon et aV~ for th e digestion of biological and
sedim ent sam ples. Sam ples required homogenization and size fractionation to
produce a u n iform slurry. Sam ples w ith stringy, fibrous stru ctu res plugged the
en try valve, and required a predigestion.
1.9.5 Literature Review: Conclusions
The microwave digestion system can be separated into two types: bomb and tube.
The bomb system s have th e potential to yield a more efficient digestion because they
can reach higher pressures and tem peratures; however, they require th e use of
expensive robotics. Tube system s, on th e other hand, have been autom ated, b u t still
require th e sam ple to be introduced as a slurry. Tube system s are plagued w ith
relatively sm all sam ple size, non-quantitative sam ple transfer, plugging of valves
a n d th e inability to deal effectively w ith production of decomposition gases.
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1.10 Objectives
The objectives of my research were therefore as follows:
To design a valve system th a t would not be
abraded by the passage of samples, and th a t
would allow efficient cleaning between samples.
To develop an autom ated high pressure digestion
system with feedback.
1.11 Summary of Thesis Contents
C hapter 1
The concept of a digestion is explained. Microwave digestion, performed m anually
and with robotics, using traditional microwave digestion bombs, is discussed. On­
line microwave digestion is introduced as a technique for autom ating microwave
digestion, and elim inating m any of th e m anual tasks of traditional microwave bomb
digestion. A literature review discusses th e aspects of microwave digestion th a t
need to be considered in developing a n autom ated microwave digestion system , such
as new m aterials, containm ent system s, and sam ple transfer.
C hapter 2
The benefits and disadvantages of narrow tube microwave digestion are explained.
A fter discussing th e problems w ith current valve designs, th e flange valve is
introduced as a solution to abrasion and memory problems associated w ith
commercially available valves. The large digestion tube design, with its cooling and
venting system s is th en presented. The use of a “Squeegee” a s a device to remove
digestate and d e a n th e digestion tube is presented.
C hapter 3
The control language M ICR02 is introduced and explained briefly.
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C hapter 4
The advantages of capsule sample introduction are detailed. The selection of a
suitable m aterial for the capsule and the m anufacture of capsules are explained.
C hapter 5
The various types of tem perature sensors used in present microwave digestion are
detailed. Infrared tem perature sensing is developed as an alternative to current
tem perature sensors. Tem perature sensing of the cooling w ater and m agnetron is
also discussed. The “In-line” pressure sensor is introduced to overcome the
difficulties w ith pressure sensors th a t use liquid pressure transfer lines.
C hapter 6
The acquisition of d a ta during a digestion “run” is explained. The digestion runs of
successively m ore complicated m aterials including water, salt w ater, acid alone, acid
and capsule, acid and soil, and finally the digestion organics are discussed. Methods
of using venting to deal w ith sam ples th a t produce decomposition gases are
developed.
C h ap ter 7
The digestion and analysis of a wide range of sam ple types are presented.
Botanical, biological, sedim ent, soil, and capsules are digested and the results
discussed.
1.12 Contributions to Thesis
T he u p d ating of M ICR02 to Borland Pascal version 7.0 and docum entation
(Appendix C) w as done by Christine Sartorus. The documentation h as since been
edited to reflect im provem ents m ade to M ICR02.
Dr. E.D. Salin provided th e focus th a t m ade th e project a success.
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2. Capsule / Tube-Based Microwave Digestion
2.1 Introduction to Microwave Tube Digestion
The last chapter provided an overview of different forms of microwave digestion.
This chapter focuses on the various aspects of tube digestion itself before
considering any alternatives. It is im portant to understand the advantages and
problems associated with tube microwave digestion in a “traditional" narrow
digestion tube. The narrow tube digestion system typically uses rotary valves or
needle valves to control exit from or entry into the digestion tube.
2.1.1 The Advantages of Narrow Tube Digestion
The digestion tube rem ains in the oven, thus elim inating the handling of the
traditional microwave bomb along with its inherent hazards. The tube design also
elim inates the need to transfer the sam ple into the digestion vessel m anually;
Slurry and acid
W a v c g u id o
in
S o lu ti o n o u t \
M a g n e tro n
G ro u n d e d
(a n s c re e n
S u p p o rt
Figure 2: McGill Slurry Tube Digestion
instead, sam ples are pumped into the digestion tube in th e form of a slurry.
The cap on a conventional microwave bomb digestion vessel can be considered as a
form of valve. The microwave bomb’s large m outh provides unrestricted access to
the interior, m aking it easy to place large sam ples in th e vessel, and, w hen th e bomb
is sealed, gases are prevented from escaping. The large opening an d th e ability to
23
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operate at high pressures are both excellent features: however, the conventional
bomb is not easily autom ated because it needs to be loaded, opened, closed, and
cleaned manually.
The latest trend in microwave digestion has been towards the use of {lowing
systems. Once the sam ple has been slurried, it is fairly simple to move it from place
to place using pum ps and valves. The bomb is replaced with a tube, and th e sample
is pumped into th e tube in the form of a slurry. The tube used is typically less than
1/4” in outside diam eter (OD) because it is not possible to pump the relatively small
volumes (-25-100 ml) through a tube of larger diam eter: valves at the entrance and
exit of the digestion tube are actuated electrically or pneumatically.
2.1.2 Narrow Digestion Tube Characteristics
The narrow digestion tube has certain characteristics which may lim it its
performance during a digestion. A study of each characteristic will allow th e system
to be improved while retaining the benefits of tube digestion.
2.1.2.1 Valve Selection: Needle Valve
K aranassios et al V" tube system used a 1/4” OD
PFA tube, which w as laid flat inside the
Valve Seat
microwave cavity (Figure 2). The digestion tube
w as closed a t both ends w ith a needle valve. The Flow>'
needle valve (Figure 3) uses a pointed stem th a t
fits into th e valve seat to control flow through
Figure 3: Needle Valve
th e valve. The se a t of the needle valve can clean
itse lf as th e valve opens and closes; however, th e needle th a t pushes against the
seat of th e valve crushes m aterial into it. W hen abrasive m aterial is present, the
action of opening and closing eventually causes it to leak. The needle valve is also
difficult to close autom atically because it not actuated by a precise num ber of turn s,
b u t is ra th e r tu rn ed to a certain torque. This type of valve is norm ally used when
fine flow control is required; it is not recommended for slurries"1, since solids can
24
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settle in the many recesses of the valve and be difficult to clean out. Any cracks or
crevices in the valve th a t come in contact with the sample slurry can trap sample,
and thus contam inate future samples.
2.1.2.2 Valve Selection: Rotary Valve
Used in system s requiring chemical inertness, the rotary valve consists of two faces
pressed against each other. Channel grooves in the two faces of the valve align,
allowing passage of liquid through the valve. A 90" tu rn closes the valve. Any
m aterial left in the groove when the valve is closed comes in contact w ith the
opposing face. Opening and closing the rotary valve with abrasive m aterial in the
channel grooves abrades the faces, eventually causing the valve to lose its seal. The
valve has very narrow passageway s which restrict th e passage of sam ple for particle
sizes g reater th an 100 pm, and of slurries g reater th a n 1% solids.
2.1.2.3 Sample Transfer
There are two ways to tran sfer a sam ple slurry into a digestion tube. The first is to
pum p all of the slurry into th e digestion tube; th is is followed by a rinse solution to
insure th a t all of the sam ple h ad been transferred from th e original container. The
second is to select a representative portion of th e slurry using an injection loop. The
injection loop is th en switched into a carrier stream , and pum ped through th e
digestion tube**’64. W hen a sam ple loop on an injection valve is used to select a
portion of th e slurry, segregation can occur if th e slu rry is not properly m ixed a s it is
being pum ped from th e sam ple container.
In both cases, th e rinse solution m ay not flush all of th e slurry into th e digestion
tube, resulting in less th a n 100% tran sfer of th e sam ple aliquot. The solution used
to rin se all of th e sam ple into th e digestion has th e added undesirable effect of
fu rth er diluting th e sam ple.
2.1J2.4 Volume Dilution and Internal Standards
In q uantitative analysis, all th e sam ple m u st be transferred from th e digestion
vessel. I f sufficient rinse solution is used, it can be assum ed th a t all of th e sam ple
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originally pum ped into the digestion tube is contained in the volume removed from
th e digestion tube. It is then simply a m atter of diluting the removed volume to a
known volume in order to allow quantitative calculations. If the sample is not
completely removed from the digestion tube, an internal standard can be mixed with
th e sam ple before digestion. The internal standard elim inates the need for a large
volume flush solution th a t can cause excessive dilution of the sam ple. When using
an in tern al stan d ard in this fashion, the concentration of the internal standard is
used to determ ine the fraction of sam ple solution volume obtained.
2.1.2.5 Deposit Formation
A fter rin sing and cleaning th e digestion tube, undigested m aterial may still be
p resen t on th e inside walls of both these systems. Generally, only physical action
will remove th is m aterial. The type of cleaning needed is much like th a t used for an
open beaker digestion. After the digestion, the beaker is cleaned w ith soap, w ater,
an d a n abrasive elem ent to remove scum th a t forms a t th e liquid surface on the
inside w all o f th e beaker. The m aterial rem aining on th e inside walls of the tube
can consist of undigested protein if biological or botanical m aterial h as been
digested. T he undigested protein deposits can accum ulate particulate m aterial.
The p rotein itself m ay be leached of all elements of interest, b u t the particulates
th a t i t collects can contribute to contam ination of future digestions. The deposits in
th e tu b e also build up and m ay eventually clog the tube.
2,13.6 Localized Heating
A nother potential problem w ith th e narrow tube design is th a t portions of th e tube
a re h eated a t different rates due to the inhomogeneity of th e microwave field
stre n g th in th e cavity. Fortunately, th is effect appears to be overcome to some
ex ten t by th e rocking motion of th e liquid in th e tube. The rocking motion is caused
by gas form ation and condensation in different sections of th e tube.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.1.2.7 Tube Temperature Measurement
The tem perature of the liquid inside the tube is difficult to m easure because of the
small size of the tube. The tem perature is usually m easured on the outside of the
tube, where it is significantly lower than on the inside because the relatively thick
and insulating fluoropolymer walls of the tube create a tem perature gradient from
the inside to outside.
A commercial system " m easures the tem perature of the tube on the outside of the
microwave oven. It is possible to calibrate such a tem perature m easurem ent system
to compensate for the tem perature difference between th e tem perature inside the
tube (inside the oven) and the tem perature on the outside of the tube (outside the
oven). However, rapid changes in tem perature would not be properly m easured. I f
the tem perature of the tube is m easured inside the microwave cavity, the
thermocouple m ay h e a t independently of the tube, thus providing erroneous results.
The conventional bomb has the thermocouple inserted in liquid shielding it from th e
microwaves; th e thermocouple attached to a narrow tube is not im m ersed in a
shielding liquid.
2.L2.8 Tube Diameter Considerations
Figure 4 depicts a narrow tube. The light area inside th e tube represents gas, while
th e d ark a re a represents liquid. The movement of th e gas is represented by arrows.
The gas can be e ith er acid vapor or decomposition products. As th e gas moves back
and forth in th e narrow tube, it pushes th e liquid in front of it. The decomposition
gases th a t a re produced during a digestion cannot be vented w ithout losing analyte.
Opening th e valves to reduce the pressure allows the compressed gases to exit,
forcing out sam ple and reagent a t th e sam e tim e. Therefore, if over-pressurization
occurs during a digestion, the only way to prevent tube ru p tu re w ithout losing
analyte is to stop microwave heating, or to cool the system.
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A nother characteristic drawback of the narrow tube
Liquid Piuq
design is th a t the mixing of liquid zones which are
separated by gas volumes is not efficient. If a liquid
G a s-
as
volume in the tube becomes depleted of reagent and it
is separated from th e rest of the reagent volume by
gas, it will probably rem ain depleted for the
Figure 4: N arrow
D iam eter Tubing
rem ainder of th e digestion, which may result in a less
efficient digestion.
2.1.3 The Disadvantages of Narrow Tube Digestion
Table 1 s u m m a r iz e s th e problems with the tube microwave digestion system s th a t
have been developed to date. These problems are brought about by the types of
valves and th e size of tubing used for the digestion tube. The first six problems
listed in Table 1 do not occur with conventional microwave bombs.
1
The inside diam eter of th e valves and tubing is not capable
of dealing w ith a particle size g rea ter th an 100 pm.
2
S lurries which are more th a n 2% solids cannot be used.
3
N on-quantitative sam ple tran sfe r or nonhomogeneous
sam pling of sample.
4
Inefficient mixing of reagent w ith sam ple in the tube during
digestion.
5
I t is not possible to physically clean th e inside of th e
digestion tube between digestions.
6
Localized heating.
7
V enting of decomposition products from th e tube is not
possible.
8
T he volume collected from th e digestion tube m u st be
diluted to a known volume or have a n internal sta n d ard
added to th e sam ple before digestion.
9
A ccurate tem peratures a re difficult to obtain.
Table 1: Main Problems with Narrow Tube Digestion Systems
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2.2 Valve Selection and Design
The principal function of a valve is to close or open a vessel. When closed, the valve
m ust prevent the contents from escaping; when open, it m ust allow m aterials to be
placed into or removed from the vessel. The sam ples m ust rem ain uncontam inated;
the valve m ust also be able to w ithstand the tem peratures, pressures, and chemical
environm ent generated during a digestion. N either the needle valve nor rotary
valve meet these operating standards. The needle valve was found to be
unacceptable m ainly because of abrasion problems; the rotary valve was elim inated
because of its narrow passageways. Therefore, other types of commercially
available valves were investigated.
2.2.1.1 Ball Valve
A ball valve (Figure 5) consists of a ball w ith a hole
“ “ S.______
through it which rotates in a valve body. The ball is
connected to a handle th a t rotates the ball 90°. O-
F,ow
rings, which are used in a ball valve, are seated in a
Ba[| ^ g0^ y
groove in the valve body; they seal the ball against
C ontact Area
th e body of the valve. The O-rings are not in direct
f j ^ ure 5* Ball Valve
contact w ith th e contents of th e digestion tube;
however, m aterial w etting th e ball is wiped from th e ball onto th e O-rings as th e
valve is opened and closed. The m aterial is also wiped from the ball into th e O-ring
groove. Likewise, slurry in th e ball passageway is brought into contact w ith th e
valve body when th e valve is closed; it rem ains betw een the valve body an d th e ball
w hen th e valve is reopened.
2J2./_3 Custom Valve
All of th e commercially available valve designs studied h ad p a rts th a t would en train
sam ple into th e valve mechanism , creating cleaning difficulties. W etted surfaces in
th e needle, rotary, and ball valves a re usually m ade of PTFE or
polychlorotrifluoroethylene (Kel-F*). These m aterials a re relatively soft, m aking it
easy for solids to embed in valve surfaces. The embedded m aterial abrades th e
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surfaces, causingthem to leak and, eventually, to fail. It thus became necessary to
consider designing a custom valve. A suitable valve design must be able to
w ithstand the tem peratures, pressures and environment of a digestion; it m ust be
easily cleaned, and actuated, and m ust not abrade when closed.
2.2.2 Design: Pressure and Temperature
P ressure is a m ajor concern in valve design. The valve m ust be able to w ithstand
th e pressures of a digestion, and to do so with a considerable m argin of safety. A
factor of two over-pressure (one h alf the actual burst pressure) is considered an
adequate safety m argin. Each valve design has a different weak point. In some
valves, th e connection of the valve to the tubing is the w eakest point. In others, the
w eak point may lie in th e O-rings or in a threaded component.
2.2.2.1 Tube Attachment to Valve
The joining o f the tubing to the valve was
S wa gel okN ut
PFATube^8 ' ^
problematic for several reasons. The main
problem lay in the strength of the
connection. In the three types of valves
^Ferrule Constriction
studied, the connection of the tubing to the
valve w as generally weaker than the valve
1/2"NPT
] ^ Swagelok
iu
j “ack F®rrule
Front Ferrule
FiSure 6: Swagelok Type Fining fo r
Attachment o f Tube To Valve
or tubing alone. The tube was usually
attached to the valve w ith a device that
crimped a ferrule onto the tube; the ferrule sealed the valve onto the tube, and held
the valve in position. W hen the tube was made of a plastic material that softened as
it w as heated, the ferrules came loose on the tube, allowing the tube to separate
from th e valve. Thus, ferrules would not provide a solid connection to a hydrocarbon
corrosion resistant tube if the tube was to be constantly heated and cooled. Also,
ferrules caused a narrowing of the inside diam eter of the tube (Figure 6), which
could cause plugging. Reaming out of the tube to remove the constriction caused by
th e ferrules reduced th e w all thickness at that point and weakened the tube.
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2.2.2.2 Closing Diameter
For a valve of a specific m aterial and wall thickness, the closing surface's inside
diam eter will determ ine the pressure rating of the valve. The closing diam eter is
the largest inside diam eter seen by the gas or liquid inside the valve. The larger the
diam eter, the lower the pressure rating for the sam e wall thickness. For example, a
ball valve may have an internal passage diam eter of 1/4"; however, the O-ring
diam eter (which determined the closing diam eter) determ ines the pressure the valve
can w ithstand. If the O-rings have a diam eter of 1", the valve will be much w eaker
than a valve w ith 1/2” diam eter O-rings. The rotary valve connected to tubing with
a flange fitting (defined below) has a closing diam eter the sam e size as th a t of the
tubing. The maximum pressure allowed for a rotary valve is generally determ ined
by th e strength of the tubing used. The needle valve has a sm all closing diam eter
and should be fairly strong, b u t th e threads th a t are used to a ttac h th e valve to the
tubing are nearly double th e inside diam eter of th e tubing; th e pressure lim iting
clem ent is th u s the threaded connection to th e valve ra th e r th e valve itself.
2 2 2 3 Valve M aterial
Most valves m anufactured for corrosive duty are m ade o f a fluoropolymer. These
fluoropolymers do not decompose a t digestion tem peratures of 200°C but instead
soften an d lose much of th e ir strength. The loss of stren g th a t these tem peratures
significantly reduces th e working pressure range.
1 2 3 Valve Access
O f all of th e valves studied, th e ball valve has th e largest access through th e valve
body. T he large internal diam eter through th e valve allows passage of discrete
pieces of m aterial th a t would not pass through a rotary or needle valve. T hus, m any
of th e sam ples to be digested would not require any grinding before en try into th e
digestion tube.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
2.2.4 Flange Valve
The “flange valve" (Figure
7) provides access to the full
inside diam eter of the
digestion tube. The valve
F lan ge V alve O p e n
body, and the screw-in
elem ents (adapter and
reta in e r nuts) th a t press the
two flanges together are
Flange Valve C losed
quite m assive and keep the
tubing in th e valve a t room
tem perature. (Tubing
m aterial kept a t room
tem p eratu re re ta in s its full
F lange Assembly
S e a le d for Pressurization
strength.) W hen th e valve
Digestion Tube
is opened, both th e tube end
an d th e flat surface can be
easily cleaned before it is
closed. T he valve
connection to th e tubing is
no longer th e w eak
component, because the
A dapter Nut
Closing A dapter
Valve Body
Retainer Nut
Figure 7: Flange Valve Showing Main Features
tube w all thickness is
m aintained an d th e closing
diam eter is th e sm allest
possible (inside diam eter of
th e tube).
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2.3 Digestion Vessel Design
The design of a pressurized digestion vessel m ust not
only enable a rapid and complete dissolution but m ust
also facilitate control and autom ation. Both
conventional bomb and tube digestion vessels have
^Valve
good features, but neither possess all the features of an
ideal digestion vessel. For example, the bomb can be
vented w ithout losing sample, and can be loaded with
samples containing large particles; the tube cannot.
On the other hand, the tube is p a rt of a flowing system
and is thus straightforw ard to autom ate. Therefore,
we began our new digestion vessel design process by
trying to combine th e best features of the bomb and
tube system.
The conventional bomb is essentially a very large
diam eter tube which is stopped a t one end; a t th e other
Figure S: Initial
Digestion Vessel
Design. using
Conventional
Bomb and Tube
Features with a
Flange Valve
end, a cap serves as a valve. To combine th e bomb w ith a flowing system , the
bottom is draw n out and attached to a narrow tube. The diam eter of th e bomb is
reduced slightly, a flange valve replaces th e cap, and, finally, a flange valve is fitted
to th e end of th e tube th a t is attached to th e bottom of th e bomb. F igure 8 depicts a
version of a new digestion vessel, one which can be vented, will mix efficiently, and
which can accept sam ples containing large particles. Since it is p a rt of a flowing
system , it can also be autom ated w ithout g reat difficulty.
The sam ple can be pumped into the new vessel as a slurry, and th en pum ped out
from th e narrow end. Gases th a t a re generated during a digestion percolate to th e
large end; they a re vented by opening a v e n t valve placed in th e w all of th e large
end. This new vessel design w as a m ajor step forward, because i t could be vented
and autom ated. However, th e drawback w ith th is vessel design w as th e difficulty in
cleaning th e vessel properly afte r each digestion. E arlier work h a s show n4* th a t a
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deposit th a t is not removed with an acid wash can build up on the inside of the
digestion tube. The vessel could only be cleaned by flushing it with liquid, since it
would not be possible to physically clean the inside surface of the vessel. This was a
m ajor drawback with this vessel design. As a result, the vessel design depicted in
Figure S was never implemented, although it was an im portant conceptual step.
The increase in diam eter a t the inlet of the digestion tube was able to resolve the
sam ple particle size problem. If the vessel were the same diam eter over the entire
length, and had a larger inside diam eter, then a cleaning device could be pushed
through the full length of the tube. To achieve this, the new vessel design was
replaced w ith a tube w ith 1/4” ID - 3/S” OD. and 60 cm in length. This 1/4" ID tube
w as larg er th an th a t used by K aranassios ct a t" or HaswelT* ct aI However, the
plug effect described earlier w as still a problem with this size of tubing, and proper
m ixing and venting did not occur.
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2.3.1 Large Bore Tubing
To allow mixing, cleaning, and
venting, a larger inside
254 mm
diam eter, 3/8” (9.5 mm), was
necessary. This diam eter
Ji-M
digestion tube (Figure 9) is
conceptually a small diam eter
conventional bomb, w ith the
j- 22.9 mm O.D. Flange
A n ti-R o ta tio n P iu s
difference th a t it can be opened
Flange N uts —
(Stainless Steel)
a t both ends; it retains the
positive features of both the
sm all-diam eter tube system
L ength o f T u b e 7 1 4 m m
.VS" (‘1.525 mm) I.D. X 1/2" (12.7 mm) O.D.
and th e wide-mouthed
conventional bombs. The tube
is fitted a t both ends w ith a
flange valve and a stainless
steel holder nut. The stainless
127 mm R.
steel holder keeps the flange in
position in th e flange valve; the
anti-rotation pins prevent the
digestion tube from tw isting
w hen th e flange valve is closed.
Figure 9: PFA Digestion Tube with Stainless
Steel Holders
The large diam eter tube,
however, does not allow th e
sam ple to be pumped out.
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2.3.2 Squeegee is the Answer
The problems with sample
Teflon Waffer
Digestion Tube
removal and cleaning for a
large inside-diam eter
digestion tube can be solved
sim ultaneously using one
Flexible Rod
RTV silicone
simple device. The sample is
pushed out w ith a Teflon
coated silicoi. £ plug, called a
“squeegee,” which is attached
Figure 10: Squeegee
to th e end of a flexible rod
(Figure 10, Photo 1). The
squeegee not only removes the sam ple solution without dilution or contam ination
but also removes any m aterial stuck to the inside walls of the tube. The squeegee is
m ade from RTV 700* silicone, a room tem perature vulcanizing (RTV) silicone'* th a t
is flexible and th u s can follow changing contours inside the tube. However, the PFA
surface prevents th e silicone from slipping, m aking it difficult to push th e squeegee
down th e digestion tube. To m ake the squeegee slip easily down the digestion tube,
an d also prevent th e acid from coming in contact w ith the silicone, a disposable
Teflon foil .005” thick and 2” in diam eter is draped over the squeegee (Figure 10).
The foil is discarded after each use, and th e squeegee itself never comes in contact
w ith th e acid. The flexible PTFE 1/8 inch OD rod used to push the squeegee
through th e digestion tube m ust be cleaned before each use since it may have
touched th e inside w alls of a n uncleaned digestion tube or outside surfaces.
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1.
2,3.
4.
5.
6,9.
7.
S.
Split mold holder.
Split mold.
Silicone Squeegee (shown a fte r being trim m ed).
PTFE, 1/8” x 80 cm.
Disposable Teflon foil.
A dapter to hold squeegee on rod.
Teflon Foil fitted over squeegee, ready for use.
Photo 1: Squeegee Molding Apparatus and Assembly
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2.3.3 Orientation
The digestion vessel is a large diam eter tube connected a t each end to a flange
valve, and m ounted on the b' 1k wall of a microwave oven. Figure 11 shows a side
view of th e digestion tube with valves at each end. The design has two key features.
F irst, the centre of the tube is lower than the two ends, thus acid flows to the center
of th e tube. Second, the two ends of the tube are m aintained a t room tem perature
by the m assive stainless steel flange valves. Sample may be pumped in as a slurry,
using a tube th a t is inserted through the opened valve: the slurry never comes in
contact w ith the inlet valve. The sample, therefore, cannot abrade or contam inate
the in let valve.
M I C R O W A V E O V E N WALL
-■ FL A N G E VALVES
LIQUID SURFACE
U-SHAPED
DIGESTION TUBE
Figure 11: Side View o f Digestion Tube with Flange Valves
Installed in Microwave Oven
Lowering th e m iddle of th e tube w ith respect to both ends keeps hot liquid away
from th e valves, an d allows gases to rise to th e flange valves. This m akes it possible
to v en t gases from th e digestion tube w ithout losing analyte a t th e sam e time.
A fter th e sam ple a nd acid have been transferred to th e digestion tube, th e flange
valves a re dosed, and th e digestion is sta rte d w ith th e application of microwave
38
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power. As the acid is heated, the sample is dispersed in the acid, and, on further
heating, the acid boils, sending acid vapor bubbling through the acid. The released
vapor condenses on the cold walls of the digestion tube near the flange valves, and
flows back down to the centre of the tube. The boiling action ensures thorough
mixing of sample and acid; any sam ple th a t may have been pushed up the tube by
the boiling action is washed back into the acid by the action of the condensate
flowing back down the tube into the acid.
2.3.4 Material Selection of Digestion Tube
The digestion tube m ust be able to sustain the rigours of m any digestions. The tube
m aterial m ust not be adversely affected by the cycling of tem perature and pressure,
and the variety of sam ples and reagents. The tube should be robust, as well as
easily replaceable. M aking the tube out of quartz or Pyrex® allows for higher
pressures and tem peratures; however, a glass tube would break more easily. The
m ain concern is the therm al stresses th a t can easily break the tube as it is heated or
cooled rapidly. On th e other hand, a tube m ade of a fluoropolymer would be very
flexible and robust. The tube m ust also be inert to all reagents used. Most
fluoropolymers allow alm ost all reagents to be used; w ith th e exception of
hydrofluoric acid, th is is also tru e for quarte or Pyrex®.
2.3.4.1 Memory
The tube m aterial should not allow reagent or sam ple to penetrate or stick to th e
surface. If m aterial is lost into th e surface, i t will contam inate future sam ples
digested in the tube. Again, fluoropolymers, quartz, and Pyrex® are good candidates.
Q uartz and Pyrex® are b e tter choices th a n plastic because they are not porous to
gases; however, glasses are known to etch and become pitted. The pitting caused by
sam ple an d reagents can tra p sam ple, m aking undigested m aterial difficult to
remove. Fluoropolymers do not become etched or pitted, b u t will eventually
decompose and allow m aterial to bind to th e surface. I t is also conceivable th a t
analyte could perm eate th e plastic tube m aterial an d contam inate fu tu re sam ples.
39
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2.3.4.2 Temperature and Pressure Considerations
To complete a digestion, a specific tem perature m ust be achieved and maintained.
The tube m ust not melt, decompose, or be otherwise affected a t digestion
tem peratures. Fluoropolymers, quartz, and Pyrex arc possible candidates.
Fluoropolymer strengths are substantially affected by digestion tem peratures. A
fluoropolymer tube capable of w ithstanding 600 psi a t room tem perature will
ru p tu re a t 220 psi a t 200°C.
2.3.4.3 Final Selection o f Digestion Tube Material
A fter evaluating all of the tube m aterials above, the m aterial finally chosen was a
1/2” OD-3/S” ID tube m ade of Teflon PFA™ 350"’ (DuPont). This fluoropolymer is
in e rt to alm ost all reagents, and is tran sp aren t to microwave energy and visible
light. I t h a s a m elting point of306"C, and a burst pressure o f200 psi a t 200°C. This
provides a digestion tube th a t can handle most operating conditions, allowing us to
build a prototype which will provide “proof of concept ”
2.3.4A Glass-Sheathed PFA Tube
The m ain concern w ith fluoropolymer tubes is the relatively low maximum pressure
available. Figure 12 shows a fluoropolymer tube w ith a glass sheath th a t was
Middle Retaining Washer
Washer
I Front Retaining
R<
/
/
Back Retainin'ig Washer
^Retaining Bolt
Swagelok Nut.
Braided Glass Sheath
7
1/2” I.D. 3/8* O.D. PFA Tube
1 /2 ” NPT - 1/2
Back Ferrule
Front Ferrule
Figure 12: Glass-Sheathed Tube with Ferrule System to Hold Glass
Sheath
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designed, built, and tested in this study. The glass sheath is a tight weave of
fiberglass, .015” thick. Figure 12 illustrates a rather elaborate scheme using a
Swagelok*' (a commercially available Ferrule system for connecting tubing) to hold
back the glass sheath; it is an earlier tube design th a t used a ball valve entry. The
sheath can w ithstand pressures in excess of 400 psi, is not affected by a rise in
tem perature, and is only .015” thick. A Pyrex'’tube, 1/16” thick and 1/2” in
diam eter, could only withstand 215 psi, even though it is much thicker th an the
glass sheath. The sheathed tube was not selected because, as will be discussed
later, the glass sheath slows the tem perature response of th e system .
23.4.5 Pyrex* Tube
Pyrex* tubes were built and tried. Pyrex** is fragile and, because th e tube is not
uniformly heated during digestion, stresses build up and crack th e tube. Also, a
Pyrex** tube is rigid, and the valves are fixed rigidly in place. Any m isalignm ent
between valves and tube ends stresses th e tube, causing it to break w hen tightened
in the valves. Furtherm ore, a Pyrex® tube th a t blows up under pressure is
extremely dangerous. Therefore, th is m aterial w as elim inated from consideration.
1 3 i Benefits of Inclining the Digestion Tube
It was found necessary to incline th e digestion tube from th e very beginning of
trials. Table 2 lists th e five m ost im portant reasons for inclining th e digestion tube.
These benefits a re obtained when th e inside diam eter of th e digestion tube is a t
least 1/2”.
Table 2: Reasons fo r Inclining Digestion Tube
1.
Allows venting of gases.
2.
Prevents liquid and solids from reaching valves.
3.
C reates head space for boiling and refluxing.
4.
Keeps liquid n e ar tem p eratu re sensor.
5.
G as phase is a t a lower tem perature th a n th e
liquid.
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2.3.5.1 Venting/Pressure
Inclination of the tube is not needed to m easure pressure during the digestion;
however, it is necessary to vent gases from the digestion tube. A hole in the
pressure ad ap ter is used to m easure pressure in. and vent gases from, the digestion
tube (Figure 13). The pressure adapter is connected to an in-line pressure sensor.
D uring a digestion, the liquid and sample reside in the middle (bottom) of the tube
(Figure 11); any gases th a t form in the liquid bubble to the surface and rise up to
the valves on each end of the digestion tube. The tube has a 60 ml total volume,
w ith a typical reagent volume of 10 ml. The gas phase volume is the difference
between th e reagent volume and total tube volume, typically 25 ml on each end.
Vapor coming off th e liquid surface rises to the gas volume next to the valve; it is
cooled by the relatively cold inside surface of the digestion tube inside the flange
Tubing to Vent
ond Pressure Sensor
Pressure A dapter
Figure 13: Flange Valve with Pressure
Adapter
valve. I f one of th e valves is opened, th e gas will escape, and the pressurized gas
from th e o ther side w ill push th e liquid out. Therefore, it is necessary to open both
valves sim ultaneous for only a short period of tim e. Sudden release of gas from th e
digestion tu b e causes th e liquid to boil vigorously, sending liquid tow ards th e valve.
I t is n ot possible to open and dose th e flange valve w ithout venting th e entire
contents of th e digestion tube. Therefore, pressure adapters on both ends of the
digestion tu b e a re joined through a Y connector to a rotary valve. W hen th e rotary
42
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valve* is opened, gases are vented from both ends of the tube simultaneously.
Because the vent has a small inside diam eter and the rotary valve can open and
close in milliseconds, it is possible to vent small portions of the gas from the tube. If
venting is done a t room tem perature, the rath e r large gas phase makes it possible to
vent several m illiliters of compressed gas a t a tim e w ithout losing any liquid.
2.3.S.2 Cool Gas Phase
In the configuration described above, one im portant difference between the sm aller
bore tube and larger bore tube is th a t the gas phase in the larger tube is a t a lower
tem perature than the liquid phase1’. Therefore, for any particular tem perature of
the liquid, the gas phase tem perature and pressure will be lower th an th a t predicted
by a system of gas and liquid in equilibrium. This is beneficial since a higher liquid
tem perature is allowed for the same gas phase pressure, resulting in lower
pressures.
23.5.3 Effective Temperature Measurement
As will be discussed later, tem perature is m easured underneath the center of the
tube. The outside tube tem perature is used as th e m easure of liquid tem perature
inside th e tube. I f liquid is always present in th e tube n ear th e detector, th e outside
tem perature will accurately reflect the inside liquid tem perature. I f liquid is not
present in th e tube n e a r th e detector, the tube will cool rapidly, giving a false liquid
tem perature reading.
2 3 3 .4 Boiling and Refluxing
The action of the boiling a d d mixes the liquid and sam ple in th e tube so no
significant reagent depletion occurs. The vapors from the boiling a d d recondense on
th e colder ends of th e tube walls and ru n back into th e digestion solution. The
refluxing action o f th e a d d in the tube d e an s th e upper p a rt of the digestion tube of
any m aterial th a t could have been pushed up th e tube by the boiling action.
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2.J.5.5 Keeping Sample from Reaching Valve
It is essential th a t no undigested m aterial reach the valves; since the fiange valve
m aintains the digestion tube close to room tem perature, m aterial lodging in this
p art of th e tube may not digest completely. Also, solid m aterial reaching the valve
could be entrained into the vent hole during venting. If m aterial blocks th is hole on
one side only, venting will push sam ple and reagent out the apposite vent hole.
2.4 Cleaning
A som etim es overlooked p a rt of a digestion is the cleaning of the vessel before use.
Previous sam ple m aterial m ust be diminished below detectable levels to ensure
unbiased results. U nless the liner of th e vessel is to be replaced, the system m ust be
thoroughly cleaned. No am ount of rinsing or flushing can guarantee complete
rem oval of debris (scum) left on th e vessel walls after a digestion; if it is to be done
thoroughly, th e cleaning of th e vessel requires th e use of mechanical action on the
vessel surface to remove debris. The vessels are first scrubbed w ith soap and w ater,
an d th e n rinsed w ith tap w ater, followed by distilled w ater. To ensure complete
cleaning, th e vessels are th en soaked in acid for a lengthy period (typically 12
hours), an d rinsed ju s t before using. This procedure m ay be followed by a blank
digestion before th e vessel is actually used w ith a sam ple.
2.5 Automatic Control
25.1 Microwave Oven Interlocks
T he microwave oven used is a domestic-type equipped w ith a muti-mode stirre r, a
1.0 ft3 (28.31) cavity, and a m axim um output of 720 W. None of the modifications to
th e m icrowave oven described affect any of th e safety interlocks originally bu ilt into
th e system . The interlocks in th e front door and th e tem perature cut-out on th e
m agnetron a re still in place and functional. To tu rn th e microwave power on, it is
still necessary to e n ter th e desired tim e on th e control panel. In a commercial
system , th is m ethod of operation would not be acceptable.
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2.5.2 Manual Control of Valves
All relays in the system arc connected to a separate three-way switch. The three
positions of the switch attach the logic level from the computer, ground or 5 VDC to
the relay control line. This arrangem ent allows any relay in the system to be
switched on or off manually, or to rem ain under computer control. In initial trials,
and in case of a malfunction, the three-way switch provides a convenient way to
override th e sta te of any relay, regardless of w hat the com puter has told it to do.
2 3 3 Digestion Tube Cooling and Venting
D uring digestion, the generation of decomposition gases can quickly pressurize a
vessel to levels beyond its rated lim it. The usual course of action is to stop the
digestion, cool the bomb, open it to vent the gases, and th en to continue the
digestion afte r closing the vessel. Recent developments place a microwave
tra n sp a re n t valve'0 on each bomb; th is can be opened to release th e gases. It is,
however, still necessary to remove th e bomb from th e oven and cool it down before
venting th e bomb m anually. To circumvent this, vessels have been designed to
w ithstand very high pressures; even w ith these newer bombs, it is still necessary to
use sm all sam ples, and to h eat slowly.
Some of th e commercially available vessels have a ru p tu re disk th a t releases a t a
certain pressure. W hen a ru p tu re disk yields, th e release of gases is sudden, and
occurs a t high tem perature, causing loss of analyte; th e digestion m ust th en be
aborted.
In order to be able to open th e system without loss of analyte o r liquid phase, the
vessel m u st be cooled to well below th e boiling point so th a t any liquid vapors
p resen t condense on th e walls, leaving only decomposition products to be vented.
The reduction in pressure w hen venting occurs will m ake th e liquid boil i f i t h a s not
been sufficiently cooled. Boiling will create aerosol th a t contains analyte. The
aerosol can th e n escape. However, heating and venting m ay be used to evaporate
reag en t from th e tube.
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Cooling is accomplished using w ater flowing through 1/S" tubing which is wrapped
around th e digestion tube (.Figure 14). The w ater in the cooling tube is Hushed out
C O O L I N G W A T E RIN '
C O O L IN G W A TER O U T
FLA N G E VALVE
D IG E S T IO N T U B E
Qorn
t
C O O L IN G T U B E
M ICRO W A V E OVEN WALL
Figure 14: Cooling Tube Wrapped Around the Digestion Tube
w ith pressurized gas so th a t it does not absorb energy during th e next h eating cycle.
V enting is achieved by rapidly opening and closing th e rotary valve. T he sequence
is rep eated un til th e pressure is sufficiently reduced. This tak es only a few seconds.
The ro tary venting valve is pneumatically*4 controlled, w ith a 120 VAC four-way
valve controlled by a system relay. T he rotary valve is opened and closed w ith a 90
degree ro tatio n o f th e valve stem . A g ear on th e valve stem is rotated by pulling a
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linear ratchet in and out with two pistons. The pistons are alternately pressurized
to 100 psi, and then vented.
It would be possible to attach an electric motor directly to the"1valve stem ; however,
current electric motors are not nearly rapid enough. Also, the rotary valve
(comprised of two faces pressed hard against each other) requires a significant
am ount of torque to tu rn it on and off.
The ability to cool the system down allows m any options to be considered. The
digestion becomes a m ulti-step process with venting and further addition of sample
and/or reagents. These decomposition products do not condense when the system is
cooled; therefore, the digestion tube m ust be vented to continue the digestion.
2.6 Conclusions - Chapter 2
The large diam eter digestion tube elim inates m any of the problems associated with
a narrow tube design. A large diam eter tube can accept a much coarser m aterial, be
vented, and is sim pler to clean. The flange valve elim inates problems w ith abrasion
and memory effects in th e valves. The squeegee will enable a thorough cleaning and
q uantitative tran sfe r of sam ple digestate from the digestion tube.
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3. Control Software
3.1 Software Control Requirements
Com puter control of a digestion system requires four basic elements: 1) a set of
instructions to follow, 2) software to interrupt the instructions, 31
tem perature/pressure data, and 4) computer control of the hardw are. The First
requirem ent is, in today’s system s, simply a tem perature/pressure/tim e program
th a t is followed faithfully from beginning to end. The set of instructions given for a
digestion should be more th an ju s t a recipe th a t is blindly followed: it m ust contain
instructions th a t anticipate events which may prevent the digestion from being
completed. Slightly more sophisticated systems have tem perature/pressure ram ps
th a t use feedback from the sensors. However, the system m ust still follow the
predeterm ined tem perature/pressure program. Any unforeseen event, such as a
vessel th a t vents, results in a n aborted run. The ideal system should be able to
rem edy th e condition, and continue to completion. A preset program is generally
good only for a specific type of sample. For example, a program created to dissolve a
pigm ent w here no decomposition gases are given off would be very different from a
program designed for a biological sam ple th a t gives off large am ounts of
decomposition products.
3.1.1 MICROZ: Language
Com puter control allows a set of instructions to be executed w ith the ability to
change th e course of th e digestion if som ething unforeseen happens. The
commercially available software evaluated could execute a program of instructions:
it also h ad a se t of param eters to control hardw are and collect data. However, there
w as no w ay to edit th e param eters or to change direction according to the
altern atin g sta te of th e system . Lab view™ is one such commercially available d ata
acquisition and control program. Labview™ h as a graphical interface th a t uses
interconnected icons on screen to represent control and d a ta acquisition functions.
In Labview™ ,programs are created by interconnecting icons in a lin ear fashion.
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Any icon can branch to one or more icons. However, it is not possible to retu rn to a
previous icon, nor can any of the previously set param eters for data collection or
control be changed while the program is executing.
To gain control of the digestion process, we developed a language called MICR02.
M ICR02 uses an ASCII text instruction file as input in order to determ ine how the
digestion should progress. The instruction file reads like a set of English language
commands. M ICR02 consists of two m ain parts. One part, the executable part, is
specific to the computer. A second part, the control unit, is common to all hardw are
configurations. The executable p art is responsible for reading and interpreting the
instruction file, sending out control instructions to the control unit, and outputting
data. The control unit insures th a t instructions from the executable p a rt will
execute in th e sam e fashion when run on two different hardw are setups. The
hardware-specific software of th e control u nit converts instructions from M ICR02
into actions, for example by turning on/off relays or reading sensors. The operation
of M ICR02 centers around output and input. Analog input, digital input, system
counters, instruction files, and com puter tim e are classed as input. O utput consists
of analog output, digital output, and relays; data files a re also classed a s output.
3.1.2 MICR02: Input
T em perature and pressure are th e m ost im portant input p aram eters. Both of these
are input into M ICR02 using a n analog to digital converter (A/D). Calibration
constants convert the A/D values to centigrade or psi respectively. The high or low
sta te of digital input lines can also be used by M ICR02 for control or interfacing.
Com puter tim e is also considered an in p u t which is used to control events. I t can
also be saved w ith the data.
3.13 M1CR02: Output
The relays control gas and w ater valves used to flush out th e cooling tube. They
also actu ate th e pneum atic rotary valve, and control power to th e microwave oven.
The pneum atic rotary valve controls th e venting of th e digestion tube, a n d allows
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liquid to be pumped in from the load valve. The D/A is an analog output which,
along w ith the digital output, controls the speed and direction of the pump used to
load reagents.
The screen output is divided into three windows. The top window displays the
column titles for d a ta in the middle window. Every tim e data is w ritten to file, it is
sim ultaneously displayed in the middle window. This feature is useful for
determ ining current tem perature and pressure, and any other param eter th a t is
being saved. The bottom window lists ten lines of code from the instruction file; the
current line is highlighted in the middle. During the digestion, it is im portant to be
able to verify w h at instructions are being executed and w hat data is being saved.
The ability to see which instruction is being processed is very useful, especially
while th e m ethod is being developed. Moreover, as th e instructions are executed, the
lines a re updated and th e pointer moved. Delays or h alts can be inserted in the
program to verify th e execution of certain instructions. Comment lines in the
program also ap p ear in tin s instruction set window; they can be used to inform the
user as to th e sta te of th e digestion.
The d a ta files created during execution a re saved according to commands placed in
th e instruction file. The files created are a double quote, comma delim ited format.
This form at delim its te x t w ith double quotes, and num bers are separated w ith
commas. T his file type will load into any commonly-used spreadsheet program.
3.1.4 MICR02: Conditional and Asynchronous Control
The loop statem en t in Micro2 allows a set of instructions to be repeated a fixed
num ber of tim es; th is is very c onstrained, w ith no allowance for changing conditions.
Conditional instructions, called triggers, set M ICR02 a p a rt from other control
software. M ICR02 deals w ith conditionals in two ways. The sim plest way is to use
a n If-Then-Else construct to redirect execution. The more efficient m ethod is to set
one or m ore trigger levels. Once th e trigger levels are set, they a re evaluated every
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time an instruction is executed; this elim inates the need for a glut of IF statem ents
in the execution path.
Triggers can change the direction of the program in response to the state of the
system. This allows digestions to be tailored to different types of sam ples while still
using the sam e routine. MICR02 is capable of monitoring conditions during the
course of a digestion and taking corrective action if the conditions exceed certain
lim its set by the user.
Most digestion methods work by m aintaining the sample and reagent a t an elevated
tem perature. Microwave energy is applied to bring the digestion tube to
tem perature, and then added periodically to keep the digestion tube between
minimum and maximum tem peratures. In this trigger statem ent, the m inim um
and maximum values are tem perature levels. Trigger statem ents point to a routine
th a t is executed when the statem ent is true. All active triggers are tested every
tim e a program instruction is executed. For example, if the m axim um tem perature
were exceeded, the trigger routine would tu rn off the microwave, and then, w hen the
tem perature fell below the minimum tem perature, the trigger would point to a
routine th a t would tu rn the microwave back on.
The instruction set and examples of program s used for digestion are provided in
Appendix C.
3.2 Conclusions - Chapter 3:
A comprehensive feedback program for the control of digestion can be achieved w ith
M ICR02.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4. Capsule Introduction
4.1 Traditional Sample Transfer Methods
The tran sfer of sam ple into a microwave digestion vessel has traditionally been done
in one of two ways: it is either weighed into the vessel or pumped in as a slurry.
Each method has its own merits: weighing directly into the vessel insures th a t all
the sam ple is transferred to the digestion vessel, while forming a slurry with the
sam ple facilitates autom ation of the digestion system because the slurry can be
pumped into the digestion vessel. Both methods of sample transfer also have
problems, as discussed earlier. The potential to autom ate a digestion system relies
largely on the method with which the sample is transferred into the system . Any
new type of sam ple tran sfer method adopted should incorporate the m erits of
m ethods previously used.
4.2 Early Development Efforts
The first approach proposed for sam ple transfer involved injecting the slurried
sam ple through th e open valve into the tube. A nother design placed a tube through
th e valve and pum ped the slurried sample into the digestion tube. In both cases,
th e sam ple had to be converted to a slurry before it could be transferred to the
digestion tube. Both these approaches prevented th e slurry from contacting the
inlet valve, b u t they were rejected because it was still necessary to form a slurry
w ith th e sam ple.
4.3 A New Approach Needed
If it is assum ed th a t th e valve on the inlet to the digestion tube gives access to the
inside diam eter of th e tube, then the sim plest method of transferring a sam ple into
th e digestion tube is to place th e sam ple directly into the tube. This approach is no
different th a n th e m ethod used to place sam ples in conventional microwave bombs,
w here sam ples a re weighed directly into th e vessel. Obviously, sam ples cannot
sim ply be scooped into the tube through the open valve since this would be
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exceptionally difficult to autom ate. But. if the sam ple were encapsulated,
autom ation of sample transfer would be greatly simplified.
4.4 Capsule Benefits
Gelatin capsules, such as the cold capsule, deliver micro-encapsulated tim e-release
medication. These tiny spheres are received by the digestive system as a u n it in a
gelatin capsule. The gelatin dissolves in the stomach, releasing th e m edication in a
single place, w hereas if the medication were adm inistered as a powder or a tablet,
th e m outh and th ro at would also receive medication. It was proposed th a t sam ples
be placed in a gelatin capsule to be inserted directly into the digestion tube through
th e opened flange valve.
The large tube design w ith capsule sam ple tran sfer has m any benefits. The m ost
obvious advantage is the 100% transfer of sam ple into the digestion tube. Sam ple is
not lost on th e walls of th e pum p tubing or the digestion tube valve. This elim inates
degradation of the input valve and cross-contamination of sam ples. The very first
test to prove out the capsule approach used empty capsules available from a local
pharm aceutical company (Merck Frosst), and a 1/2” PFA tube fitted w ith two large
ball valves a t each end. The capsule, size 00 (cold capsule size), was filled w ith
dried pulverized vegetation (V85- MOEE) and loaded into th e middle of th e tube
along w ith 10 ml of w ater. The tube was placed inside th e microwave and power
was applied for 1 m inute. After 1 m inute, th e capsule w as completely dissolved and
th e sam ple w as evenly dispersed in the w ater.
4.5 Capsule Concept
This sim ple dem onstration showed th a t capsule sam ple
CAP
introduction w as feasible. The sam ple and capsule stayed in
th e liquid an d were not pushed to e ith er end of th e tube. The
dispersion of sam ple w as fast, effective, and used a Tmrnmmn
BODY
of liquid. The dispersion of a dried powder sam ple in th e
sam e q u an tity of w a ter in a n open beaker w ith h e a t applied
Figure 15: Capsule
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from a hotplate would have been more difficult, requiring stirring and ultrasonic
agitation to achieve complete sam ple dispersion.
4.6 Sample Transfer
The capsule, which is 35 mm long and 9 mm in
diam eter, is m ade up of two parts, body and cap
(Figure 15). The body and cap are tapered .005” over
th eir full length. The body is .003” sm aller in
diam eter th a n the cap, allowing it to fit into the cap.
The body contains the sample, and the cap fits over
th e body, forming an a ir tig h t seal. The flange valve
and tube inside diam eters are large enough to accept
th e capsule. The capsule slips easily down tubes,
m aking it sim ple to design a sam ple introduction
system th a t can ru n unattended. Any sample th a t
can fit into a capsule can be transferred to the
digestion tube. Figure 16 shows a scheme to load
capsules into th e digestion tube; here, capsules are
loaded sequentially from a stacked tube.
IMJSII
Figure 16: Automatic
4.7 Capsule Selection
T he commercially available gelatin capsules were ideal for proof of concept;
however, th ey were found to contain high levels of elem ents of interest, such as zinc,
calcium, m agnesium , copper, and sodium. These levels were higher th a n the
concentration in th e sam ples being analyzed; it w as th u s necessary to fin d a clean
capsule. Several capsule m anufacturers were contacted, none o f whom had a
capsule th a t w as completely free of elem ents of in terest. Capsulgel®, a m ajor
capsule m anufacturer, m akes all of th e ir capsules from th e sam e base formulation,
consisting of calf bones, pork skin, S i0 2, sodium laurel sulphate, plus other
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proprietary ingredients, a formulation th a t is designed to be easy to m anufacture,
and to dissolve when in contact with stomach acids. Another capsule m anufacturer,
G.S. Technologies, has a product called Vegecap' which is m ade from
hydropropylmethyl cellulose. Analysis of this type of capsule revealed very high
levels of sodium with lesser concentrations of calcium, m agnesium and silicon. The
high level of sodium is due to a 1% sodium chloride solution used in the
m anufacture of this type of capsule. Thomas Scientific sells a polycarbonate capsule
for bomb calorimetry. Polycarbonate can be made very “clean”. However, these
capsules w here not obtained because they are available in a 200 pi size only, which
was too sm all for our needs.
4.8 Clean Capsule Material
The search for a commercially available clean capsule was not successful, and it
became a p p aren t th a t we would have to m anufacture our own capsules. Many
m aterials were tested, including starch, m ethyl cellulose, polyvinyl alcohol, and
Dextran”; however, these m aterials were either insufficiently clean or they could not
be m ade into a capsule. Another difficulty w as th at, even if a suitably clean
m aterial capable of being m ade into capsules was found, no capsule m anufacturer
w as willing or able to m ake custom capsules.
Torpac, a capsule m anufacturer, initially expressed an in terest in m aking capsules
from a custom m aterial. However, they were unwilling to find th e proper m aterial,
to develop a m ethod for m aking these new capsules, or to quote a fixed price for th e
job. By th e ir own admission, these m anufacturers deal w ith a very lim ited range of
m aterials for m aking capsules. The process is a tightly guarded secret, and is
strictly regulated, w ith little room for change.
55
with permission of the copyright owner. Further reproduction prohibited without permission.
Acrylamide is commonly used for
electrophoretic analysis, and it is
easily made into a gel. As a monomer,
DIPPING
MOLDING
acrylamide is m anufactured in bulk at
extremely high levels of purity;
S utM llO tQ O
D tp
analysis of a 1% solution of
acrylam ide monomer using ICP-MS
So p o ta to
D rip
detected no m etals of interest. This
m aterial seemed ideal for the
m anufacture of capsules because
existing technologies th a t m ake
Dry
capsules from gel can be used, and the
m aterial is very clean.
4.9 Methods of Manufacture:
Molding
G.S. Technologies, another capsule
m anufacturer, w as able to shed some
light on th e process of m aking
(ZD
capsules and supplied u s w ith racks of
pins th a t a re used commercially to
Figure 17: Capsule M anufacture Processes
m ake capsules. According to this
m anufacturer, th e pins a re dipped in
th e capsule form ulation, and then air
dried while th e rack of pins is tumbled
to keep th e form ulation from falling off the pin before it has dried. The pins are
highly polished a n d tapered, allowing the capsules to release easily after they have
dried.
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Molding of the polyacrylamide was first used to m ake capsules, since the dipping
process did not seem suitable. The formulation could not be retained on the pins.
The pins were cut from the rack of pins supplied by G.S. Technologies, and fitted
into a mold (Figure 17: Molding). The acrylamide was polymerized inside the
mold, and the mold was then separated. The gel on the pin was air dried while
being rotated. The rotation prevented the gel from flowing off the pin. W hen the gel
was sufficiently dry, the capsule was released w ith a twisting/pulling action.
The capsules formed in this fashion were thick, heavy (,5g), ill-fitting, and
occasionally perforated. Efforts to try and reduce the thickness by decreasing the
clearances inside the mold were unsuccessful. Frequently, w hen the mold w as
separated, the gel rem ained inside the mold. A slight m isalignm ent of pin an d mold
resulted in one side of th e capsule being very th in and th e opposite side very thick.
To prevent the introduction of a ir bubbles into th e mold, the mold w as assem bled
while submerged. As a resu lt of these difficulties, th e molding process w as
abandoned.
4.10 Methods of Manufacture: Dipping
The dipping process is th e m ethod of choice in commercial m anufacturing; however,
the application to polyacrylamide had to be designed from first principles. P ins
m ade solely from plastic w ere selected in favor of stainless steel pins in a n effort to
reduce th e possibility of contam ination of th e capsules. PTFE is a logical choice
since it is chemically inert, and will not allow m aterial to stick to its surface.
Kel-F®, a fluoropolymer sim ilar to PTFE, w as chosen to m ake “cap and body” pins
because it is more rigid and m achinable. Pins w ere m anually dipped in
polyacrylamide, dried on a rotating wheel, an d released w ith a tw isting action
(Figure 17: Dipping). The process is less w asteful th a n th e molding process because
th e gel is polymerized in advance, and only th e gel needed is used. In th e molding
process, th e mold was subm erged in a n excess of polymerizing acrylam ide. The gel
outside th e mold w as discarded.
57
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The preparation of single capsules was used to empirically adjust all of the
im portant param eters and design features of the pin. The most im portant
p aram eter was the consistency of the polyacrylamide. If it was too thick, the gel did
not stick to the pin; if it was too thin, the gel dripped rapidly from the pin. It was
found th a t when the surface of the pins were highly polished, no consistency of
acrylam ide prevented the gel from falling off. The gel formed a ball on the end of
th e pin and fell off. A latex dental retainer placed a t the base of the pin solved this
problem. Gel covering the retain er served as an anchor to prevent the gel from
clum ping on th e end of th e pin.
4.11 Polyacrylamide Formulation
T he w a ter and acrylamide are mixed in a beaker th a t has been cleaned with
laboratory soap, rinsed thoroughly with tap w ater and Milli-Q w ater (deionized
w a ter Millipore Corporation), th en drip dried. The solution and ammonium
persulfate are degassed under vacuum for 10 m inutes after boiling sta rts (m illitorr
vacuum ). A fter degassing is completed the N,N ,N\N1- Tetram ethylethylenediam ine
(TEMED) is stirred in, followed by th e ammonium persulfate (the order is
im portant). The solution is then fu rth er degassed un til th e solution thickens (30
m inutes). I t is fully set a fter 90 m inutes. The gel is stirred w ith a clean, thick glass
rod, an d tran sferred to th e dipping trough.
4.12 Polyacrylamide Capsule Manufacture
T he Kel-F® pins were m ounted on the pin rack, and fitted w ith latex retainers. The
pins w ere th en dipped, ends first, into the gel trough, un til the retainers were ju s t
covered (Photo 2). The pins w ere slowly pulled out of th e gel; any excess on th e ends
w as removed by passing th e tips over the surface of th e gel. The pins were held,
ends down, un til they had dripped three tim es. The pin holder was then m ounted
on a ro tatin g pum p wheel contained in a Plexiglas enclosure (Photo 3). Once all four
p in holders h ad been m ounted on the wheel, the door w as pulled down, th e fan
tu rn e d on an d drying started . The wheel rotated a t 7 rounds p e r m inute (RPM)
58
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during th e entire drying time. The tem perature was kept a t 30°C using a 100 w att
light bulb equipped with a rheostat. The humidity rose over the next 40 m inutes
and then slowly started to fall. The gel on the pins dried and had acquired a glassy
look. A fter all the pins looked dry, a pneumatic nebulizer was used to generate a
fine m ist which filled the enclosure. The capsules were removed as soon as they
were dry since they would otherwise be difficult to tw ist off the pins. To remove a
capsule from a pin it was mounted on the pin holder, punctured on the end to allow
air in so th a t the capsule would not collapse when pulled off the pin, and then was
pulled off with a twisting motion. The capsules were then trim m ed with a pair of
ceramic scissors and fitted together. All work with capsules w as done using low
powder nitrile gloves (Fisher Scientific Inc.) th a t had been rinsed w ith distilled
w ater.
59
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Photo: 2 Capsule Dipping Rack with Pins
1.
2.
Pin holder (DWG B-1021*).
L atex re ta in e r (DWG. A-1020*).
3.
Kel-F® pins cap and body (DWG. A-1020*).
4.
5.
Polyacrylamide gel.
Gel dipping tray.
*
See draw ings in Appendix D
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Photo: 3
1.
2.
3.
4.
5.
6.
Capsule Drying Apparatus
Relative hum idity m eter.
P m holder.
Cap pins (Stainless Steel).
Body pins (Stainless Steel).
Argon gas flow control.
100 w a tt light bulb and rheostat.
7.
Fan.
S.
9.
10.
Plexiglas box w ith door flipped up.
11.
*
Rotating wheel on pum p drive (DWG A-1020*).
Pum p speed and direction control.
Mounted pin rack.
See draw ings in Appendix D
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4.13 Conclusions - Chapter 4:
The selection of polyacrylamide as the capsule m aterial produces a light wei
clean, and strong capsule, making it a very effective sample delivery system
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5. Sensors
5.1 Temperature Sensors
5.1.1 Introduction
The digestion tem perature is the most im portant operating param eter in
determ ining the efficiency of a digestion!l.
In a system open to the atm osphere, the boiling point is determ ined by the reagent
used, and atmospheric pressure. In a closed system, a t therm al equilibrium ,
pressure can be calculated from the tem perature for any particular reagent.
Likewise, if the pressure of such a vessel/reagent combination is known, so is the
tem perature. However, most microwave digestion vessels are not in therm odynam ic
equilibrium , and thus the pressure cannot be used to determ ine th e tem perature
inside the vessel. The upper walls of a conventional microwave bomb are cooler
because only th e liquid (in the bottom) is heated. Therefore, th e vapor is cooler th an
the liquid, and the pressure is lower th a n th a t calculated from th e liquid
tem perature.
Traditionally, the tem perature in a bomb is m easured w ith a therm ocouple (TC) or a
fiberoptic fed through the cap into the bomb. The thermocouples a re encased in a
Teflon-covered m etal tube (Figure 18). The stainless steel tube is long enough to
reach from inside the bomb to th e outside of the microwave cavity. The Tefloncovered end prevents contam ination of th e acid solution while protecting th e
stainless steel case. The TC end is imm ersed in th e liquid inside th e bomb to obtain
a direct tem perature reading.
5.1.2 Fiberoptic Temperature Sensor
W hen a fiberoptic probe is used, th e probe is held ju s t above th e liquid surface. The
fiberoptic probe is m ade alm ost entirely of quartz, and is not heated directly by th e
microwaves; it is, however, m ore expensive and fragile compared to a stainless steel
sheathed thermocouple. The infrared radiation from th e surface activates a
phosphor on th e quartz fiber end of th e tem perature probe inside th e bomb. The
63
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fiberoptic probe collects infrared radiation emitted from the liquid surface: the
emission of the phosphor is related to the tem perature of the liquid. The fiberoptic
probe itself is not compatible with hydrofluoric acid, and the phosphor is
decomposed by concentrated nitric acid.
5.1.3 Thermocouple Temperature Probe
The m easurem ent of tem perature inside a microwave oven is complicated by the fact
th a t any electrically conductive m aterial used to m easure tem perature will be
heated by microwave energy. The metal sheathing protects the TC from the
microwaves; however, the sheath can cause other problems. Self-heating of the
sheath is caused by different field strengths and direction over the length of the
sheath (inside th e microwave cavity), resulting in a flow of current over the surface
from a re as of high field strength to areas of low field strength. Also, because the
surface of th e probe h as a finite resistance, the flow of current results in heating. If
the resistance on th e surface of the sheath is too high, arcing can occur. The heating
of th e TC itself, which gives a false tem perature reading, is not observed in a
conventional microwave oven because the TC end is immersed in a liquid th a t acts
as a large h e a t source.
Both sheathed therm ocouples and quartz fiber optic tem perature probes a re used
_
, „
Therm ocouple Hanoi,}
l/lfi’ ODStoinlessSteoi
Sheath
Insulatln
PowderFiH
C u ta w a y
/
—7—.
Nickel-Chrome /C o n sra n ta n
C onnectors
Nickel-Chrome / C onstanton
Wires
Figure 18: Stainless Steel Sheathed, Grounded Thermocouple
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successfully in commercially available, conventional microwave digestion bombs.
We selected the sheathed TC tem perature probe because it was inexpensive, rugged,
and not damaged by microwave energy. This was a good startin g point; however, it
was discovered early in this work th a t a tube system has special requirem ents not
met by the TC.
5.1.4 Tube Temperature
The digestion vessel chosen for this work is a large diam eter tube w ith a flange
valve attached at each end. M easurem ent of tem perature inside th e tube presents
new problems, since it cannot be accomplished in the sam e way as in the
conventional bomb w ithout compromising the advantages of th e large tube design.
If a thermocouple probe is inserted through the valve and into the digestion tube
during digestion, it creates a serious cleaning problem: m aterial sticks to the probe,
requiring a m anual washing. Also, certain types of sam ple introduction are
hampered. Nearly all tube digestion system s have a thermocouple attached to the
outside of the tube; the portion of tube to which the thermocouple is attached is
outside th e oven cavity.
5.1.5 Thermocouple Approach To Temperature Determination
The first tria ls to determ ine tube tem perature used a stainless steel sheathed Etype thermocouple w ith a grounded junction. The E-type TC (p art # CSXX-116G12E from Omega) had an IS ” long sheath, with a 2’ wire extension (Figure 18).
Thermocouples are usually used in m atching pairs and attached in series; one TC is
placed in ice w ater, and the other is used for m easurem ent. Millivolt readings
across the p a ir of thermocouples are used in a lookup table to obtain the
tem perature. The lookup table gives accurate tem peratures over a wide
tem perature range (-270°-1000°C), b u t th e values are accurate only w hen a
m atching reference thermocouple is used to negate the EMF’s generated from the
connection of dissim ilar m etals tc. th e thermocouple w ires used to m easure th e
voltage. The necessary accuracy (±2°C) can easily be achieved w ith a two point
calibration for th e tem perature range required (20°C-200°C).
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5.7 5.7 Thermocouple Calibration
The signal level response of the E-tvpe TC used is described with a straight line
equation (±.1°C) for the entire tem perature range of interest (20e,C-200°C'>.
Readings which were taken with a single TC using ice w ater (0°C1 followed by
boiling w ater (100°C) were used to generate a linear calibration curve. It was not
necessary to use a cold junction TC because a calibration curve, rather than a
lookup table, was used to calculate the tem perature.
5.1.5.2 Thermocouple Installation
The TC w as passed through the wall of the microwave cavity and held against the
wall of th e middle of the digestion tube with Teflon tape. The stainless steel sheath
was silver soldered to the wail of the microwave cavity to prevent the TC leads and
sh eath from radiating microwave energy out of the cavity (Figure 19-A).
5.1.5.3 Thermocouple Self-Heating and Arcing
The TC attached to the tube was used to m easure the tem perature of th e tube as
microwave energy was applied. Initially, heating rates observed for w ater and nitric
acid were higher th a n had been calculated, eventually giving readings well above
300°C. The heating ra te of the liquid was calculated assum ing th a t the total power
o utput (720W) would be absorbed solely by the liquid in the tube. The tube
tem p erature could not reach 300°C since the PFA tube would have melted. Arcing
betw een the TC and the digestion tube was also observed, resulting in blackening
an d p itting of th e digestion tube wall where th e TC had been attached. W rapping
th e TC m ore tightly against the tube with Teflon tape brought th e heating rates
closer to calculated values; however, arcing w as still present. W hen microwave
power w as applied w ith th e TC detached from the tube, the TC became hot to touch,
and it became evident th a t th e stainless steel sheath of the TC was absorbing
microwave energy and heating.
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SIDE-ON
SIDE-ON with glass
sheath protection
Toflon T a p e
Stiver ,
Soidor
/
GiossShectri'
Floor
END-ON
c
PFA H older
D
SIDE-ON with large
diam eter
braid or
c o p p e rtu b e
Brcld or C o p p e r T ube
END-ON
en la rg ed tu b e
full length
SIDE-ON
gold p la te d
Brcld or C o p p e r Tube
Gold Plating
Figure 19: Thermocouple Trials
67
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5J.5.4 Methods to Eliminate Self-heating and Arcing
In an effort to elim inate arcing and self-heating of the TC, several approaches were
tried. Figure 19 (A-F) illustrates some of the attem pts at elim inating the self­
heating and arcing of the TC. Figure 19-B depicts a glass sheath draped around the
digestion tube th a t insulates the TC from the tube. The glass sheath increased self­
heating and did not prevent arcing. In Figure 19-C, the TC is held against the tube
with m inim um contact. In this configuration, it was necessary to place a holder
m ade
01
PFA between the TC end and the digestion tube to prevent the TC from
puncturing th e tube. No appreciable difference was noted using the end-on
approach. In the next tria l (Figure 19-D), copper braid was used to cover the TC.
This w as of no help in reducing self-heating or arcing.
A surface can have localized areas of very high potential if the surface is not
completely smooth'’'’. The areas of high potential exist because of the very small
radius of curvature on the rough edges of the surface; potential is inversely
proportional to the radius of curvature of the surface. For this reason, the relatively
sm all rad iu s of th e stainless steel TC sheath (1/32”) could be th e cause of selfheating. Figure 19-D shows th e outside diam eter of the TC enlarged over p a rt its
length. The full length w as not enlarged since this would have moved the
m easuring end of the TC aw ay from the tube. A copper tube placed over the TC
(Figure 19-E) increased th e diam eter of the TC over the entire length. N either the
Figure 19 D or E configurations were able to elim inate the arcing; however, the self­
h eatin g w as reduced. Unfortunately, th e mass of the probe increased the response
tim e. Arcing from the TC end w as still intense, causing irreversible dam age to the
TC end. Gold-coating th e stainless steel sheath (Figure 19-F), as suggested by
Ja ssie et a lu, h ad no appreciable im pact on self-heating or arcing.
5.LS.5 Thermocouple Tube Temperature Abandoned
The fundam ental difference betw een th e tem perature probe arrangem ents depicted
in Figure 19 and the traditional microwave bomb arrangem ent is th a t the TC in
Figure 19 is not im m ersed in liquid. In a conventional bomb, th e TC is im m ersed in
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a conductive solution or water. The liquid shields the immersed portion of the probe
from microwave radiation, keeping the probe end a t the liquid tem perature.
Because the TC could not be prevented from self-heating, the TC was abandoned as
the tem perature sensor for the digestion tube, and alternate technologies were
evaluated.
5.1.6 Infrared Detection for Temperature
5 .1.6.1 Infrared Viewing o f Temperature: Pliotoconductive Detectors
Using an infrared detector placed in line-of-sight of the digestion tube, it is possible
to “view” the tube tem perature. Infrared detectors made of photoconductive PbSe or
HgCdTe are capable of sensing tem peratures from 10°C to well over 2000°C. This
type of detector requires m any components, including physical signal chopping w ith
support circuits to obtain a noise and am bient tem perature corrected signal, a
system of lenses th a t can transm it the infrared radiation to th e detector, and a bias
current to th e detector, along w ith an amplification system for reading th e bias
current. This type of detector is very expensive to im plem ent correctly and is not
ideally suited for reading tem peratures in the range of in terest (20-200°C).
Infrared therm opiles (IR/TC) are detectors th a t have been developed specifically to
read am bient tem peratures. Applications th a t require sensitivity to sm all
differences in body tem perature or night vision use infrared detectors which work
well a t am bient tem peratures. A full discussion on infrared tem p eratu re
m easurem ent can be found in Appendix A.
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5.1.6.2 Infrared Vie wing o f
Temperature: Thermopile
Detectors
R a d ia tio n trom Liquid
\
Holder
R ad iatio n from l u t x 1
The relatively inexpensive
Difjostion Ktbo
infrared therm opile (IR/TC)
m ade by Omega (model num ber
OS36-E-240-GMP) was chosen
for th is work. The IR/TC is
m ounted 1/8” inch below the
middle of the digestion tube
through a hole in the bottom of
th e microwave cavity (Figure
M ic r o w a v e C a v ity
F loor
IR/TC
Figure 20: Thermopile Temperature
Measurement
20, Photo 4). The IR/TC is held
in place w ith a m etal base and a set screw. A holder made of PTFE secures the
digestion tube over th e IR/TC to prevent the tube from moving away from the
detector as the tube h e ats and expands. Aluminum tape seals the m etal base to the
floor of the microwave cavity to prevent microwave leakage.
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5.1.6.3 Infrared Tube Temperature Considerations
The PFA tube is opaque between 6.5 and 14 microns so th a t the surface tem perature
of the tube is m easured, rath e r than the tem perature of the contents of th e tube.
The tube does not have an emissivity of 1, nor is the surface viewed by the
thermopile flat. Therefore, the thermopile cannot use a calibration curve generated
from a black body such as graphite to m easure the contents of the PFA tube.
Photo 4: IR/TC Attached to the Digestion Tube
1. Cooling tube.
2. Holder.
3.
M etal base.
4.
IR/TC (not visible).
5.
Aluminum tape.
6.
Microwave cavity floor.
5 .1.6.4
IR/TC Mounting
The other consideration is the very large viewing angle of the therm opile. This
m eans th a t the surroundings of the tube m ay become p a rt of th e tem perature
m easurem ent. N evertheless, w hen th e therm opile is placed next to th e PFA tube
during a digestion, th e IR/TC reading increases as th e acid in the tube is heated.
Therefore, if the tem perature of th e liquid inside the tube_ adjacent to th e IR/TC can
be determ ined independently, the response of th e IR/TC can be calibrated.
»
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5.1.6.5 IR/TC Calibration
The tem perature of the liquid in th e tube can be determ ined using a shielded
therm ocouple placed inside the tube. A J-type TC was fitted through the pressure
adapter, and extended to the middle of the tube into 10 ml of nitric acid (.Figure 2 1 J.
Thermocouple
Inside Tube
Connected to Digital Display
Front View
IR/TC
Connected to Computer
Figure 21: IR/TC Calibration with TC Inside Digestion Tube
T he nitric a d d w as heated to 200°C; tem perature m easurem ents were tak en w ith
th e IR/TC and TC as th e a d d cooled. The IR/TC readings in millivolts were plotted
ag ain st th e TC tem peratures; a quadratic least squares fit provided first and second
order coeffidents and a n offset (Figure 22). Millivolt readings of the IR/TC could
72
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200
U
n
o
5
160
120
0
5
80
1
*
a
i—
-0.4
o
0.4
0.8
1.2
1.6
2
2.4
2.8
D e t e c t o r S i g n a l (m v )
Figure 22: Quadratic Least Squares Fit o f
Signal Response of Tube Signal
from IR/TC
now be used to give accurate tem perature readings (±2°C) of th e contents of the
digestion tube.
5.1.7 Magnetron Temperature
To prevent overheating, the m agnetron is factory-equipped w ith a therm al device
th a t cuts power to the microwave oven when a preset tem perature is reached; th is
acts as a fail-safe and keeps th e system from destroying itself. This “cut-out” device
m ust cool down before power is restored to th e microwave oven. The m agnetron
supplies microwave energy to th e oven through a wave guide. I f th e energy is not
absorbed in th e microwave oven, it can be reflected back through th e wave guide,
and h e a t th e m agnetron. A m agnetron operating a t a higher tem perature h as a
considerably reduced operational lifetime.
A therm ocouple w as placed underneath th e therm al cutout switch. A stainless steel
jacketed E-type thermocouple w as used for th e m easurem ent of m agnetron
tem perature. The thermocouple w as calibrated w ith ice and boiling w a ter responses
to g en erate a two-point calibration curve.
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One way to m onitor reflected energy is to m easure the tem perature of the
m agnetron w ith a TC, and insure th a t a maximum tem perature is not exceeded.
For sm all loads placed inside the microwave cavity, the equilibrium tem perature of*
the m agnetron will be higher than for large loads. This does not elim inate the
reflected microwaves but does allow operation under controlled conditions.
5.1.S Cooling Water Temperature
The cooling w ater flows through a 1/S” cooling tube, which is coiled around the
digestion tube from one end to the other inside the microwave oven. Tap w ater
flows through th e cooling tube and out past a tem perature sensor a t a rate of 50 ml
per m inute. The inside diam eter of the cooling tube is 1/16"; it is not large enough
to use norm al-sized thermocouples, so a thermocouple imbedded in a tiny
hypodermic needle w as inserted into the “T” connection of the 1/S" tube. The low
m ass an d shielded thermocouple gives excellent response, is not affected by am bient
microwave radiation, and is not corroded by the cooling water.
Teflon M embrane
To Vent Valve
To Digestion Tube
To Pressure Sensor
Figure 23: Pressure Line with Liquid Transfer Line and
Membrane
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5.2 Pressure Sensors
The fast and accurate m easurem ent of pressure is essential since th e vessel can
easily become over-pressurized and be destroyed. The sim plest way to m easure the
pressure inside the digestion vessel is to run a tube from the digestion vessel to a
pressure sensor. In a tube system, it is not feasible to connect the pressure transfer
tube directly to the digestion tube inside the microwave. M aterial entering the
pressure tran sfer line cannot easily be flushed out, and any connection m..de w ith
the digestion tube will weaken the digestion tube wall. In previous w ork4", an
expanding tube pressure sensor (Burdon Gauge) was used. This type of pressure
sensor was connected to a “T” in the vent or digestion line. W hen pressurized, the
gauge would increase in volume, taking in sam ple th a t would be difficult to clean
out. In a pressure sensor w ith volume displacem ent, the sensor is usually protected
by a flexible diaphragm ; a liquid is used to tran sfer the pressure. The liquid used to
tran sfer th e pressure m ust not expand w ith increasing tem perature, or react w ith
the chemicals used. The most practical choice is w ater as it does not expand w ith
increased tem perature (within limits), and will not decompose on contact w ith the
strong acids used.
5.2.1 In-line Pressure Sensor
D uring digestion, both flange valves are fitted w ith a vent ad ap ter (pressure
adapter). Each vent adapter is m ated to a 1/8” tube th a t is connected by a “Y”
connector to th e vent valve. An in-line pressure sensor (Figure 23) is placed on one
of th e vent lines between th e “Y” connector and th e vent adapter. T he vent tube
flows through th e pressure sensor assembly, allowing liquid into th e digestion tube,
venting of gases from th e digestion tube, and flushing of th e pressure assembly.
The use of a diaphragm and liquid tran sfe r h as m any problems. The diapnragm
m ust be perfectly flexible, impermeable, and re sistan t to all chemicMs used, an d be
able to resist sudden pressure changes, including rapid decompression. The in-line
pressure sensor w ith th e m em brane and th e oil tran sfer line w as not very reliable.
Sudden pressure changes would easily push in or blow out th e m em brane.
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Pressurized gases would be absorbed by the oil through the m embrane and later boil
out, breaking the m em brane if there %vas a sudden drop in pressure. Oil was used
as a tran sfe r medium: however, it was soon replaced with w ater because the oil
would h eat, expand and, eventually break the membrane.
The in-line pressure sensor with liquid pressure transfer line was replaced by a
flush stainless steel front diaphragm pressure sensor. The flush face of the pressure
sensor is covered w ith a th in PTFE disk, which is sandwiched between the stainless
steel face of the pressure sensor and a cavity m anufactured from Kel-F (Figure 24).
BNC C onnector
Pressure Sensor
Ke!-F Body,
fla t Front Surface of Sensor
Teflon Wafer
Figure 24: Front Surface Pressure Sensor In-Line
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The PTFE disk on the face of the pressure sensor protects it from the acid in the
digestion system. The pressure line enters on one side, and exits on the other. The
main advantage of the front surface pressure sensor arrangem ent is th a t it has
alm ost zero displacement. The front surface in-line pressure sensor has no liquid
tran sfer line, and needs no flexible diaphragm.
5.2.1.1 Pressure Calibration
Calibration is accomplished by pressurizing the entire system in increm ents from
am bient to 200 psi, using a gas supply which is fitted with a readout gauge. The
pressure sensor selected is rated to 500 psi, is accurate to ± 1 psi, and has an over­
pressure ratin g of 1000 psi.
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5.3 Capsule-Based Microwave Digestion System: Hardware
Overview
The following photographs (Photos 5.6, and 71 show the various components of the
system. Photo 5 shows the front view of the microwave with the door open. The
microwave unit is a Eaton Viking Model num ber RE-77STC, discontinued in 1992.
The inclined digestion tube is attached to the flange valves. The cooling tube
entering on the left is wrapped around the digestion tube: it exits on the right, on
th e back wall of the microwave cavity. The back wall of the microwave was cut out
and replaced w ith a panel to support the flange valves. Aluminum tape was used to
seal any holes th a t could leak microwave radiation.
Power lines to th e two transform ers which supply the magnetron arc routed through
a m anual switch box on th e side of the microwave oven, and down underneath the
oven to th e com puter controlled relays. None of the microwave interlocks were
defeated. To obtain power from the oven, it is still necessary to use the original
controls.
Photo 6 provides a view of the complete system from the back. A m etal box w as
added to th e back of th e microwave oven to protect the operator from microwave
radiation and pressurized components. The door allows the user to view the back of
th e microwave oven through a m etal screen and 1/4” Plexiglas. The box also
m inim izes th e stray microwave radiation th a t can interfere w ith the analog signal
lines.
Photo 7 is a closer view through the back of the protection box.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Photo 5:
1.
2,4.
3.
5.
6.
7.
8.
Front View of Microwave Oven
PFA digestion (1/2” OD) tube wrapped with
cooling tube (1/8” OD).
IR/TC.
Cooling tube in and out.
Flange Valves.
Microwave Oven control (original).
Microwave override control switches.
Microwave power sw itching lines.
79
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Photo 6:
Overall View of D igestion System
1.
2.
3.
4.
Back protection box.
Microwave Oven.
2S6 Computer.
Instrum entation board.
5.
6.
7.
Valve/relay control box.
Back protection box door.
Com puter screen showing three
windows.
S.
Microwave Oven Door.
80
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
Photo 7:
1.
2.
4.
5.
6.
7.
8.
9.
10 .
Back View of Microwave Oven
Flange Body (opened).
Flange Valve N u t screwed into Flange Valve with
pressure/vent line.
Pneum atic actuator for vent valve.
Flange Valve N u t w ith pressure/vent a d ap ter for left side.
M anual rotary valve for load line.
Vent line (right side).
Replacement back wall.
Flange Valve N u t w ith capsule loading adapter.
Flange Valve N u t sam ple removal adapter.
In-line pressure sensor.
11 .
12 .
Vacuum line.
13.
14.
Cooling w ater in.
Cooling w ater out.
15.
16.
17.
Gas lines for pneum atic actuator.
18.
Pressure Analog/power line.
Vent vessel.
A dd w ash bottle.
Flange Valve m ounting bracket.
Flange Valve wrench.
Aluminized bubble w rap for back door seal.
81
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5.4 Conclusions - Chapter 5:
The infrared tem perature sensor works effectively as a replacement for the
thermocouple probes normally used, elim inating problems with arcing and self­
heating. The in-line pressure sensor is an effective way of m easuring pressure while
a t the sam e time using the access to vent and deliver reagents. The pressure sensor
will not contribute to contamination since it can be thoroughly flushed.
82
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6. Operation and Characterization of a Capsule-Based
Microwave Digestion System
6.1 Introduction
The last chapters described the design process for the capsule micrtnvave digestion
system. This chapter will discuss the characterization of the many design features
incorporated into the system. This will allow comparison of the large tube design to
traditional microwave bomb and tube systems.
Pn eu m atic'
Actuator
rum p
V e n ts
L o a n in g Valvr
In-Line
P ressure
S ensor
V ent Valve
M a n g e V alves
Coolin
C ooling
Wator Out
Coolin
M agnetron
Flush G as
In
Solenoid
Valves
Wator
Digestion &
Cooling
T ubes
Counter
Relays
Controls
Microwave
M ag n etro n
TC
C om puter
Control
Tube
te m p eratu re
M agnetron
T em perature
Signal Buffering/ Amplification/ Digitization
Figure 25: Total System Sensor Configuration
83
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W ater Cooling
T em perature
6.2 Data Acquisition
The capsule microwave digestion system (Figure 25) has four sensors: 1) Tube
Pressure, 2) Tube Temperature, 3) Magnetron Temperature, and 4) Cooling W ater
Temperature. The cooling and magnetron temperatures are obtained with stainless
steel sheathed TC’s. Digestion tube temperature is acquired with an IR/TC. Pressure
is measured with an in-line pressure sensor. The analog s ig n a l from these sensors are
buffered and amplified on a multichannel instrumentation board (Trulogic Systems
Inc.). The signals are then multiplexed to a single 12-bic analog-to-digital converter,
and digitized.
The data acquisition is controlled by MICR02. Instructions are contained in a text file
th a t is read when the digestion is started. The instructions for a digestion are normally
those referred to as a “run”; they can also be for calibration or diagnostics. The
instruction file used for controlling the behavior of MICR02 through a n m was changed
continually during the development of the digestion system. Nevertheless, regardless of
the purpose of the nm , data for temperature, pressure, experiment time, computer
Milli-Q Water
?.
Tube Temperature
200
jvtognetrorr--.
Temperature
"Cooling Water
lemperarure
Microwave
Percent
Tkne-On
.
. n'i.lO ; ' ■
"
•! i rs. n i r . . :•
0
100 200
300 400 500 600
Time (s)
700
800
900
Figure 26: Heating o f Water in Digestion Tube
84
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tim e, instructional data file nam e and power w ere alw ays stored. C ollecting all of t he
data all of the tim e allowed the data to be presented in a consist ent fashion.
Figure 26 illustrates the data collected with the four sensors for a digestion sequence
with water. The data for all future "runs” is plotted in an id e n tic a l fashion. This first
run, using water, is used to describe each plot. The run was started by cycling the
microwave On/Off (25%/75%) until a tube temperature of 90°C was reached (ISO s); the
tube tem perature was then ramped a t O.S°C/s to a target tem perature of 1S0°C, and
held for 5 consecutive minutes. The five minute digestion is a much shorter time than
normally used in microwave digestion; designed to reveal differences between
traditional microwave digestion and capsule-based microwave digestion. The
microwave heating was then stopped, and the cooling water turned on until the tube
tem perature reached 70°C. The cooling water was then flushed from the cooling tube
with argon gas. The data collected during the run was transferred from computer
memory to a data file, and displayed as in Figure 26. The instruction file for this and
all other “runs” can be found in Appendix C.
6.2.1 Tube Temperature
Tube tem perature is the controlling param eter for the nm shown in Figure 26.
Software control loops use tube tem perature to ramp to a target tem perature, maintain
a target tem perature for a fixed amount of time, and cool down.
6.2.2 Tube Pressure
Although tube pressure was not used to control the run, this would have been possible.
It is more im portant to monitor pressure to insure th at critical pressure levels are not
exceeded. Tube pressure increases are a combination of reagent vapor pressure effects
and gaseous decomposition product released. In Figure 26, the pressure is seen to rise
as the tem perature increases; it then falls as the tem perature decreases. The pressure
returns to zero a t the end of the run, indicating th at uncondensable decomposition
products were not formed during the run; this result was expected for w ater alone.
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6.2.3 Percent Microwave Time On
The "Percent Microwave Tim e On” is a plot of th e microwave power supplied to the oven
cavity. Microwave power is supplied to the oven cavity w hen high voltage and filam ent
current are applied to the m agnetron. The high voltage and filam ent current are each
controlled by separate relays. The logic levels used to control the relays are also fed
through an AND g a te th at controls the clock input to a high resolution counter. W hen
either or both o f th e relays are turned off, the clock to the counter is turned off. The
clock is turned on only if both relays are turned on. The counter is decrem ented by one
for each clock pulse which reaches th e counter. A second identical counter is connected
directly to th e sa m e clock and is continually counting. The tw o counters are read a t th e
beginning and end o f each integration period. The difference in clock counts betw een
the two counters m ultiplied by th e clock frequency gives th e tim e th a t th e m icrowave
power w as turned off during th e integration period. A n in tegration period o f two
seconds w as found to be adequate, and is u sed throughout th is w ork u n less otherw ise
stated.
6.2.4 Cooling Water Temperature
At the end of a digestion cycle, or before venting (as will be seen later), the digestion
tube is cooled. Tap water passes through a cooling tube th a t is wrapped around the
digestion tube, and flows out past a tem perature sensor to the drain. The exiting water
tem perature is called the cooling w ater tem perature. The “Cooling W ater Temperature”
trace in Figure 26 shows the tem perature to be constant until near the end of the run,
when the cooling w ater is turned on. The tem perature rises rapidly a t first, then peaks,
and drops rapidly.
6JL5 Magnetron Temperature
The magnetron tem perature was taken on the outside of the magnetron next to the cut­
off sensor. The tem perature of the magnetron, when measured in this fashion, is only
useful for relative comparisons, since the actual internal tem perature of the magnetron
is probably much higher. Nevertheless, the outside tem perature can yield information
about w hat is happening during a run. In Figure 26, the magnetron tem perature rises
and slowly starts to fall near the middle of the run. A rise in tem perature of the
86
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
m agnetron indicates th a t a portion of the microwave energy is being reflected back to.
the m agnetron and absorbed, heating the m agnetron.
6.3 Microwave Heating Characteristics: Digestion Tube
O ne o f th e m a in r ea so n s th a t m icrow ave en ergy is im p ortan t to th e d ig estio n
process is th a t th e rea g e n t and sa m p le are h eated directly by th e m icrow aves. T he
d ig estio n v e s s e l is h ea ted in d irectly by th e reagen t. T h e h ea tin g can be tu rn ed o ff
an d on a t w ill; in com p arison , in resistiv e h ea tin g , h e a tin g e lem en ts in contact, w ith
th e v e s s e l first h e a t th e v e sse l w hich, in tu rn , h ea ts th e reagen t in sid e. W hen
r e sistiv e h e a tin g e le m e n ts are tu rn ed off, th e rea g e n t co n tin u es to be h ea ted
b eca u se th e v e s s e l r em a in s hot.
For microwave absorption, the reagent m ust either have a dipole moment or else be
conductive. W hen a combination of different types of absorbers are combined in the
digestion tube, it m ay be difficult to determ ine which component is responsible for
the behavior observed. One way to better in terpret observed behavior is to fully
understand how each of th e components behaves when heated in the digestion tube.
63.1 Microwave Heating of Water in Digestion Tube
The coupling of microwave energy to w ater occurs m ainly through the dipole of the
w ater molecule. Repeated alignm ent and random ization of the dipole generates
h e at through friction in the bulk liquid. This coupling is not very efficient; little of
th e energy supplied to the microwave cavity is absorbed by the w ater load in the
tube.
The energy absorbed heats the water and raises it to its boiling point, generating water
vapor. The vapor rises to the ends of the tube and condenses. The condensate collects
as w ater droplets, and flows down to the bulk liquid.
The orientation and placement of the digestion tube and flange valves was intended to
favor this vaporization and recondensation process in the digestion tube. The digestion
tube was bent in the form of a “U”, with the ends higher than the middle. The large
inside diam eter of the digestion tube allowed liquid placed in the tube to flow down to
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th e center, and gases to rise to th e ends. The ends of the tube were attached to large
sta in less steel valves th at kept the ends of the digestion tube near room tem perature
w hile the liquid contents of th e tube were being heated.
In Figure 26, 10 ml of Milli-Q water was taken through a digestion cycle. The rise in
temperature and pressure was followed by an increase in magnetron temperature. The
rise in magnetron tem perature suggests that microwave power was being reflected back
from the microwave cavity, and heating the magnetron. A portion of the reflected
energy was also absorbed by the cavity walls. This domestic oven was designed to
operate with a 1/4” thick Pyrex" plate in the floor of the oven. The Pyrex ' plate was
removed to make room for the IR/TC. It had been noted th at when this oven was
operated without a load, the aluminum tape used to seal the flange valves to the back
wall heated and crackled. The crackling inside the microwave cavity indicated th a t the
water load was not absorbing all of the microwave energy entering the cavity.
The Percent Microwave Time On trace in Figure 26 shows that, as the tem perature
rises, so does the power required to further increase the tem perature. The required
power continues to increase until it reaches 100% power-on. When 100% Percent
Microwave Time On is reached, the tem perature continues to rise, but more slowly.
Upon reaching the target tem perature of 180°C, the power required decreases, and the
magnetron tem perature starts to drop.
There are two explanations for the poor absorption of microwave energy by water.
First, the absorption coefficient of w ater is known to decrease1:1with a rise in
temperature; secondly, p art of the load is removed as w ater is converted to vapor which
does not absorb microwave energy13.
6.3.1.1 Tube Temperature Determination Using Water Vapor Pressure
If the system is in thermodynamic equilibrium, the pressure can be used to determine
the tem perature of the liquid. Previously tabulated values of tem perature and pressure
for water** can be used to determine the tem perature for any given pressure. A
“System” is defined as being in thermodynamic equilibrium if there is no energy
entering or exiting the “System”. However, when the digestion tube is defined as the
88
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‘‘S ystem ”, it is not in therm odynam ic equilibrium because there is energy absorbed
(m icrowave energy entering) and radiated (heat energy exitin g the tube). N evertheless,
it is still in stru ctive to try to determ ine liquid tem peratures in the tube from the
pressure.
The tube pressure begins to rise once th e w ater boils ( 1 0 0 ° 0 , and cont inues to rise for
th e duration o f th e tem perature ram ping stage. The tem peratures derived from the
200
M easu red
O
c. 150
w
o
0
100
o
0
-
Temperature
\
»V ,f 1f I
.-vfvfil
V
I i 1 . 1 . 7
i' ’
>'*1
>V *.
ly-""
K
'I '
T ’ 11 '
'
C a lc u la t e d
T em p eratu re
£ 50
0
0
200
400
600
800
1000
Time (s)
Figure 27: Comparison: Measured Temperature vs
Calculated Temperature
pressure using the steam tables (Figure 27: Calculated Temperature) follow the
tem perature m easured with the IR/TC (Figure 27: Measured T em perature). However,
th e calculated tem perature is always lower than the actual tem perature, with the
difference decreasing a t higher tem peratures, especially on the cooling cycle.
In a system where the maximum tem perature obtainable is limited by the pressure
lim it of the digestion tube, reducing the vapor pressure allows higher tem peratures to
be reached. If the tube system were in therm al equilibrium, the measured pressure
would be higher.
63.2 Salt Solution Heating in Digestion Tube
The data for a n m executed with a 1% sodium chloride (NaCl) solution is presented in
Figure 28. The tem perature rose steadily until the first pressure maximum of 100 psi
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w;is reached. A first pressure trigger set at 100 psi executed a cooling and ven tin g
routine before returning to the tem perature ramp. The low pressure trigger w as
designed for ven tin g th e decomposition products that come off rapidly at th e beginning
of a digestion. (The evolution of decom position products w ill be discussed at length
later.) The pressure trigger w as then set to 170 psi, and the tem perature w as ramped
to th e target digestion tem perature.
The magnetron tem perature rose only slightly and much less power was required to
reach the target tem perature with a 1r/r salt solution than was required to heat water to
the same temperature.
The pressure obtained was higher than th at obtained for water alone. This
phenomenon has not been fully explained. One theory is th at a hotter vapor results
because the energy supplied to the salt w ater is absorbed more rapidly and the cooling
is at the same rate. The hotter vapor generates a higher pressure.
Tube Temperature
...
..
.Magnetron
\
Temperature :\\
Cooling Water
Temperature
Microwave Percent Time-On
94112207
0
T-----------------r
200
400
600
800
1000
1200
Time (s)
Figure 28: Salt Water Run
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The resu lts obtained w ith a salt solution are important because they give insight into
w h at happens w hen a sam ple dissolves, releasing salt into th e reagent solution.
<5.0.2. / Integration o f Microwave Percent Tinte-On
The M icrowave Percent Tim e-On trace o f Figure 2b is inform ative because it show s that
th e m icrowave power w as being used in several w ays. At the beginning o f th e run.
power w as cycled on and off in a constant duty cycle loop, giving evenly spaced pulses.
In th e n ext part o f th e run, power w as supplied on dem and. The pulses w ere not evenly
spaced, and it can be seen th at, a s the tem perature reached th e target tem perature,
pow er w as supplied 100CF o f th e tim e. Once the target d igestion tem perature w as
reached, it w as m aintained a t th a t tem perature for 5 m inutes; a t th e end o f th e h eatin g
cycle, th e power trace fell to zero, and stayed there till th e end o f th e run.
500g 400O)
<D
300c
Hold
T em perature
O 200-
o
R am p To
T em p eratu re
ii 1
0
0
0
25%Time-Ori
Duty Cycle
0
200
400
600
800
Time (s)
1000
1200
1400
Figure 29: Integration o f Time Microwave Percent Time-On
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Figure 29 presents the Microwave Percent Time-On information in a different fashion,
as an “integration" of Microwave Percent Time-On. The Full Power line in Figure 29 is
the integral time plot which would be obtained if the microwave power were on all of
the Lime. The Milli-Q W ater plot is the integration of Microwave Percent Time-On
from Figure 26. A steeper curve, indicates th at more power is being used. Initially, a
26',; time-on duty cycle gives a flatter slope than the middle of the curve, w here power
10 ml HN03
lu b e Tem poratufo
200
o
lu b e Pressure
Magnetron
Tem perature
Q
"Cooling Water
Temperature
ioo-
' i Microwave Percent Time-On
c
©
y
5
CL
94112103
200
600
400
800
Time (s)
Figure 30: Blank Nitric Acid Run
is applied almost 100% of the time for ramping to digestion tem perature. Once the
digestion tem perature is reached, the slope of the plot decreases, showing th a t less
power is required to maintain a tem perature than is needed to raise the system to th a t
tem perature. At the end of the curve, the slope is zero because no power was applied.
A third power-time integral, the 1% NaCl plot, is the integral of the Microwave Percent
Time-On of Figure 2S. This time integral is sim ilar to th a t for Milli-Q’*' water, but it is
not as steep overall. The flat portion in the middle of the plot represents the cooling
92
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
period in w hich no microwave power is applied to allow a venting sequence. tThe
v en tin g sequence w ill be explained in greater detail later.)
M illi-Q w ater and lfr NaCl behave in a sim ilar fashion. More power is used to ramp to
a tem perature th an is needed to m aintain a tem perature. The main difference is that
1r/r N aC l u ses less than h a lf th e power needed by Milli-Q w ater to attain or m aintain a
tem perature.
In later work, w hich is not presented here, runs using Milli-Q
W ater taller rinsing out
th e d igestion tube several tim es w ith Milli-Q W ater! showed th a t even m inute traces
o f s a lt left behind im proved th e coupling o f m icrowave energy’ hv the w ater.
6.3.3 Acids Heating in Digestion Tube: Nitric Acid
Nitric acid (70%v/v) is more dissociated than water, has half the heat capacity, and
decomposes when heated. These characteristics allow nitric acid to couple efficiently
with the microwave energy, and increase the tem perature more rapidly. However,
HNO., generates greater pressures for an equivalent tem perature than w ater alone.
The pressure a t the digestion tem perature averages 50 psi, which is higher than th a t
obtained with Milli-Q Water, but lower than the 90 psi achieved with 1% NaCl. The gas
volume and liquid in the digestion tube turn a red/brown color during the digestion.
The color persists after cooling, and at the end of the digestion cooling period, no
residual pressure was measured. When the acid and gas are removed from the
digestion tube, the brown gas, which was heavier than air, immediately sank to the
bottom of the receiving beaker. The brown gas is NO. '" released from the nitric acid; a
reversal of the process used to make nitric acid'’*.
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When th e h eatin g characteristics o f nitric acid and Y'/t N aCl are compared, th ey are
found to be nearly identical. F igu re 31 shows a ram ping integral low er than th at for
Milli-Q W ater. The slope o f th e integral curve for nitric acid (42.3%) is sta tistica lly no
different than th e 44.6% for 1 % NaCl solution.
</3
300
Holding
Temperature
O 200Ramp To
Temperature
25%Time-On
Duty Cycle
400
Time (s)
Figure 31: Nitric Acid Power Integral
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ISO
Cooling Tube On
tutV'IY
H U PM
Temperature (° C)
Pressure (psi)
100
‘turn
C o o lin g Iutx> l»'m p**rnfut^
4 psi
200
400
600
800
tutv> !om p»voh«i>
\ Residual
/ Pressure
150-
Cooling Tube
Removed
T u b o Pr<»ssuto
-<Jt psi
100 -
50-
C o o l i n a l u b o H jm p o ia H m >
200
600
400
10 psi
800
Time (s)
Figure 32: Cooling Configurations A, and B
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Cooling lubo on
lop Portion
l l j ! >*• h * * W J K *
h;*pm
Temperature (° C)
Pressure (psi)
100-
50C o o lin g lutx*
3 psi
200
800
600
200
Residual
Pressure
■
lu t» Ti'mporafuro
ISO
Cooling on
all (ho time
100
3 psi
200
600
800
94112101
Time (s)
Figure 33: Cooling Configurations D, and C
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The fact th a t 1% N aCl and n itn c acid have the sam e h eatin g profile is significant. 'Phis
m ean s th at a high or low sam ple load in the acid, w hether added or generated during
th e d igestion process, w ill not greatly affect the h eating profile during a nitric acid
digestion. T his would not be th e case for a "digestion" usin g Milli-Q water; in this
case, th e h eatin g profile would change dram atically as th e sam ple started to dissolve in
th e w ater.
6.3.4 Digestion Tube Cooling
6.3.4.1 Tube Cooling During Digestion
Since th e digestion tube is not therm ally insulated from its surroundings, it is
continually being cooled during the digestion process as it is being heated with
microwaves. It has already been shown th a t the liquid in the digestion tube is a t a
higher tem perature th an the gas phase. Therefore, since the microwave energy is
being absorbed by th e reagent and sample and not by the tube, it is conceivable th a t
th e liquid can be a t one tem perature while the outside surface of the digestion tube
is a t a lower tem perature. This therm al gradient is enhanced by the excellent
in su latin g properties of Teflon PFA.
Since th e tube loses strength a t higher tem peratures, the m ain advantage to having
th e outside surface of the digestion tube at a lower tem perature is the improved
b u rst stren g th of th e tube, allowing for higher pressures and, therefore, higher
tem peratures.
The p artial vapor pressure of nitric acid m akes up a large component of the total
pressu re a t th e digestion tem perature. Reducing the pressure of nitric acid vapor
will allow higher digestion tem peratures to be used. Figure 32 and Figure 33
display th e d ata for four ru n s using 10 ml of concentrated nitric acid (70% m/m), a
digestion tem perature of 180°C, and different cooling configurations.
R un A w as performed w ith th e cooling tube wrapped along the entire length of the
digestion tube (norm al configuration). The cooling tube was flushed w ith gas prior
to th e run. The pressure w as seen to rise as heating began, overshoot w hen the
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
d ig estio n tem p era tu re w a s reached, and o scillate ab ou t 50 psi u n til th e en d o f th e
d ig estio n . C ooling th e tu b e a t th e end of th e d igestion left a resid u al 4 psi to be
ven ted .
R un B w a s done w ith th e cooling tu b e com pletely rem oved from th e d ig estio n tube.
A s F igu re 32-B sh ow s, th e p ressu re p eak s h a v e th e sa m e p a ttern a s run A b u t are
le ss pronounced. A t th e en d o f th e run, a resid u al p ressu r e o f 10 psi n eed ed to be
ven ted .
Run C has the cooling tube wrapped around the top portion of both ends of the
digestion tube, adjacent to the vapor in the tube. Cooling w ater was run through
the cooling tube for the entire duration of the digestion cycle. The pressure profile is
not much different from Run B, except th a t the m agnetron tem perature rose
continuously during the heating. The cooling w ater tem perature was raised for the
duration of the digestion because it w as being heated directly by th e incident
microwaves. At the end of th e digestion, a 3 psi residual pressure was vented.
The physical configuration for R un D was identical to Run A; however, w ater w as
ru n through the cooling tube during th e entire digestion cycle. The cooling w ater
tem perature was raised as it passed through the cooling tube, and th e m agnetron
tem perature increased continuously through th e heating period.
The pressure curves for th e four cooling configurations of Figure 32 reveal some
interesting facts about heating nitric acid. In ru n A, th e m agnetron tem perature
increased during th e tem perature ram ping, then levelled off w hen th e digestion
tem p eratu re w as reached. In ru n s C and D, th e m agnetron tem perature increased
(at a slower rate) after th e digestion tem perature w as reached. In ru n D, th e cooling
tube w as w rapped around th e entire length of the digestion tube, so, to a certain
extent th e cooling w ater shielded th e nitric a d d from th e microwave energy. The
fact th a t w a ter is a poorer absorber th a n nitric a d d explains th e rise in m agnetron
tem p eratu re, since th e microwave energy sees m ostly w a ter in th is configuration.
However, in n m C, th e w a ter in th e cooling tube w as only shielding th e p a rt of th e
98
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d ig estio n tu b e th a t con tain ed vapor, and th e nitric acid ttho m ain m icrow ave
ab sorb er in th e cavity) w a s still fu lly exp osed to th e m icrow ave field. T h e
ob servation th a t cooling w a ter con trib u ted to an increased m agn etron tem p eratu re
rise s u g g e s ts th a t, reg a rd less o f w h a t is already p resen t in th e ca v ity , w a ter
in c r e a se s th e reflected m icrow ave en ergy. T he increased rise in m agn etron
tem p era tu re ca n n o t be exp lain ed by th e am ou n t o f acid and w a ter p resen t in sid e th e
ca v ity b u t m ig h t be ex p la in ed by th e geom etry o f th e load in sid e th e cavity.
The m ain reason for trying the different configurations for the cooling tube during
th e h eatin g cycle was to find the most effective way of reducing the vapor pressure
of th e nitric acid. Run B, with the cooling tube completely removed, relied on air
cooling to cool th e outside of the tube. The maximum pressure obtained was 91 psi.
reduced from th e 104 psi reached in ru n A. The cooling a t the end of the digestion
tube w as not a s effective, and a residual pressure of 10 psi was left a t th e end of the
run. T his suggests th a t the cooling tube (I/S’*OD PFA) wrapped around the entire
length o f th e digestion tube in ru n A therm ally insulates the digestion tube from the
surrounding air. In ru n C, a cooling tube was wrapped around the top portions of
th e digestion tube containing th e hot vapors, and w ater ran through th e tube during
th e h eatin g cycle. This w as more effective th an in run B, w ith a maximum pressure
of 83 psi, and a residual pressure of only 3 psi. In ru n D, th e cooling tube was
w rapped over th e entire length of th e digestion tube, w ith w ater running through it
during heating. Run D w as the m ost effective of the four configurations tried, with
th e lowest m axim um pressure, 76 psi, and a residual pressure of 3 psi. The only
draw back w as a slowed tem perature ram ping rate, taking 400 seconds to reach th e
digestion tem perature instead o f300 seconds.
Cooling of th e digestion tube during h eating is a viable and sim ple way of reducing
th e vapor pressure during digestion. This m ethod will work as long as enough
microwave power is available. I f th e digestion tube exterior were brought to a lower
tem p erature, as, for example, w ith liquid nitrogen, it is possible th a t insufficient
microwave power would be available to m aintain the digestion tem perature.
99
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Traditional microwave bombs have been cooled with liquid nitrogen during the
digestion cycle’’’', but the magnetron was unable to supply sufficient energy and the
digestion was incomplete because the digestion tem perature was never reached.
The pressure plots for the four configurations are presented together in Figure 34.
All of the plots show a rapid rise in pressure; they level out once the digestion
tem perature is reached. Published tem perature/'partial vapor pressure data"' for
nitric acid and w ater show an exponential rise in pressure when tem perature is
increased a t th e higher tem peratures; a sm all change in tem perature results in a
much larger pressure difference than a t lower tem peratures. Thus, th e tem perature
ram p m ust not be too steep, since the pressure may exceed lim its before the system
is able to adjust. This is especially true when using a microwave oven th a t uses a
120 -r
- (A) Cooling Tube O
Pressure (psi;
'
80 -- v
Cooling Tube
Removed
B
Cooling Tube On
Top Portion
40 --
^ Q ) Cooling Tube On
200
250
300
-i
350
1--------1400
450
Time (s)
Figure 34: Pressure Cooling Comparison
100
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500
system th a t has varying levels of power output. In run D, the tem perature ram p
rate is lower than the other Lhree; the initial overshoot in pressure when the
digestion tem perature is reached (seen in runs A, B, and C) is not seen in run D.
It can be concluded from the study of these four cooling configurations th a t a cooling
tube wrapped over the entire length of the digestion tube with w ater running
through it during the digestion should be used, and th a t the tem perature ram p rate
should be kept to a m inim um. These recommendations will increase the digestion
tim e; therefore, it may be sim pler to apply a closer control of tem perature and
pressure.
63.4.2 Cooling After Digestion Completion
The tim e needed for cooling of the digestion vessel after a digestion is completed, or
in preparation for venting, can take up a significant portion of the total digestion
tim e. After the digestion sequence of 10-20 m inutes is completed, traditional
4.5 ;
C ooling Tube Using Wafer
k= -1.092
Cooling
Cooling Tube
B Airwith
Removed
k=-0.955
1.5
0
0.5
1.5
2
Time (min)
Figure 35: Newton Cooling Curves o f the Digestion Tube
101
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m icrow ave bom bs a re left to cool in th e oven for 10 m in u tes or more. W hen th e
bom bs are cool en o u g h to h an d le (w ith g loves, apron and sa fety visor), th ey are
tran sferred to a cooling b ath , u su a lly ice w ater. T he coolin g procedure ta k e s n ea rly
3 0 m in u tes to com p lete. In m u ltip le step d ig estio n p rocedures, sev era l coolin g
periods m ay be required. For ex a m p le, th e d ig estio n o f organic sa m p le s ty p ica lly
c o n sist o f a pred igestion p h a se, follow ed by cooling, v en tin g , and th e m a in d ig estio n
cycle.
There are two factors th a t affect cooling: m ass and surface area. The digestion tube
is designed with these factors in mind. Its long slender shape, low m ass, and larger
surface area allow it to cool much more rapidly th an a traditional microwave bomb.
The much larger external diam eter and shorter height of a traditional microwave
bomb (4.7 cm OD, 3 cm tall)'" gives an exposed surface area of 64 cm', w hereas the
digestion tube (1.25 cm OD, 60 cm long) has a surface area of 94 cm*. The large
internal diam eter of th e bomb m eans th a t a m uch thicker wall is needed to
w ithstand the sam e pressure. The thick walls effectively insulate th e hot liquid
inside the traditional bomb from th e cooler exterior. I f th e bomb is left to cool
surrounded by circulating am bient a ir inside th e microwave oven after th e
digestion, nearly 40 m inutes m ust pass before th e bomb reaches room tem p eratu re71.
The digestion tube takes 15 m inutes to air cool to room tem perature after reaching a
digestion tem perature of 180°C. The m ethod of cooling each vessel w as designed to
allow direct comparison of cooling characteristics of th e tu b e and bomb.
6,3.4.3 Newton Cooling o f Digestion Vessels
The cooling of a n object can be described using Newton Cooling71.
(6-1)
ln(T-Tf) = In (T0-Tf) - k t
T, and Tr a re th e sta rtin g and final tem peratures respectively; t is th e tim e, T is th e
tem p eratu re a t tim e t, and k is th e slope of th e cooling curve when In (T-Tf) is
plotted versus t.
102
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The tube tem perature data from plots A and B of Figure 32 were used to plot the
Newton Cooling curves in Figure 35 for two cooling configurations. Al cooling tube
using w ater, and B) using air cooling only. These gave Newton Cooling constants k
of 1.092 and 0.955 respectively. A large k means more effective cooling. W ater
cooling (A) is seen to be more effective than air cooling alone (B); both configurations
are alm ost an order of m agnitude better than the air cooling of a traditional
microwave bomb. Jassie et al has used this relationship to describe cooling of
traditional microwave bombs and obtained a cooling constant k of 0.129 '.
The rem ainder of the digestion work w as performed using a cooling tube wrapped
around th e entire length of the digestion tube. Tap w ater a t an average
tem perature of 12°C flowed through the tube during cooling. The cooling w ater was
removed from th e cooling tube using compressed argon gas.
63.5 Polyacrylamide Capsule Digestion
The discussion so far has dealt with heating different types of reagents and solutions.
10 ml HN03+ Capsule
200-
Tube Temperature
j—'
Magnetron
Temperatun
; 23 psi
Microwave Percent Time-On
; :
yr
l , 1
1 1
Cooting Water
Temperature
t
94112501
0
100
200
300
400
500
Time (s)
600
700
800
Figure 36: Polyacrylamide Capsule Digestion
103
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900
This work has given insight into the heating and cooling characteristics of w ater, acid,
and salt solutions. The next steps deal with the dissolution of an empty capsule.
An em pty capsule was placed in the middle of the digestion tube, using the squeegee to
push it into place. Concentrated nitric acid was then pumped into the tube from both
open ends of the digestion tube. Adding the acid from both sides of the tube keeps the
capsule submerged in th e acid. If the capsule is not immersed in the acid when
microwave power is turned on, boiling of the acid will push the capsule to one end of the
tube and out of the acid.
The capsule digestion data is shown in Figure 36. The digestion is started by adding
the microwave energy in short pulses. This allowed the tem perature of the acid to rise
slowly without any violent boiling, giving the capsule time to dissolve in the acid. The
slow addition of microwave energy raised the tem perature in the tube until the capsule
was completely dissolved; a tem perature of 90"C was found to be adequate.
The tem perature was then ramped from 90"C to a digestion tem perature of 180"C. The
ramp was implemented using a program loop containing time delays and instructions to
increment the minimum and maximum tem perature triggers as the loop was executed.
Each time the loop was executed, the minimum and maximum tem peratures in the
trigger statem ents were incremented, and the triggers were reinitialized a t the new
minimum and maximum temperatures. The minimum and maximum tem perature
triggers were typically 5"C apart, and incremented by 2”C. The loop had a total delay
time of two seconds, giving a tem perature ram p of l"C/sec. Because the triggers were
checked each tim e an instruction was executed, the 2 second tim e delay was divided up
into 20-100 millisecond time delays. The duration of the ram p could be determined by
the num ber of loops executed, or an additional trigger could have been used to exit the
loop once the digestion tem perature was reached. The latter was found to be the
preferred method, since this insured th a t the targ et tem perature had actually been
reached. Once the digestion tem perature was reached, control was passed to the
digestion control loop. The digestion loop m aintained the tem perature between the
digestion minimum and maximum trigger tem peratures (typically ± 5°C) for the
predetermined digestion time (5 minutes for all experiments).
104
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Pressure was also monitored to ensure that design specifications were not exceeded.
The digestion was deemed complete when the tem perature had been held at digestion
tem perature for the prescribed amount of time.
6.3.6 Polyacrylamide Digestion
It was initially thought that the dissolution of polyacrylamide would generate large
volumes of decomposition products in the form of CCV., but. as can be seen from Figure
36, this was not the case. The average pressure during the digestion was only 7a psi.
25 psi greater than th a t in a simple nitric acid run. At the end of the final cooling
Curve 1: DSC
F i l e in fo : P4D_p«p
S s a p le Height: 2.350
Wed Dec 07 14; 37:44 1934
sq
poly tacryloaide) cepsule fragment*
♦ 1 poly tacrylaaldel cepsule fraga
Heat Fiov W/gl
2 4.00 -
2
22.50 22.00
125.0
7 5 .0
T a a p ara tu re f*Q
fo r Guy Legere; lOdeg/aln; 25-250C seen
aoo' 2
S
TO»U
0 .0
«•»
l&.O
C/M l.
175.0
225.0
K .S ingflaid
PERKIN-€LMER
7 S e r i n T h i n t l u ta ly a la Symtm
n m Dec 07 1 4 :0 :3 7 1994
Figure 37: DSC Analysis o f Polyacrylamide Capsule Fragment
stage, a residual pressure of 20 psi was measured. The presence of polyacrylamide in
th e digestion solutions was suspected since large quantities of carbon were found in
105
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solution. The decom position products associated w ith the low pressure rise probably
resulted from decom position o f a combination o f th e following: 1) monom er not reacted
during polym erization, 2) am m onium persulfate, 3) TEMED. The latter two are present
in the finished capsule, but a t very low concentrations since they are both added
sparingly.
The fact th at polyacrylamide does not decompose at 1S0°C should have come as no
surprise, since it is known “ that to break down organic compounds fully, a
decomposition tem perature of 230°C' or greater must be reached. To verify th a t this
was actually the case, samples of capsule material were subjected to differential
scanning calorimetry (DSC) analysis. The DSC instrum ent was a Perkin-Elmer 7
Scries Thermal Analysis System , which was run in power compensation mode, using
Al crucibles under a nitrogen atmosphere, and a sample weight of 2.35 mg. The
reference cell was heated from ambient a t 10 6C/min.; it was aborted a t 260°C to avoid
IN S E R T CAPSULE
M IC R O W A V E OVEN WALL
LJ
T EM PER A TU R E (IN FR A R E D )
Figure 38: Capsule Pushed in by the Squeegee.
106
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exploding the sample cell and damaging the instrument. The sample cells were
noticeably swollen when removed from the instrument. In power compensation mode
the reference cell tem perature is ramped and power is supplied to the sample cell to
maintain the same temperature.
The DSC data confirms th at polyacrylamide requires a tem perature well in excess of
1S0°C to decompose. A tem perature of 260°C would insure complete decomposition.
The data for the thermal analysis (Figure 37) shows a steep rise at 230°C. and again
260°C. The rise in heat flow can be attributed to the energy required to break the
carbon-carbon bonds. The side-chain carbons break a t 230°C, and the main chain
carbon-carbon bonds break a t 260°C.
6.3.7 Capsule/Sample Digestion Sequence
In the empty capsule digestion discussed in the previous section, the capsule was
completely dissolved, and no m aterial was left in the ends of the digestion tube when
the digestion was complete, even though there was vigorous boiling during the
•
ADD REAGENTS
Figure 39: The System After Reagent Addition.
107
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digestion. The next step was to determine how a partly insoluble sample would behave
during a digestion. A soil CANMET Energy, Mines and Resources Canada, Certified
Reference Material SO-4 was loaded into a capsule and digested. Soil was chosen
because it is mostly silica, does not dissolve in nitric acid, has an inorganic soluble
component, and an organic fraction (4.4%C). A 0.25g encapsulated sample of SO-4 was
transferred to the digestion tube (Figure 38). Concentrated nitric acid was then
pumped slowly into the digestion tube so th at the capsule remained in the center of the
digestion tube, and fully immersed in the acid (Figure 39).
Valves a t either end of the tube were then closed, sealing the system, and microwave
energy was applied (Figure 40). From this point on, tem perature was monitored to
ensure th a t the sample was maintained a t the digestion tem perature for the specified
length of time (5 min).
Figure 40: Microwave Energy is Applied.
108
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Pressure was also monitored to ensure th at design specifications were not exceeded.
When the sample/solvent mixture had completed its digestion sequence, the sample was
cooled and vented.
The digestion mixture was cooled by room-temperature water flowing through a tube
wrapped around the digestion tube (Figure 411.
When the mixture had cooled, the system was vented through a computer-controlled
valve system, and the cooling-water tube was blown free of water. The flange valves at
the ends of the tubes were opened, and the sample/acid mixture, presumably digested,
was pushed out of the digestion tube using the squeegee system (Figure 421.
63.8 Capsule Digestion Observations
Since the digestion tube is made of sem i-transparent PFA, it was possible to observe the
system operation visually in ways th at would not be possible with an opaque material.
The digestion process was quite interesting to watch. As microwave power was applied,
COOL a n d VENT
Figure 41: The Tube is Cooled and then Vented.
109
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the capsule opened well before the capsule body was digested. This was probably due to
heating of the gases inside the capsule. As the capsule popped open, the solid sample
was distributed throughout the acid, with some sample at the solution surface, and a
small amount deposited slightly above the solution surface. These particles were
deposited by the bubbles th at formed as the acid boiled. One might be concerned about
the digestion of these particles, and th at concern leads to another observation on the
system.
The valves did not rise appreciably above room tem perature, and the upper portions of
the tube were ju st warm to the touch. This setup provides an enormous benefit from
the point of engineering high-pressure fittings. This approach has considerable
advantage in th at vaporized solvent is continually being condensed on the upper
portions of the tube wall, and washed down over any particles deposited on tube wall
surfaces. Unlike the narrow-bore tube systems th at we have observed, here the entire
REM OVE SOLUTION
Figure 42: Squeegee used to Remove D igest
110
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solution is constantly mixed by boiling. Boiling under constant pressure is not a
problem; however, venting, when necessary, must be done with some care to avoid
sample loss. This loss can be prevented by first cooling, and then using small, fast
pressure drops to avoid having the solution boil up through the secondary vent valves
as pressure drops. The pneumatically actuated valve system allows this venting to take
place without difficulty.
O
10 ml HNO3 + Copsule + SO-4
200-1
lubo lomporQtut,
96 psi (avp )
2
© 100
Tube Pressur
M opnetron
T em perature
£ *
C oolino W ater
Tem perature
M icrow ave P ercen t Time-On
100 -
§9
OQ
0 £
a_ i=
38 psi
[
5 )il
i.
I I S I I U
11 1 1 i
t ! !
.
1 1 1 1 1 ii
I ■ ,1
'I 'I
■! '| jl II ‘l
094112404
r-
0
100
200
300
400
500
600
700
800
900
Time (s)
Figure 43: Soil Digestion
6 3 3 Soil Digestion
The digestion of th e SO-4 soil sam ple proceeded like a n em pty capsule digestion.
T he d a ta for th e digestion ru n is presented in Figure 43. The average pressure
during digestion and th e residual pressure were both higher th a n those observed for
th e digestion of an em pty capsule. The increased pressure levels can probably be
a ttrib u te d to th e m odest am ount of carbon in th e reference m aterial.
W hen th e sam ple digest w as removed from th e digestion tube, th e liquid w as a
m urky brown color. T he next day, th e liquid w as clear, though still brown, w ith a
111
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white precipitate a t the bottom of the test tube (probably undissolved silicate
material).
6.3.10 Vent cycle
The digestion of capsules and soils did not generate large am ounts of decomposition
gases th a t would have required a venting sequence before the end of the digestion.
The venting sequence is im portant, because the digestion of botanical and biological
samples is capable of generating quantities of decomposition products which can
destroy the digestion tube if not controlled properly.
If at any time during the digestion the pressure exceeds the maximum limit allowable,
the “maximum pressure” trigger is tripped, initiating a venting sequence. If the
pressure is about to rupture the digestion tube, the vent valve is opened immediately.
Otherwise, the tube is cooled to below 70”C before it is vented. There are several
reasons for cooling the digestion tube before venting, the most important being the
presence of volatile elements in the hot gases. Cooling allows the volatile elements to
condense on the walls of the tube, or to be absorbed by the acid. The other reason is the
loss of analyte as aerosol: venting causes a sudden release of gas from the hot liquid,
forming an aerosol. Also, when the liquid in the digestion tube is hot, the vapor
pressure is much higher, and acid is lost in vapor form on venting. A loss of add
volume up to 40% has been observed if venting occurred repeatedly while the liquid was
still hot.
The venting is accomplished by opening and dosing the vent valve quickly. When the
vent valve is opened, pressurized gases escape from both sides of the digestion tube.
This sudden release of gas causes the a d d to boil, producing a fine aerosoL Time is
given for the aerosol in the tube to settle before the venting sequence is repeated. The
venting sequence is repeated until the pressure has dropped to 10 psi or lower. The
minimum and maximum tem perature triggers are reset to below 90°C, and the
digestion is restarted. Once the tem perature has ramped to the digestion tem perature,
and held a t tem perature for the required amount of tim e w ithout a venting sequence,
the digestion is considered complete.
112
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6.3.11 Sucrose Digestion
Sucrose, a su g a r m ade up of easily digested glucose units, w as selected to test the
v en tin g sequence u nder digestion conditions. A 0.5g sam ple of g ra n u la r sucrose was
en cap su lated an d digested in 10 ml of concentrated nitric acid (F igure 44). The
sucrose w as expected to react violently w ith th e acid, so the te m p e ra tu re ram p rate
lutx- te m p e ra tu re
M ag n etro n d i t c h
t ollov.vi.x1 bv
te m p e ra tu re P ro p
lu b e Pressure
M ag n etro n
Cooling W ater
te m p e ra tu re
Temperature
Microwave Percent Timo-On
94120902
0
200
400
600
800
Time (s)
1000
1200
1400
1600
1800
Figure 44: Sucrose in Capsule Digesiion
w as decreased by 25%, and th e m aximum pressure trigger was set a t 100 psi. The
p ressure rose to 100 psi, and th e cooling started; however, th e pressure continued to
rise, an d overshot to a pressure of 155 psi. If th e pressure trigger had been se t a t a
higher level th a n th e digestion tube m axim um, the pressure would have been
exceeded, w ith obvious results. Therefore, th e first pressure m axim um w as set a t
100 psi; a fte r th e first venting sequence, the maximum pressure trigger w as set to a
h igher p ressu re of 170 psi. A digestion tube bu rst pressure of 215 psi a t 180°C has
been determ ined through previous experience.
113
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The digestion of sucrose required two venting sequences. A fter th e second venting
sequence, th e digestion tem p eratu re w as m aintained for 5 m inutes w ithout th e
pressu re exceeding th e m axim um p ressure trigger level of 170 psi. A residual
pressure of 30 psi rem ained upon cooling th e digestate to room tem p eratu re.
The sucrose digestion run proved th at the system was capable of handling the
evolution of decomposition gases w ithout hum an intervention or aborting of the run.
The evolution of gas and application of power was m anaged in a fashion th a t
avoided over-pressurizing the system. This sequencing of power cannot be
programmed in advance as a fixed power program. It should be noted that, if the
system were capable of containing all of the gases produced during the digestion, there
would be no need to vent the system during the digestion. However, the pressure
requirement would exceed 1000 psi even for samples as small as 200 mg.
63.12 Temperature Fluctuation Sources
A close look a t th e tem perature trace (Figure 44) reveals a severe glitch a t 115S
10 mt HN03+ Capsule + (botanical) Orchard Leaves
200 1
Tube Temperature
■n I. S
*I
Cooling Water
Temperature
Microwave Percent Time-On
:<WW,n v : 5 '! <i1
!!Si H
R e 94090701
500
1000
1500
Time (s)
Figure 45: Orchard Leaves Digestion
114
with permission of the copyright owner. Further reproduction prohibited without permission.
2000
seconds, coinciding with a glitch in the magnetron tem perature at 1156 seconds. It
is quite unlikely th a t this sudden drop in either tem perature is real. Thermocouples,
being high impedance devices, are prone to interference from RF coming from AC
power lines. The thermocouple signals have been found to be very sensitive to
placem ent of signal wires leading to the instrum entation board. Moreover, the
spikes in the tem perature signal are consistently negative. In some instances, the
sudden drop in tem perature is associated with a rapid rise in pressure. If the rise in
pressure is caused by boiling, it could be responsible for the drop in tem perature if
th e liquid was superheated.
63.13 Digestion of a Botanical Sample
The digestion of 0 .4 g o f the SRM Orchard Leaves 1571 from NIST (Figure 45)
proceeded m uch like th e sucrose digestion. T here w as a rapid rise in pressure.
10 ml HN03+ Capsule + (Biological) Bovine Liver
200-1
©o
Tube Temporaturo
Venting
While Hot
Tube Pro
Magnetron
Temperature
JUKW
Cooling Water
Temperature
'
,
- il
|? J j
! i !:
Microwave Percent Time-On
600
Time (s)
■’
i )
!i *
94112502
800
Figure 46: Bovine Liver Digestion
115
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requiring a series of venting sequences before the digestion w as completed. T here
was, however, an unexplained v enting w ithout cooling on th e first pressu re rise.
The solution obtained after the digestion was clear, with m inute floating strands.
These strands are thought to be made of undigested silica. It is known from other
work (NIST Certificate for 1571) th a t low values for iron are obtained if HF is not
used in the digestion.
63.14 Digestion of a Biological Sample
The digestion of Bovine Liver serves as a good example of an organic sam ple th a t
has a readily digested component. At time zero, microwave power was applied; a
steady increase in tem perature w as observed as the software controlled microwave
energy to produce a predefined tem perature ramp. D uring th e first 200 s, a linear
tem perature rise was observed; a m arked pressure increase occurred after about 100
s as th e bovine liver began to digest and release carbon dioxide. The increase was
quite rapid and a t about 250 s, th e software noted th a t a pressure threshold had
been reached. At this tim e, all microwave power was cut off, and th e cooling w ater
flow w as initiated. A drop in both tube pressure and tem perature was observed
while th e cooling-water tem perature rose and dropped as th e solution cooled.
At about 500 s, the vent threshold was reached (70°C), and th e system w as vented
over a short tim e to atm ospheric pressure. The heating process th en began again;
however, the startin g tem perature was higher, and th e system now plateaued a t our
designated maximum tem perature of 180°C. Once again, th e pressure rose, b u t
m uch more slowly, and th e cycle continued. There are additional subtle features of
th e trace, shown in Figure 46. Each tim e the sam ple w as cooled, it dropped to a
lower an d lower pressure, suggesting th a t th ere w as less carbon dioxide in th e tube
atm osphere. O ther gaseous components, like w ater vapor, condensed out on cooling.
63.15 System Cleaning
Sample solution removal left a thin film of the solution on the inside of the tube volume.
The digestion tube was cleaned by filling it with dilute nitric a d d , and bringing it to a
116
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boil using the microwave energy. To do this, both ends of t he digestion tube were fitted
with adapters th a t were open to the atmosphere and emptied into a waste container.
The tube was then filled with acid, and boiled. The violent bubbling pushed the boiling
acid back and forth through the tube and finally out into the waste container. The
squeegee was then used to remove the remaining wash liquid. This cleaning cycle was
repeated a t least twice, to thoroughly wash out the tube. The analysis of the blank
digestion runs confirms this (see later Table 71.
117
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6.4 Conclusions - Chapter 6:
The large diam eter digestion tube equipped with cooling and venting can be used to
digest organic sam ples unattended. The power, tem perature and pressure profiles
arc sim ilar to those obtained with the traditional microwave bomb. The large
diam eter digestion tube should digest samples and yield results sim ilar to if not
identical to traditional microwave bomb digestion. It should be able to do this
without operator intervention.
118
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7. Analysis
7.1 Introduction
T he previous ch ap ter discussed th e digestion sequence of reagents and different
sam ple types. T his ch ap ter will present and discuss th e analysis results of a variety
of sam ple types. All digestions w ere performed a t 1S0"C. for a m inutes, w ith 10 ml
of concentrated n itric acid. T he acid w as added volum etrically w ith a dispenser;
capsule, sam ple + capsule, d ig estate bottle, digestate bottle + digestate and, 1 ml of
th e d ig esta te w ere all w eighed. T he w eights w ere used to calculate th e total
recovered volum e of d ig estate removed from th e digestion tu b e w ith th e squeegee.
T he recovered volum e, sam ple w eight and. concentrations in th e digestate w ere th en
used to calculate th e concentration in th e sam ple.
All digestates and reagent blanks were diluted 10 tim es in 15% H N O , for analysis.
Two m ulti-elem ent standards (QC-19, QC-7 SCP Science. LaSalle) were diluted in
15% HNO.,. A P, Y and C standard were made in-house using potassium phosphate,
y ttriu m n itra te and m annitol and these were also diluted in 15% HNO.,.
T he m ajority of th e analyses w as performed on a PE/SC1EX 5000 ICP-MS
(C anadian Geological Survey, Ottaw a), others on the SCIEX 250 ICP-MS, JarrellAsh 25 ICP-AES (Scanning), and ARL Quantometric A nalyser ICP-AES (Direct
reading). ICP-MS w as used for all trace elem ents and ICP-AES for m ajor elem ents
th a t w ere too concentrated to ru n by ICP-MS. The isotopic m ass values a re listed
next to th e elem ent nam e w hen ICP-MS was used.
7.2 Detection Limit Determination
To determ ine detection lim its a blank solution (15% HNO.,) w as run 10 tim es (Table
3). The intensities (counts) statistic’s are presented in Table 4. The average (AVG)
and relative stan d ard deviation (RSD) are calculated using th e values from Table 3.
The AVG and RSD a re reported in Table 4.
#
119
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Detection limits determined in solution are a useful indicator of performance;
however, the only meaningful detection limit is th a t for the sample analysis. To
obtain detection lim its in the sample, the RSD was converted to concentration usin
Table 3: Blank Intensities in 15% Concentrated Nitric Acid
Run #
Element/
Mass
1
2
3
4
5
[counts]
6
7
8
9
10
34
41
36
Be 9
32
29
39
35
30
29
23
657
B 10
688
708
665
646
663
681
657
668
636
9149
9396
9205
9341
C 13
9259
9251
9215 9220 9122 9094
Na 23 44678645431 44981 44874 45045 45121 45297 44329 44628 44182
1
3
6
5
3
9
5
7
6
Mg 24
1453 1425 1469 1431 1485 1466 1434 1443 1414 1489
AI27
28311 27724 27384 27849 28021 28273 27834 28183 28466 28784
Si 28 70902672828 73334 72421 73160 74009 73966 74351 73516 73028
4
7
8
2
4
5
8
6
6
15914 16291 15951 15984 16051 16226 16526 16573 16694 16872
P 31
Ca 44
15069 15504 15028 14917 15088 15086 15109 14866 15155 15018
Ti 49
79
62
61
64
76
68
93
66
63
56
571
608
594
587
V 51
584
562
579
564
608
576
Cr 53
46224 46612 46944 46169 46473 46848 48139 47169 46511 47087
Mn 55
930
931
884
874
878
953
933
926
916
923
Fe 56 69927970469 70077 70208 70398 70177 70387 70262 70219 70178
8
6
1
8
0
8
8
4
9
Ni 60
314
275
329
361
300
281
299
331
319
298
89
111
84
Co 59
78
81
99
87
90
98
95
Cu 63
1972 1938 1866 1851 1906 1861 1848 1970 1968 1876
Zn 66
89318 89621 88384 88668 88955 87989 89853 90479 89880 90753
64
64
59
79
54
As 75
60
51
49
50
48
49
S e 77
54
38
46
49
53
45
51
51
43
Se78
3885 4028 3993 3976 4104 4083 4037 4049 4069 3988
Y 89
9303 8986 9228 9319 9362 9066 9451 9183 9395 9266
24
17
14
Mo 95
23
26
26
21
22
21
23
Ag 107
33
25
33
23
36
28
29
33
28
26
24
Cd 111
11
17
14
13
21
14
24
15
18
Sb 121
22
25
29
21
21
17
22
24
19
26
Ba 137
79
74
109
79
99
73
78
83
76
82
Tl 205
18
24
27
32
23
30
23
25
28
28
Pb 208
142
160
145
134
129
143
148
136
140
131
120
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p rev io u sly d eterm in ed s e n sitiv itie s, m u ltip lied by th ree and ad ju sted for a sa m p le
w e ig h t o f 0 .5 g an d d ilu tion o f 200 tim es. T h e d etection lim its for ICP-M S in th e
sa m p le a re liste d in th e la s t colum n o f T ab le 4. T h e v a lu e s rarely rise above 1 pg/g
(N a , S i, P , a n d Zn).
The ICP-MS w as run for a total of
4 hours to analyze all the
solutions and th e raw intensities
were saved for all runs. The
Table 4: Blank Study for
ICP-MS
Element/
Mass
AVG
RSD
blank solution w as ru n several
[counts][counts]
tim es during the course of the
analyses. D uring a 3 hour period
th e in stru m e n t slowly drifted in
th e sam e direction for each
elem ent. The equivalent drift for
a 0.5 g sam ple diluted 200 tim es
w as S i" (-39 pg/g), Cu,a (+0.011
pg/g), Pb**0* (<.01 pg/g) and N a"
(0.3 pg/g). M inor torch
breakdow n (quartz) is th e source
of high background and drift for
Si an d N a. The Cu and Pb drift
is insignificant. The high
detection lim it for Fe is a resu lt
of th e ArO m olecular ion.
Be 9
33
667
B 10
Na 23 448571
Mg 24
1451
AI27
28083
Si 28 731521
P 31
16308
Ca 44
15084
Ti 49
69
V 51
583
Cr53
46818
Mn 55
915
Fe 56 702308
Ni 60
311
Co 59
91
Cu 63
1906
Zn 66
89390
As 75
58
Se 77
48
4021
Se 78
Y 89
9256
22
Mo 95
Ag 107
29
17
Cd 111
Sb 121
23
Ba 137
83
Tl 205
26
141
Pb 208
5
21
4030
26
405
9842
341
172
11
16
576
27
1607
26
10
52
897
10
5
64
145
4
4
5
4
12
4
9
Detection
Limit in the
Sample
[pg/g]
0.009
0.25
0.950
0.006
0.089
3.90
1.5
0.76
0.022
0.002
0.080
0.003
0.20
0.013
0.001
0.013
1.1
0.009
0.072
0.29
0.21
0.002
0.001
0.004
0.001
0.008
<.001
0.001
121
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The variability of the instrum ent background levels is not a lim iting factor in the
analysis of the digestate solutions. O ther more severe effects, such as molecular
ions and inter-elem ent interferences determ ine the detection limit for any particular
solution. For example, ArC1,1will limit Cr""’ and ArCI the As" determ ination.
Table 5: Multi-Element Standard Repeat Runs
Element/ %RSD
Mass
%
Be 9
Mg 24
Ca 44
Ti 49
V 51
Cr 53
Mn 55
Fe 56
Ni 60
Co 59
Cu 63
Zn 66
As 75
Se 77
Se 78
Mo 95
Cd 111
Sb 121
TI 205
Pb 208
1.4
1.2
3.4
1.5
1.3
1.2
1.1
2.1
1.0
1.2
1.4
2.7
0.9
1.0
1.7
1.7
1.3
1.4
1.3
1.8
1
2
3
4
Run #
6
5
7
8
9
10
[Arbitrary Concentration]
7846
7640
5199
7559
7841
7709
7649
7661
8108
7862
8305
9114
7648
7735
7943
7976
7983
7665
8032
8044
7696
7446
5014
7744
7639
7734
7694
7685
8116
7847
8021
8967
7740
7757
7946
8033
8025
7774
7899
8193
7771
7489
5002
7692
7705
7653
7795
7420
8032
7636
8115
9080
7663
7706
7906
7905
7688
7706
7921
8192
7882
7420
5100
7510
7583
7637
7593
7577
8130
7620
8185
8697
7716
7788
7950
7762
7887
7708
7935
8025
7804
7548
5004
7475
7654
7761
7532
7265
8056
7743
7932
8912
7599
7568
7799
8064
7806
7972
7760
7928
7821
7462
4840
7514
7748
7610
7555
7370
8135
7803
8119
8942
7686
7708
7771
7914
7834
7726
7884
8094
7994
7440
4926
7489
7533
7519
7573
7430
7931
7727
8065
8341
7525
7761
7898
8126
7784
7916
7896
8373
7697
7507
4743
7436
7606
7554
7602
7578
8060
7736
8149
8908
7589
7789
7598
7711
7867
7607
7687
7989
7660
7625
4795
7377
7592
7523
7534
7242
7915
7655
7960
8843
7563
7632
7743
7827
7960
7780
7741
7901
177
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7659
7372
4652
7559
7770
7719
7675
7364
7989
7593
8043
8577
7706
7595
7636
7846
7874
7772
7868
7931
7.3 Precision Determination
The m ulti-elem ent standard QC-19 (19 elements a t 100 pg/gl was diluted 600 times
(0.167 pg/g) and run 10 times to determ ine the precision of the instrum ent (Table 5 V
The percent RSD (%RSD) ranged from 0.9 (As' '1 to 3.4 (Ca4\
For the most part, the
%RSD’s average 1.5%; a value consistent with the performance of a cross-flow
Table 6: Commercial Capsule Digestion and
________ Analysis *_____________________
Element/
Mass
Be9
Na 23
Mg 24
A! 27
P 31
Ca 44
Ti 49
V 51
Cr 53
Mn 55
Fe 56
Ni 60
Co 59
Cu 63
Zn 66
As 75
S e 77
Y 89
Mo 95
Ag 107
Cd 111
Sb 121
Ba 137
TI 205
Pb 208
Red Capsule
Capsugel
Vegecap
Black Capsule
Capsugel
G.S.Technologies
fog/g]
[pg/g]
[pg/g]
<0.1
226.2
11.0
7.0
21.8
45.8
43.2
0.1
0.8
0.2
1.6
0.4
<0.1
1.1
-8.6
<0.1
<0.1
1389.5
<0.1
<0.1
<0.1
<0.1
0.2
<0.1
0.1
<0.1
231.5
14.5
6.9
22.4
52.4
5.8
0.3
2.2
29.9
103.4
1.8
0.2
0.5
-6.9
<0.1
<0.1
1403.8
0.1
<0.1
<0.1
<0.1
0.3
<0.1
0.2
<0.1
246.7
52.0
1.4
8.5
137.2
0.1
<0.1
0.6
0.2
4.0
1.0
<0.1
0.8
-6.1
<0.1
0.1
<0.1
0.1
<0.1
<0.1
<0.1
0.3
<0.1
0.1
* N ote: Values are reported for a 0.5g equivalent weight.
123
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nebulizer used to introduce the sample. A closer look at the Ca“ data shows a
consistent drop in signal, 10% in total. Zinc1’', Fe"*' and others, also show a sim ilar
drop in signal while other elements of the sam e m ass range (Mn ") are not affected.
The cause was not determined.
7.4 Commercial Capsule Digestion and Analysis
Capsules from several m anufactures were obtain and digested. The analytical
results are tabulated in Table 6. The analyses reported in Table 6 were performed
after polyacrylamide was chosen as the prefered capsule m aterial. The aim of these
results is to show th a t the commercial capsules are not as clean as th e
polyacrylamide capsules. Therefore, th e polyacrylamide capsule digest was used as
a blank. Also, any m atrix effect caused by the presence of carbon will be subtracted
out, since the polyacrylamide and commercial capsules are alm ost th e sam e weight.
The capsules w ere selected to represent the different types available; th e red capsule
contains an opaque pigm ent, the black capsule uses an opaque dye and th e Vegecap
capsule is m ade of cellulose.
The m ain concern w ith using a commercial capsule is contam ination. To evaluate
th e level of contam ination, th e concentrations in Table 6 are reported for a sam ple
weight of 0.5 g. All th e capsules analyzed weigh approxim ately 50 mg, so th e actual
concentration in th e capsule m aterial itself is a t least 10 tim es higher.
These capsules (m ade for hum an consumption) are void of all toxic trace elem ents
determ ined, w ith th e exception of lead which is ju s t detectable. Sodium, Mg, Al, Ca,
P, Fe, and, Ni however, a re present in all three capsules well above th e detection
lim it. The Y present in th e two Capsugel capsules is thought to be co n ta m in a tio n .
Vegecap, th e cleanest of all three capsules, is still not clean enough for use in trace
analysis.
124
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7.5 Polyacrylamide Capsule Digestion and Analysis
The analysis of six
polyacrylamide capsules
Table 7: Polyacrylamide Capsule Digestion
(Table 7) revealed the
________ and Analysis *_______________
presence of trace quantities
C ap su le #
of Fe. Ca, Na, Al, and Mg.
Percent Capsule Weight Recovered.
The traces of Zn, Cr and Y
1
2
3
4
6
7
88.0
77.9
93.7
84.7
100.3
97.9
<0.1
1.4
0.2
0.2
20.5
0.3
12.3
0.1
0.1
0.9
0.1
1.3
0.2
<0.1
<0.1
-9.4
0.1
0.3
27.3
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
8.3
0.4
0.9
8.1
5.1
11.2
0.1
0.1
11.7
<0.1
0.4
0.1
<0.1
<0.1
-7.1
0.1
0.3
29.9
<0.1
<0.1
<0.1
<0.1
0.1
<0.1
<0.1
<0.1
13.8
0.9
17.9
9.0
9.5
14.9
0.2
0.1
26.5
<0.1
0.2
0.3
<0.1
1.1
232.4
0.1
0.3
30.9
<0.1
<0.1
<0.1
<0.1
0.2
<0.1
0.1
are thought to be due to
contam ination, introduced
after th e m anufacture of
th e capsules. These digests
w ere obtained as regular
sam ples. Two w ashings
w ere done before every
analysis, blank or sam ple.
Therefore, th e analyses
presented here are
representative of any
m em ory in th e digestion
tube itself.
A reag en t blank (without
polyacrylam ide present)
w as used as a blank
solution. The
contam ination levels vary
greatly. C apsule # 3 is th e
m ost contam inated while
capsule # 7 is very d e an .
T he m aterials used to m ake
Element/
Mass
Be9
Na 23
Mg 24
Al 27
Si 28
P 31
C a44
Ti 49
V 51
Cr 53
Mn 55
Fe 56
Ni 60
Co 59
Cu 63
Zn 66
As 75
Se 77
Y 89
Mo 95
Ag 107
Cd 111
Sb 121
Ba 137
TI 205
Pb 208
[pg/g]
<0.1
0.4
0.1
1.0
4.4
0.7
12.3
0.1
0.1
2.4
<0.1
-0.2
0.1
<0.1
0.1
10.6
0.1
0.4
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
8.4
0.5
0.2
0.8
0.5
8.2
8.0
19.3
1.2
3.0
19.4
9.0
0.1
0.1
<0.1
<0.1
0.3
1.5
<0.1
0.1
-3.7
-0.4
0.1
0.5
<0.1
<0.1
<0.1
0.3
-10.0
-8.5
<0.1
<0.1
<0.1
<0.1
1202.9 42.5
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
<0.1
0.1
<0.1
<0.1
<0.1
0.1
’ N ote: Values are reported for a 0.5 g
equivalent weight.
125
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the capsules can m ake a very clean capsule, but the capsule m anufacture and
squeegee are open to the a ir leaving the system vulnerable to airborne
contamination.
As discussed earlier, the digestion tem peratures never reach the 230°C needed to
break down the polyacrylamide and remove it from solution as CO.. The analysis of
the capsule digestate revealed th a t alm ost all of the capsule m aterial is still present
in solution after the digestion. Ail th e carbon found in solution is converted to
polyacrylamide and used to calculated the recovery of capsule m aterial (Table 7).
This is consistent with the low volumes of decomposition gases generated during th e
digestion
It w as initially thought th a t aspirating the digestate solution directly would
simplify th e analysis process, reduce the risk of contam ination and not increase th e
dilution factor, however, th e digestate solution would not nebulize well. Upon
investigation it was determ ined th a t the polyacrylamide w as responsible. The sam e
q u an tity of acrylamide monomer in solution does nebulize, suggesting th a t th e
polymerized form of acrylam ide is responsible for th e ab ru p t change in nebulization
efficiency. W hen an equivalent quantity of polyacrylamide gel (5 mg/ml) is placed in
w ater or nitric acid; th e solution does not nebulize. T his is thought to be due to th e
extrem ely long polyacrylamide molecule th a t does not allow th e liquid to sh e a r
a p a rt d uring nebulization. D iluting to 0.5 mg/ml polyacrylamide allows th e solution
to be nebulized and no change in nebulization efficiency is observed. A 50 m g
capsule digested in 10 ml of nitric a d d m ust be diluted 10 tim es to avoid problem s
w ith nebulization. Therefore all digestates were diluted 10 tim es in 15% H N O , for
analysis.
7.6 Tort-1 Digestion and Analysis
To determ ine th e accuracy a n d precision of th e digestion system , N ational Research
Council of C anada (NRCC) Certified Reference M aterial (CRM’s ) : NRCC TORT-1,
lobster hepatopancreas, w as digested seven tim es. This p a rticu la r reference
126
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m aterial was chosen because it was used to evaluate the CEM SpectroPrep tube
digestion system 1'. In th at evaluation, the lobster m aterial required a predigestion
before it could be pumped as a slurry into the tube digester. Apparently this
m aterial is very stringy, clogging the inlet valve if not predigested ' when
concentrations greater lfr w/w were used. Obviously, predigestion was not required
for th e capsule system.
The resu lts for the seven digestates analysed are listed in Table S and Table 9. The
elem ents selected and accepted values arc those employed in an evaluation study of
the CEM system by Sturgeon ct a l'\
The elem ental concentrations were obtained without using internal standardization,
m olecular ion corrections or adjustm ent for m atrix effects. The first elem ent listed.
A s 1, h a s a n acceptable %RSD w ith a high recovery. The high levels of Cl in this
m arine sam ple produces an ArCl m olecular ion (40+35) interference giving
consistently h igher results for As71. The accepted value for Cd is debatable: the
Table 8: Lobster Hepatopancreas TORT-1
Individual Digestion Run Values
Digestion Run #
Element/
Mass
As 75
Cd 111
Cr 53
Cu
Ni
Pb
Zn
1
29.9
26.6
3.8
364
2.6
9.5
156
2
3
4
5
6
7
30.3
26.1
7.4
354
2.8
16.9
156
[pg/g]
32.9
27.0
3.6
338
2.4
10.5
159
31.6
27.5
3.7
351
2.5
9.8
157
30.4
25.5
3.4
334
2.5
16.9
152
30.6
25.8
4.6
334
2.9
9.0
148
31.0
26.6
4.2
344
2.8
13.3
151
127
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Tabic 9:
Element/
Mass
As 75
Cd 111
Cr53
Cu
Lobster Hepatopancreas TORT-1 Statitics
Accepted
Value
Average
Determined
[pg/g]
[pg/g]
Ni
Pb
24.6±2.2
21.1±0.1
2.8±0.3
367±1
2.75±0.26
9.7±1.4
Zn
145*1
31.0
26.5
4.4
346
2.6
12.3
154
RSD
Recovery
0/
/O
%
3.3
2.6
32
3.3
7.0
28
2.6
126
126
150
94
102
137
104
m aterial selected may have come from a different lot th an th a t actually used for the
evaluation study. Previous work done on th is m aterial by the sam e au th o r yielded a
result of 23 pg/g and originally certified TORT-1 m aterial has a value of 26.1 pg/g
for Cd (NRCC certificate). The Crv results a re high for both RSD and recovery. The
CrMhas a n A rC '1m olecular ion interference th a t is relatively intense: coupled w ith
the low level of Cr, th is results in a high RSD. The recovery is high for th e sam e
reason. Nickel, Cu, and Zn all have excellent RSD’s and recoveries. The dual level
Pb results in Table S are either a result of contam ination or th e heterogeneous
character of th e sam ple itself. At this tim e not enough inform ation is available to
m ake a decision72.
The TORT-1 sam ple is organic and generated decomposition products during th e
digestion w hich required th e system to be cooled and vented th ree tim es. The
recovered volum es removed from th e digestion tube a re all w ithin 3% o f initial
volumes added. The venting did not result in a significant loss of analyte or
reagent. This is im portant because it m eans th a t it is not necessary to calculate th e
recovered volume. This way it is possible to sim ply remove th e digestate from th e
digestion tube, dilute an d analyze. The CEM system required th e addition o f an
in tern al sta n d ard to th e sam ple slu rry in p u t a n d also dilution to a known volume on
exit from th e digestion tube, although th e la tte r is performed autom atically.
128
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7 .7 Bovine Liver Digestion and Analysis
Bovine liver is a sam ple type th a t is difficult to make into a slurry. As a result, this
sam ple type is not easily digested in a narrow tube digestion system th a t requires
th e sam ple to be pumped in as a slurry. Liver is organic, requiring a venting
sequence to complete the digestion. When bovine liver is digested in a traditional
microwave bomb, a two step digestion is needed to remove decomposition products
th a t could form during the final digestion.
Table 10: Bovine Liver 1577 Digestion and Analysis
Element/
Mass
%RSD
1
2
3
[pg/g]
Cd 111
Cr 52
Co 59
Cu 53
Pb 208
Mn 55
Mo 95
Zn 66
Fe
Na
K
Ca
P
Mg
0.3
5.4
0.3
22.3
0.3
3.7
0.4
0.2
0.2
187
190
188
0.4
0.4
0.3
17
15
15
4.9
3.6
3.6
134
137
135
264
336
255
2345 2261 2314
10037 10175 9364
109
113
118
9801 10067 10141
639
563
600
Average
Certified
Determined * Value
Recovery
[pg/g]
[pg/g]
%
6.1
98
0.3
4.6
104
520
42
0.8
7.4
9.7
19
1.1
16
1.9
4.4
4.0
1.8
6.3
0.2
188
0.37
15
3.6
135
260
2307
9859
113
10003
601
0.3 ±0.04
0.088
±0.012
(0.2)
193 ±10
0.34 ±0.08
10.3 ±1
(3.4)
130 ±13
268 ±8
2430 ±130
9700 ±600
124 ±6
(11000)
604 ±9
100
97
108
146
106
104
97
95
102
91
91
99
* Average with digestions 1&3 only, %RSD with 1,2,3.
T able 10 contains th e analysis sum m ary of 0.5 g bovine liver digestions. A
prelim in a ry look a t th e concentrations obtained reveals th a t C r and Fe in th e second
digestion a re relatively high, and a closer look shows th a t Mo, Co and Mn a re also
m a rg in a lly higher. All of these elem ents a re associated w ith stainless steel CSS).
A dding u p all th e differences in concentration betw een digestion 2, and digestions
129
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1&3, gives a total of 107 pg/g. If this is normalized to 100, the percent concentration
for each element is Fe (71%), Cr (16.8%), Ni (8.7%), Mn (1.9%), Mo (1.3%), V (0.2%)
and Co (0.2%). This agrees favorably with a 316SS th a t has Fe (66%-74%), Cr (16%18%), Ni (10%-14%), and Mo (2%.-3%). This is an excellent m atch considering an
estim ated 0.5 mg 316 SS contamination. The bovine liver was dissolved in capsules
th a t were fabricated using a pair of vise grips to hold the pins while removing the
capsules. The contam ination was evident in the blank capsules (Table 11) ru n with
th e Bovine Liver samples. The steel grips were replaced w ith a plastic pin holder to
m ake the last batch of capsules and elim inate this contamination.
The C r w as determ ined a t m ass 52. The very high C content of the digestates
m akes m ass 53 a b etter choice for C r determ ination because th e relative abundance
Table JI: Capsule Made Using Stainless Steel Grips
Blank Capsule #
Mass
1
2
3
4
[Mg/g]
0.7
0.1
2.9
13.4
1.2
[pg/g]
1.1
0.6
9.3
21.2
4.3
[Mg/g]
1.5
0.7
12.6
28.4
5.6
[pg/g]
0.2
<0.01
0.6
3.0
0.2
/Element
Mn 55
Mo 95
Cr 53
Fe
Ni
of C13(yielding ArC13, 40+13=53) is 100 tim es less th a n C 5‘ (yielding A rC12,
40+12=52). The combination of low C r in th e sam ple an d high m olecular ion
interference yields completely erroneous values even w hen m ass 53 is used. T he M n
a t 15 pg/g is 50% higher th a n it should be. O ther researchers73,74 obtained th e
certified valve of 10.3 ±1. The only resonable explanation, for th e high Mn, is a
130
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co n ta m in a tio n o f th e stand ard reference m aterial. All oth er e lem en t d eterm in a tion s
fall w ith in th e certified v a lu e range.
7.8 Orchard Leaves Digestion and Analysis
The O rchard Leaves 1571 reference m aterial was chosen because it does not totally
dissolve. The Si m atrix of these leaves contains Fe which is not released into
solution unless HF acid is used. The results of the analysis in Table 12 show th at
Table 12: Orchard Leaves 1571 Digestion and Analysis
Element/
Mass
/oRSD
1
2
3
[pg/g]
K
Na 23
Mg
P 31
Ca
Cr 53
Mn 55
Fe 56
Ni 60
Cu 63
Zn 66
As 75
Mo 95
Cd 111
Sb 121
Pb 208
13533
174
5477
2396
22338
5.2
85
217
1.6
13
35
11
0.3
0.26
2.7
46
13140
68
5390
2362
22112
4.5
84
218
1.6
12
28
11
0.3
0.13
2.8
44
13107
69
5336
2355
26416
4.5
83
224
1.7
12
25
11
0.3
0.12
2.6
44
1.8
59
1.3
0.9
10
8.5
1.0
1.8
2.5
2.5
18
2.4
0.9
45
34
2.5
Average
Certified
Determined Value
Recovery
[pg/g]
[pg/g]
%
13260
104
5401
2371
23622
4.8
84
219
1.6
12
29
11
0.3
0.17
2.7
44
14700±300
82±6
6200±200
2100±100
20900±300
2.6±0.3
91 ±4
300±20
1.3±0.2
12±1
25±3
10±2
0.3±0.1
0.11±0.01
2.9±0.3
45±3
90
127
87
113
113
185
92
73
123
100
116
110
100
114
93
98
not only Fe is low b u t also Mn and Mg a t approxim ately 90% of the certified value.
O th er w orkers74 using only H N 0 3 for digestion of Orchard Leaves 1571 also obtained
low resu lts for Mg and Mn b u t gave no specific reason. Visual observation of the
digestates ag ain st a light revealed th in strands th a t settled a fte r 24 hours.
131
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U n lik e th e B ovine L iver 1577, th ere is very little Cl p resen t in th e Orchard L eaves
1571 g iv in g a ccu ra te v a lu es for A s. A rsen ic is an im p ortan t elem en t b eca u se it can
v o la tilize and be lo st d u rin g v en tin g . T he v a lu e o f 11 pg/g for a ll th ree d ig estio n s
d em o n stra tes th a t A s rem a in s in solu tion a fter v en tin g .
7.9 Soil SO-2 Digestion and Analysis
One of the main concerns stated earlier was the damage caused to the inlet and
outlet valves by abrasive samples. SO-2 soil is an abrasive m aterial th a t does not
dissolve completely in nitric acid. Samples of slurried soil pumped into a digestion
tube will come in contact with the inlet and outlet valves. The use of a capsule
elim inates contact w ith the inlet valve but the digestate containing undigested
abrasive m aterial m ust still pass through the outlet valve. The squeegee
successfully removes th e digestate, because th e recovered solution w eight is th e sum
of th e acid and soil added. The removal of the soil sam ple is followed by a rinse, and
second pass of th e squeegee, to remove all abrasive m aterial from the digestion tube
and valves.
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The recoveries (Table 13) are low for most elements as expected,because HNO, does
not totally dissolve SO-2". This is the case for soils in general ' which have a
silico-alumino m atrix th a t rem ains after the digestion. The values of Or are high
because of the molecular ion interference explained earlier.
Table 13: SO-2 Digestion and Analysis
Element/
Mass
Average
Determined
%RSD
Recommended Recovery
Value
[pg/g]
%
1010
19000
249900
1.2
0.18
16
9
7
21
5400
720
2
8
8600
64
124
3000
55600
19600
24500
80700
16
1.6
0.1
43
83
2563
32
70
33
76
77
86
86
3.1
40
45
80
70
22
4
31
fog/g]
Ba 138
Na 23
Si 29
Be 9
Cd 11
Cr 53
Co 59
Cu 63
Pb 208
Mg
Mn
Mo 95
Ni 60
Ti
V 51
Zn
P
Fe
Ca
K
Al
157
306
285
1
0
410
3
5
7
4098
553
2
7
270
25
56
2377
39119
4354
891
25305
9.3
8.0
9.2
9.2
22
5.0
5.5
4.6
0.3
6.9
3.9
25
43
14
6.3
3.8
9.9
11
1.3
16
0.7
7.10 Marine Sediment MESS-1 Digestion and Analysis
MESS-1 is soil w ith a botanical component. This type of sample is not only abrasive
b u t also generates decomposition products when digested. The digestion required a
133
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venting sequence to complete the digestion. The recoveries are low for most
elements. Those th a t are close to 100% recovery are probably from the botanical
fraction of the sediment. The Pb and Zn show full recovery and fall w ithin the
certified value range. The Cr for this sample is not higher than the certified value
because it is a relatively high concentration in the sample a t 71pg/g, compared to
0.088 pg/g in bovine liver. A comparison of the Cr values for SO-2 and MESS-1
shows how crucial a correct selection of m ass num ber for analysis can be. SO-2 Cr
was determined a t m ass 52, and MESS-1 a t 53, mass 53 is a much b etter choice.
Table 14: MESS-1 Digestion and Analysis
Element/
Mass
%RSD
1
2
3
lpg/g]
B ed
Na
Mg
Al
Si
P 31
Ca
Ti 49
V 51
Cr 53
Mn
Fe
Ni 60
Co 59
Cu 63
Zn 66
As 75
Se78
Cd 111
Sb 121
Pb 208
K
0.9
5741
4932
13009
388
665
2578
173
32
32
292
25051
24.9
10.5
24.1
232
9.8
0.4
0.7
<0.1
31.2
2567
0.9
5885
4370
13296
291
638
2359
166
30
29
301
22021
24.5
10.1
22.5
205
9.6
0.3
0.7
<0.1
30.1
2538
0.9
5630
5104
13155
515
655
2686
217
33
29
301
26023
24.9
10.7
23.6
175
10.1
0.1
0.8
<0.1
32.7
2612
1.1
1.8
8.5
1.5
20
3.0
6.3
2.7
4.0
7.1
2.1
9.1
1.2
2.4
4.8
8.6
1.5
6.1
5.0
2.6
0.8
Average
Determined
Certified
Values
Recovery
Ipg/g]
[pg/g]
%
0.9
5752
4802
13153
398
653
2541
185
32
30
298
24365
24.8
10.4
23.4
204
9.9
0.3
0.7
0.0
31.3
2572
1.9±0.2
18547±1113
8685±543
57825±1992
315522±8881
637±61
4817±457
5425±168
72.4±17
71±11
513±25
30495±1749
29.5±2.7
10.8±1.9
25.1±3.8
191±17
10.6±1.2
0.34±0.06
0.59±0.10
0.73±0.08
34.0±6.1
18596±332
47
31
55
23
0.1
100
53
3.4
44
42
58
80
84
96
93
107
93
88
119
<10
92
14
134
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7.11 Conclusions - Chapter7:
The digestions obtained for all of the sample types digested resemble those obtained
w ith th e traditional microwave bombs. The difference is th at, if a narrow digestion
tube had been used, coarse sam ples would require extra grinding, and organic
sam ples would require cooling of the bombs before opening to vent gases. Memory
effects w ere negligible.
135
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8. Conclusion
It was concluded early in this work th a t the tube system could be more efficiently
autom ated th an the traditional microwave bomb vessel.
The flange valve design proved to be the simplest, safest and m ost reliable valve for
the digestion tube. All parts th a t came into contact with sam ple could easily be
cleaned, elim inating any memory effects coming from the valve. The valve does not
abrade because no parts rub across each other, they are sim ply pressed together.
A large diam eter digestion tube design was found to be the best choice for several
reasons. F irst, a large diam eter tube in the shape of a U and inclined w as found to
be th e best configuration for venting decomposition gases.
Secondly, th e large internal diam eter allowed sam ples to be transfered directly into
the tube. This avoided having to form a slurry with the sam ple and pum p it in.
Sample transfer, in th e form of a capsule, guaranteed 100% sam ple tran sfer into th e
digestion vessel. Sam ples th a t have a large particle size can be added to the
digestion tu be w ithout fu rth er grinding. Sample types th a t clog valves or cannot
form a slu rry can be added directly.
The th ird m ain advantage of th e large internal diam eter tube is th a t digestate can
be pushed out w ith a squeegee. Sample transfer out w ith th e squeegee was always
97% or better. This elim inated the need for an internal stan d ard to account for loss
of sam ple volume.
A nother advantage of th e large tube design w as th a t it could be cooled rapidly. This
is crucial w hen venting is required. The traditional microwave digestion bombs are
bulky and tak e alm ost ten tim es longer to cool.
The m easurem ent of tem perature using shielded therm ocouples a s abandoned and
replaced by a n infrared therm opile. The non-contact m easurem ent of tem perature
136
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avoided th e arcing and self-heating when a thermocouple was attached to the
digestion tube.
Pressure m easurem ents taken with an in-line pressure sensor allowed pressure
m easurem ents, venting and loading of reagent from the same line leading form the
digestion tube. This design eliminated memory effects from the pressure line.
The ability to vent the system under software control allowed digestions to progress
w ithout over-pressurizing the system or loss of sample. It was found th a t no analyte
(volatile or in solution) was lost during venting. Cooling of the digestion tube to
prevent loss of analyte also prevented loss of reagent.
The squeegee w as also used to remove wash solution from the digestion tube. Two
w ashings w ere sufficient to remove any memory from the digestion tube.
M ICR02, a n interpretive computer language, was developed for control of the
digestion. The control language had the flexibility to change its mode of operation
to account for changing conditions in the digestion; such as over-pressurization of
th e digestion tube.
The digestion of biological, botanical, soil, and sedim ent sam ples, using a 5 m inute
digestion a t 180°C w ith 10 ml cone HNO.„ showed th a t th is digestion system
achieves th e sam e type of digestion as the traditional microwave bomb digestion
system s. The autom ated heating, cooling and venting allowed digestion of all these
sam ple types w ithout any operator intervention. This system design can therefore
easily be autom ated.
137
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9. Future Work and Limitations
9.1 Future Work
The polymer digestion tube loses strength as tem perature increases, which is a
problem because of the solvent vapor pressure. A tem perature of 2S0"C for nitric acid
alone will give a pressure greater than 1400 psi. This can be overcome by encasing
the digestion tube in a metal tube, allowing the digestion tube to reach tem peratures
close to its melting point without bursting. Microwave would be fed in from th e ends
of the metal casings.
A multi-tube design would increase the sample throughput. A single m agnetron with
waveguides th a t feed individual tubes could be used. The waveguides would be
designed so th a t an increase or decrease in load in one tube would not affect the power
level in another tube.
The cooling of the digestion tube m ust be made more efficient to reduce th e cooling
tim es from m inutes to seconds. A set of cooling fins on the tube cooled by a ir flow
would improve the cooling without having to w aste water.
The squeegee is pushed through th e digestion tube w ith a long rod th a t is exposed to
th e atmosphere. The rod should be enclosed to elim inate contam ination from the
atmosphere. A collapsible rod (like a car antenna) would reduce the space required to
house the rod. The rod would have a rinsing unit built in to clean after each use.
The system is capable of taking samples to dryness to remove solvent. The classic use
of sulfuric arid to remove lower boiling point solvents could be done using th e venting
system. However sulfuric arid itself could not be evaporated w ithout dam aging the
digestion tube; sulfuric arid boils a t 330°C, while PFA m elts a t 304°C
The sensors can be built directly into the digestion tube to improve accuracy and
response.
The tem perature m easured using the ER/TC is th e outside tube tem perature.
M easuring the tem perature using a param eter specific to th e solvent would give a
more accurate tem perature of the contents of th e tube. The linew idth of emission
138
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lin e s from w a ter or nitric acid is one such param eter.
Li now idth increases w ith
tem p eratu re and calibration w ould be independent oi'the physical s i tup.
T he m ateria l PTFE-TFM'* w ould be a good replacem ent for th e PFA. T his m aterial
h a s a m eltin g point above 3 4 0 ”C. essen tia l for th e com plete decom position o f organic
m a tter th a t requires a tem p eratu re o f 320"C.
A dapters would be placed automatically using a m ulti-adapter turret.
Cleaning,
capsule loading, reagent addition and venting adapters can all be placed on a single
tu rret. A robot mounted on a rail would sendee several turrets. The flange valve can
be held closed using pressure or a 1/4 turn block breach lused for artillery gunsh A
capsule feeder th a t contained standards, blanks, samples, and other types would bo
fed autom atically to the adapter turret.
Capsules would be manufactured in a
completely controlled and clean atmosphere. Humidity control is the most im portant
param eter.
The system has so far only been tested with nitric acid. As this study showed, nitric
acid digestion in this system w as no different than the traditional microwave bomb
digestion. O ther acid digestions need to be verified. It is possible to do multi-step
digestion using several different reagents to do the chemistries of "old".
9.2 System Limitations
The system has pressure and tem perature limits th a t do not allow complete digestion
of organic samples. W hatever lim it the acid digestion has is also a lim itation of the
system. The m ain deficiency of the system is the single tube arrangem ent. A less
im portant lim itation is the tim e needed for cooling. It m ust be reduced from m inutes
to seconds. The capsule m aterial is not decomposed a t the tem peratures used in this
work. A capsule m aterial th a t would easily digest and could still be m ade into a
capsule and rem ain d e a n would be a n asset.
139
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10. Bibliography
I.
Fresenius' J. Anal. Client. (I 9 9 4 j. v o l.
3 4 9 , pp. 4 2 8 -3 3
11. M a iu sic w ic z and R. E. S tu rg eo n . 19 9 4 .
C o m p arison o f the e ffic ie n c ie s o f o n lin e and high-pressure c lo se d v e sse l approach es to
m ic r o w a v e heated sa m p le d e c o m p o sitio n .
2.
J. Anal. At. Spectrom. v o l. 6 , pp. 2X 3-287.
H. M a iu s ic w ic z . 19 9 1.
V ap o r-p h a se acid d ig estio n o f in organ ic and organic m atrixes for trace elem en t a n a ly sis u sin g a
m ic r o w a v e heated bom b.
3.
Analyst (Cambridge. U. K.
J, v o l. 1 19. pp. 1 0 17 - 10 2 1.
D . A m arasiriw aru den a. A . K ru shcvska. M . A rg en tin e, and R . M . B arn es, 1994.
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4.
Trace Sttbst. Environ. Health, vol. 9. pp. 297-301.
A. Abu-Samra. J. S. Morris, and S. R. Koirtvohann. 1975.
Wet ashing of some biological samples in a microwave oven.
5.
Spectra 2000 [Deux MilleJ. vol. 146. pp. 44-50.
D. Didenot. 1990.
Wet digestion method using a microwave source.
6.
Patent France # 2560529 AI 6 Sep 1985
R. Commarmot. D. Didenot. and J. F. Gardais
Apparatus for wet chemical reaction on samples for analysis.
7.
Patent France # 2560686 A1 6 Sep 1985
R. Commarmot. D. Didenot. and J. F. Gardais
Oven for automatic digestion of samples in individual containers.
8.
Spectra 2000 [Deux MilleJ. vol. 146. pp. 44-50.
D. Didenot. 1990.
Wet digestion method use.
9.
Am. Lab. (Fairfield, Conn.). vol. 23. pp. 40@42-40@45.
B. D. Zchrand M. A. Fedorchak, 1991.
Microwave acid digestion of inorganics using a bomb vessel.
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AnaL Chim. Acta, vol. 226. pp. 1-16.
M. Wuerfels. E. Jackwerth. and M. Stoeppler. 1989.
Residues from biological materials after pressure decomposition with nitric acid. Part I. Carbon
conversion during sample decomposition.
140
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11.
J. Anal. A!. Spectrom. vol. 7. pp. S45-S50.
A. Krushevska. R. M. Barnes. C. J. Amarasiriwaradena. H. Loner. and L. Martinos, ll>l>2.
Determination of the Residual Carbon Content by Inductively Coupled Plasma Emission
Spectrometry after Decomposition of Biological Samples.
12. J. Anal. Ai. Spec:rani. vol. R. E. Sturgeon. S. N. Willie. B. A. ■lelhven, and W. U. Lam.
1995.Continuous Flow Microwave Assisted Digestion of Environnuvial Samples.
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H. M. Kingston. L. B. Jassie, and Editors.
Introduction to Microwave Sample Preparation: Theory ••>ul Practice, Washington. DC: ACS.
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14.
Prog. Anal. Spectrosc. vol. 12, pp. 21-39.
H. Matusicwicz and R. E. Sturgeon. 1989.Present status of microwave sample dissolution and
decomposition for elemental analysis.
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4 0 , pp. 1 9 2 7 -1 9 3 6 .
T. Gut) and J. B aasner, 1993.
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v o l. 119, pp. 1 0 0 3 -1 0
L. J. M . S tew art and R. M . B arn es. 1994.
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D o cto ra l T h e sis
L. B. Ja ssie.
19S9.
O rder N o . D A 9 0 14653
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The composition of the residue of biological materials after pressure digestion with nitric acid.
147
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11. APPENDICES
11.1 APPENDIX A: IR/TC Calculations
11.1.1 Blackbody Radiation: Thermopiles
To understand the response characteristics of infrared detectors, it is instructive to
understand the spectral layout of radiation of objects near room tem perature. The
wavelength distribution of the radiant energy (Spectral R adiant Exitance -M J of
objects from 20-200°C can be described using Planck's function (Equation 1), plotted
in Figure 47 for the three tem peratures 20,100, and 200°C having maxim a between
6 and 10 micrometers. The three curves have peak intensities th a t shift to lower
Equation I: P lanck's Function
^
> watts/cm* micrometers
c, = 2jre'ti
c, = — = l.43SScmK
'
k
c = velocity of light 2.99793 x 10" cm / sec
h = Planck's constant 6.6256 x I0'u watt see’
k = Boltzmann constant 1.38054 x 10'1' watt sec / K
X. = wavelength in centimeters
wavelengths, and increase in a rea tinder th e peak w ith increasing tem perature. I f
Planck’s function is integrated from 0 to co, th e total ra d ia n t energy is a function of
T4; th is is called th e Stefan-Boltzmann law. Therefore, th e intensity of th e radiation
incident on a detector coming from objects (20-200°C) will be a function of T \ To
obtain th e theoretical response curve of a detector as a function of tem perature,
Planck’s function is integrated over th e spectral window of th e detector for each
148
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
tem p era tu re m easu red . In te g ra tin g Planck's function for a fin ite w a v elen g th
in terv a l (E q u ation 2-1) can n ot be carried out in a closed m a th em a tica l form'’".
T h e in teg ra l o f P lanck's fu n ction for d ifferen t tem p era tu res and w a v elen g th
in te r v a ls a re a v a ila b le in ta b u la r form in a book com piled by C zerny e t al ’
Th e
ta b le o f in te g r a ls w a s com piled in d ep en d en t o f P lanck's co n sta n t h sin ce, a t th e tim e
(19 6 1 ), th is co n sta n t w a s n ot firm ly esta b lish ed . To accom p lish th is. C zerny defined
th e d im e n sio n le ss v a ria b le v (E q u atio n 2-2), and collected a ll co n sta n ts in cr
(E q u ation 2-3) to d erive th e eq u a tio n E q u ation 2-4 ’. E q u ation 2-4 is th e S tefa n B o ltz m a n n la w m u ltip lied by B(v). B (v) is a d im en sio n less fraction (0..1) o f th e to ta l
in te g r a l o f P lan ck 's fu n ction for th e w a v elen g th in terval 0-X for T. V a lu es o f B(v)
are ob ta in ed b y fir st ca lcu la tin g th e v a lu e o f v, for \ and T (E quation 2-2), an d th e n
fin d in g th e a sso c ia ted v a lu e in th e tab le. To ob tain th e in teg ra l b etw een tw o
700
Lower D e te c to r
Limit 6 urn
High D etector
L lm ltl4um
100
0
m ic ro m eters
Figure 47: P lanck's Blackbody Radiation Curves
149
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
v a lu es o f th e in tegral for th e sh o rter w a v elen g th
w a v e le n g th s
su b tracted from th e in teg ra l v a lu e o f B (v J a t th e longer w a v e le n g th
B(v,) are
(E q uation 2-
5). U sin g th e s e com piled resu lts, th e th eoretical power d e n sity for a sp ectral
w in d ow 6 to 14 m icrom eters in cid en t on th e IR/TC w a s p lotted in F igu re 4 8 , u sin g
E q u ation 2-5
11.1.2 Choice of Infrared Thermopile
T h e d etector w id ely u sed for th is tem p era tu re ran ge is a th erm o p ile w h ich co n sists
o f a n etw ork o f th erm ocou p les con n ected in series. M easu red ra d ia tio n is in cid en t
on th e h ot ju n c tio n s, and th e cold ju n ctio n s are a tta ch ed to a referen ce tem p era tu re
source. T h e th erm o p ile h a s m a n y a d v a n ta g es over a p h otocon d u ctive in frared
d etector for m e a su r in g tem p era tu res n ea r am b ien t. T h ey a re in e x p e n siv e , s e n sitiv e ,
self-p ow ered , a n d h a v e no m o v in g p arts.
Equation 2: P lanck's Integral
o (x, r) - 2c, T - V * - 7 W ) *
X|H1£ 'At
(1 )
XT
v = ---- with 0 < v < «>
(2 )
2
c C\
—r
15
ci
(3 )
a =—
=
J E{X.T)dX = —T4B{v)
k\=o
(4 )
*
jE(Wya.-=— r 4[B(is)-B(r,)]
Xi»I
ft
(5 )
150
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
T h e IR/TC d raw n in F igu re 49 is a reconstruction o f th e particu lar d ev ice chosen
u sin g d e ta ils su p p lied in a d v ertisin g litera tu re, th e in stru ction m an u al for th e
device, co n v ersa tio n s w ith O m ega en g in eers, and patent. U S # 5.229,612'"". T h is
IR/TC (F ig u re 4 9 ) h a s a silicon w indow w ith a v ie w in g a n g le o f 90° an d resp o n se
from 6.5 to 14 m icron s, m ak in g it id eal for tem p era tu re m ea su rem en ts ra n g in g from
250
<-> 200
150
®
100
50
0
0
0.05
0.1
0.15
0.2 0.25
0.3 0.35
0.4
In c id e n t P ow er (w a tts /c m ')
Figure 48: Integration o f Blackbody Curve
through 6-14 jjm Window
room tem p eratu re to well over 200°C. It has a 1% precision over th e entire working
range, an d a lin ear response (±2%) between 95°C and 135°C. This IR/TC is
inexpensive, sensitive, rapid, and has a linear output over the specified range of use.
The plot of th e integral of Planck’s function in Figure 48 shows th a t th e incident
radiative power coming from an object as a function of tem perature is non-linear.
T he purpose of m ost of th e design features was to linearise th e output.
151
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
The therm opile window is elemental silicon, which has a spectral bandpass of 7-20
pm. The silicon window removes all wavelengths below 6.5 pm. The m anufacturer
claims th a t this improves the linearity of the output, and, as one can see in Figure
48, this still allows most of the radiation to reach the thermopile. The m anufacturer
also claims an upper wavelength limit of 14 pm, but does not state how the
wavelength limit is reduced to 14 pm from the silicon upper wavelength lim it of 20
pm.
The tem perature of the thermopile assembly can also affect the output levels. The
signal is generated by hot and cold thermocouples connected in series. The cold
SC P‘*tfaofV*owL.in»>\
tt'ccto
in n m D
D oo M
to rr Body
Body
DotoOotBocy
$*J»ccr» W indow
SoparohnflO-nncs
Incident Radiation
Cold Junction Notwork
lR/fCS*Qnol load*
Thormlstomotwork
‘HOT J u n c ti o n NotwOfk
CottitnannQ Mofaf f«ns-
T-Obcon ThormoO'l*
Tnormoodo Hoot Sink
Figure 49: Thermopile Schematic
junctions a re attached to th e body of the device, and, i f th e body changes
tem perature, so does th e signal, even though th e ra d ia n t energy input m ay no t have
changed. T herm isters connected in series w ith th e therm opile are placed on th e
body of th e device to correct for changes in signal caused by changes in tem peratu re
of th e body. The therm isters w ith negative and positive tem perature coefficients are
combined to com pensate for a n increase or decrease in tem perature. The
therm ocouple is also attached in series to a passive resistive netw ork to correct for
152
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
non-linearity of the device, and adjust the signal level. Thermopiles are notorious
for th eir variability, therefore, laser trimmed resistors are used to adjust the signal
level during m anufacture to insure consistency from device to device.
The linearity obtained w ith the combination of resistors and therm isters is effective
over a sm all tem perature range, typically 40-70°C. The particular device chosen for
evaluation has a linear output between 95°C and 135°C, which was the closest
m atch to our experim ental needs out of the 6 tem perature ranges available from the
m anufacturer. The m anufacturer states th a t the only change made for the 6 ranges
w as a change in the resistive network.
The double-barrel arrangem ent used to encas*' the T 05 can therm opile allows for
240
(J
200
S:
160
■i i
40
millivolts IR/TC
Figure 50: IR/TC Response Viewing Graphite
rap id response while, a t th e sam e tim e, reducing th e effects of am bient tem peratu re
changes. The cold junctions o f th e thermopile a re connected through the body of th e
T 0 5 can to th e h e a t sink. H eat absorbed by th e hot junction is transferred to th e
cold junction, an d on to th e h e a t sink. Effective h e at tran sfe r away from th e hot
junctions prevents h e a t buildup a t th e hot junctions, an d allows th e hot junctions to
change rapidly in tem perature w ith a change in incident power.
153
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T h e rela tiv ely in ex p en siv e infrared th erm op ile (IR/TC) ch osen for ev a lu a tio n w a s
m odel n u m b er O S36-E -240-G M P, m ade by O m ega. T h e m a n u fa ctu rer su p p lied th e
p a ten t n u m b er (U S # 5 ,2 2 9 ,6 1 2 ) th a t th e device w as b ased on, and sta ted th a t a
re sistiv e n etw ork w a s used to lin ea rize th e output. H ow ever, th e ex a ct com p on en ts,
layou t, a n d d ra w in g s for th e IR/TC w ere not su p p lied .
T h e IR/TC relies on a s e t o f co llim a tin g fin s to reduce th e field o f v ie w to 90°, a n d
d oes n ot req u ire a focu sin g len s.
I I . 1.3 Characterization of IR/TC
The IR/TC w as characterized using a heated alum inum block, heated on a hot plate,
and covered w ith graphite powder. The graphite powder is black, and gives the
alum inum surface a n emissivity of 1 for the range of tem peratures studied. The
window of th e IR/TC was placed 1/4” above the alum inum surface, next to an J-type
TC embedded in the m etal surface. The J-type TC w as connected to a digital
readout, tem perature readings were tabulated m anually, and th e IR/TC millivolt
output w as read with the microwave d a ta acquisition hardw are under Micro2.
Readings w ere acquired for the IR/TC and the J-type TC as the tem perature of the
m etal w as raised to 220°C from 30°C. The millivolt readings of th e IR/TC were
plotted versus the tem perature of th e alum inum block. The resulting curve
consisted of three straight line sections th a t were best approxim ated w ith a
quadratic equation. The quadratic calibration gave tem peratures th a t were w ithin
one degree centigrade over the whole range 0-200°C, which w as found to be
adequate for this work.
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11.2 APPENDIX B: Flange Valve and Digestion Tube Flanging
The entire digestion system is centered around the Hanged valve and digestion tube.
The exploded view in Figure 51 identities the different components that make up
the flange valve. When a component-label has a DWG number, an engineering
draw ing for the component can be found in Appendix G. The assembled flange valve
in Figure 51 is fitted with a pressure adapter. The pressure adapter is attached to
the pressure/vent tube with the 1/S" flange nut. The pressure/vent line is used to
m easure pressure, to vent, and to add reagents to the digestion tube. The digestion
tube is held in the flange valve housing by the PFA tube holder, and the pressure
ad ap ter is pushed against the digestion tube with the flange valve nut. The pin in
th e PFA tu be holder is provided to prevent rotation of the digestion tube when the
valve is closed. I f th e valve is pressurized on the digestion tube side, opening the
Flange Valve Nut
DWG. A'1001
Flange Valve Housing
DWG. B-1001
Pressure Adapter
DWG. 1006
Pressure/Vent Tube
PFA Tube Holder
DWG. A-1011
\
\*
PFA Diqestlon Tube
DWG.A-1014
I
1/8
I
1/8" ID Range Nut
DWG. A-l 002
BackWash O-Ring
ffTl--
Swagelok® Wash Tube Fitting
Fully Assembled
Ready for Digestion
Figure 51: Exploded View o f Flange Valve with Pressure Adapter
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valve could sen d g a s e s back to th e operator. To avoid th is, a h ole is placed a t th e
ju n ction o f th e p ressu re ad ap ter and d igestion tu b e, allo w in g g a se s e x itin g to flow
down through th e w a sh tu b e fittin g rather th a n through th e back o f th e v a lv e to th e
operator. T he b ack w ash O -ring is added a s ex tra protection.
The flange valve and tube components are fabricated in-house. The steel and PFA
components th at m ake up the valve are made on a lathe, but not the digestion tube
with the flange nuts attached. The flanged digestion tube could have been made
from a PFA tube 1” OD x 3/S” ID, machined down leave flanged ends: the PFA tube
holders would have been split in two and attached as halves. However, PFA tubing
th a t size is not available commercially. If 1” rod stock had been used to m ake the
digestion tube, the 3/S” ID hole drilled through the center would not have been
smooth enough. This left flanging a straight piece of tubing using conventional
means as the next alternative. The standard tube flanging technique for tubing less
th an 1/S” OD uses a heated steel mold pushed up against the end of th e tubing. The
heated mold has a flat surface w ith a pin protruding from the middle; the pin guides
the flat surface onto the end of the tube. The heated PFA tube end m elts and flows
outward, forming a flange. The flange th a t is formed is much th in n e r th a n the tube
wall and, as a result, has a lower bu rst pressure a t the joint.
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Rem ovable
Back Panel
DWG. B-1003
M icro w a v e C avity
Vent/Pressure Tube
Flange Valve Assembly
Waste Tulb e
//
//
"Q s?
*
Digestion Tube
Double Flange
Valve Mount
DWG. B-1002
Figure 52: Flange Valve and Digestion Tube Mounted on Back
o f Microwave Oven
In a effort to increase th e flange thickness, a heated flanged-shaped mold w as used
to form th e flange. The tube end melted as it was pushed into a heated mold. The
PFA m elted into th e mold, b u t did not flow to fill the mold because of its high
viscosity. Increasing th e tem perature formed bubbles in th e PFA, causing
decomposition of th e PFA open to the air, and serious deformation of th e PFA tube.
W hen th e mold w as cooled to remove th e flange, the PFA flange stuck to th e mold.
The mold w as fabricated from copper, stainless steel, and graphite; none of these
released th e PFA easily. It w as discovered m uch la te r th a t silicon grease wiped onto
th e surface w as very effective a s a releasing agent.
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Photo 8:
Flanging Setup
Hot Air Gun.
Flange Mold (DWG. B-1022).
Digestion Tube Holder (DWG. A -1011).
4
Digital Thermocouple Readout.
5
Adapter for Inert Gas Use.
6
Fully Assembled Digestion Tube.
7
Thermocouple.
The front surface mold w as replaced w ith a mold m ade from two pieces, form ing a
cavity th a t would fill w ith m elted PFA to shape th e flange. T he cavity mold w as not
successful, since bubbles still formed.
To overcome th e problems of flanging w ith a mold, it w as instead decided to weld a
piece o f PFA in th e form of a flange (called a “D onut”, 1/2” ID x 0.90” OD x 0.200”
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th ick ) onto each end o f a PF A tu b e 11/2" OD x 3 /8 “ ID x 60 cm ). H e a tin g th e tu bing
w ith th e d on u t a tta ch ed to th e end o f th e tu b e did not w eld th e tw o togeth er. E ven if
both th e tu b in g and donut had m elted , th e PF A w a s too v isco u s, and th e tw o m elted
su rfa ces did n o t m ix to g eth er and bond. I f th e tem p eratu re w a s in creased , bubbles
sta rted to form , follow ed by decom position.
M ost p olym ers a re w eld ed u sin g h ot air. T h e su rfaces are h eated w ith h ot air, and
bond w h e n p u sh ed to g eth er. T eflon PFA'*', a fluoropolym er. d oes not h ea t or cool
rap id ly, an d req u ires h ig h er w eld in g tem p era tu res th an oth er polym ers su ch a s
p olyp rop ylen e or polyvin yl chloride. T h e w eld in g tem p era tu re m u st be h igh en ough
for th e flu orop olym er to m elt, b u t n ot so h ig h a s to cau se d ecom p osition .
W hen th e tube and donut were heated in a ir and then pushed together to bond, the
surfaces did not bond. It was not possible to keep the PFA hot enough in free air for
bonding to tak e place. Increasing the hot a ir tem perature caused the PFA to
decompose and form a wax-like covering. Talking to the distributors of th e hot air
gun (Leister™ , Karl Leister, Switzerland) revealed th a t inert gas is used in the
w elding of fluoropolymers. However, using th e hot a ir gun w ith argon gas flowing
through th e gun did not improve th e bonding.
Close inspection of th e bonds formed revealed th at, if the two bonding surfaces had
slid onto each other to create a fresh surface, the two pieces would have bonded
properly.
The setu p in Photo 8 shows a mold used for welding the donut and PFA tube
together. T he arrangem ent holds th e two pieces rigidly in place as they are heated,
welded, a n d cooled. A stainless steel m andrel holds the PFA tube, the mold holds
th e gun, donut, and thermocouple. Photo 9 provides a closer look. A ir from the gun
flows th ro ugh th e middle of th e mold, through the center of the donut, over the end
of th e PFA tube, and out th e exhaust holes on the side of th e mold. The PFA holder
n u t is screwed into th e mold during th e welding process. A ir tem perature is
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controlled by th e gu n , and air flow is controlled u sin g a stop down ring th a t covers
th e ex h a u st h oles o f th e mold.
11.2.1 Flange Welding Instructions
To weld the donut and PFA tube together, a donut is first loaded into the mold, then
the PFA tube is held I/S’*above the donut with a mandrel. The PFA holder is
screwed into the mold ju st above the exhaust holes, and the gun is turned on. A
thermocouple inserted into the mold m easures the hot air tem perature next to the
inside of the donut. The mold is heated to the melting point of PFA. 306°C, and
m aintained for 3 m inutes. The PFA tube is then pushed into the donut. At room
tem perature, the donut fits tightly on the PFA tube. When heated to the m elting
point, PFA expands, and m aterial is pushed out of the joint, joining the tube and
donut. The fresh surfaces generated on the inside of the donut and the outside of
the tube form a bond through the full thickness of the donut. The process is
repeated for th e opposite end of the digestion tube, insuring th a t both PFA holders
are on the tube.
160
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Photo 9:
1.
2.
3.
4.
5.
6.
7.
Close-Up of Flanging Setup
PFA Tube Mounted on Stainless Steel Mandrel.
Air Exhaust Hole (one of 6).
Stop Down Ring (DWG. A -102 H.
Flange Mold (DWG. B-1022X
Hot Air Gun.
PFA "Donut".
Hot Air Passage Hole.
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11.3 APPENDIX C: Computer Control
11.3.1
MicroZ Instructions
F o rm a l:
- T h e instruction file c o n sis ts o f m ain com m an d tok en s, sub to k en s and param eters w h ich can
be separated by sp a c e s, carriage returns, lin e fee d s or co m m a s.
- A line m ust not e x c e e d 2 5 tokens.
- T h e file m ust not e x c e e d 2 5 5 tok en s.
- T h e file m ust in clu d e an E X IT m ain com m an d token su ch that the program can term inate
upon co m p letio n .
C om m en ts:
- C o m m en ts arc a llo w e d in the instruction tile.
- T h e y are d en o ted by q uotation m arks and p laced at the end o f a line.
- W hen the quotation m arks are en co u n tered , the rest o f the lin e is e x p e c te d to be a co m m en t
and th erefore is ign ored .
- C o m m e n ts a ls o lim it file siz e .
E xam ple:
R I ON
R 2 ON " This turns relay 2 on
EXIT
S p e llin g :
- A misspelled main command token will be treated as a label.
- A misspelled sub token or erroneous data could result in program failure or incorrect
program operation.
Windows: (during execution)
• The screen is divided into three windows.
- The top window contains the headers for data of the output file.
- The middle window shows the data of the output file into which the data collected will be
stored and saved.
- The bottom window follows the progress of the instruction file as it is being executed. It
shows the untokcnized instructions and the comments (i.e. the original instruction file)
Variables:
-Variables can be declared at anytime during the program when a main command token is
vaiid. Simple equations involving +. x, / are used to combine the variable other variables
and constants. The variables arc used mainly in trigger statements. The variables can
change value if they past through certain routines. They can not be saved as can channels.
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Instruction set (Main Command Tokens)
I
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
R
DOUT
DIN
AD
DA
SYSTEM
GOSUB
RETURN
IF
TIME
SAVE
TRIGGER
LOOP
END
HALT
GOTOLABEL
EXIT
Paste
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169
169
170
172
173
174
175
176
177
17 8
179
181
182
184
185
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Com m and:
OPERATION OF RELAYS
P u rp o se: T urns on or o lT a s p e c ific relay channel.
Syntax: R X V
where X is the relay channel number from 1 to the number of channels available and
V is either ON or OFF
Example: R 1 ON
Comments: The above example illustrates how relay one is turned on.
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C om m and:
DOUT
OPERATION OF DIGITAL OUTPUT CHANNELS
P u rp o se: Turns a s p e c ific d igital output channel on or off.
Not implemented
Command:
DIN
OPERATION OF DIGITAL INPUT CHANNELS
Purpose:
Sub token:
NAME
not a main command token)
Parameters:
anv name (which
Svntax: DIN X NAME Y
where X is the instrument board channel number and Y is anv name
Not implemented
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Com m and:
CALIBRATION OK ANALOG TO DIGITAL CHANNELS
P u rp o se: S e ts the n am e, g a in , slo p e and offset o f a s p e c ific a n alog to digital ch a n n el.
S ub tokens:
Parameters:
NAME
GAIN
nam e (not a sub token )
a real
a real
a real
a real
SLOPE
SQR
OFFSET
number
number
number
number
Syntax: AD X NAME GAIN Z SLOPE IP SQR Y OFFSET V
where X is the physical analog to digital channel number.
Example 1: AD 1 TUBE_TEMP GAIN 100 SLOPE 1 OFFSET 0
Comments: The analog to digital channel m ust be nam ed since an unnam ed
channel is assigned th e nam e "undefined". A physical analog to digital channel
num ber can have different nam es associated w ith different slope, gain, and offset.
The commas are ignored during execution. Sub tokens can be placed in any order
except for th e nam e of th e channel. The nam e of a channel is used to save d a ta and
operate triggers w ith the param eters associated w ith th a t name.
Channel Calibration:
The gain value assigned is used to obtain a first reading. If th e reading is g reater
th an 4095, th e gain is reduced by ten and th e reading acquired. It will rep eat th is
until a value w ithin range is obtained or th e gain is equal to 1. The reading
acquired is always corrected for the gain before applying the slope, sqr, and offset.
L inear C alibration.:
W hen a lin ear calibration is used, the value of SQR is not given and a default of 0
(zero) is used. The linear calibration requires two points. In th is exam ple values
are obtained a t am bient pressure and 200 psi. The gain is set to 1000, slope 1000,
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offset 0 and readings are taken at both pressures. The values returned are the
values th a t would have been acquired with a gain of 1 multiplied by 1000. This does
not m ean th a t the readings were taken at a gain of 1000, because all readings are
first adjusted to a gain of 1 before applying the slope and offset.
The two pressures gave readings of -3.53 and -11.64 for am bient and 200 psi. The
200 psi difference gives -S.11 so the psi/rcading is 200/-S.11 = -24.66, the slope and
th e offset is (offset - (-3.53:i:-24.66)) = 0. offset = -S7.05.
F or a physical channel num ber 3, named Tube_Pressure. the param eter line would
be:
AD 3 Tube_Pressure Gain 1000 Slope -24,66 Offset -87.05.
T his p articular example had a large DC offset and signal lines polarity th a t gave a
decreasing signal with increasing pressure. The param eter line will adjust the
readings of channel num ber 3 to provide output in psi with a reading of 0 for
am bient pressure and 200 a t 200 psi.
Q uadratic C alibration:
SQR is th e squared term a , b is th e SLOPE and c the OFFSET in ax'+bx+c= y. A
q uadratic least squares fit w ith a t least ten values is required. The values from the
fit can be plugged straight in.
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Cmnmanil:
DA
CALIBRATION OF DIGITAL TO ANALOG CHANNELS
Purpose: Sets the value, slope and offset of a specific digital to analog channel.
Suh tokens:
Parameters:
SLOPE
OFFSET
a real number
a real number
Syntax: DAX SLOPE IVOFFSET V. Z
where X is the digital to analog channel, and Z is the channel value.
Example I: DAI, SLOPE40, OFFSET 0. 100
Example 2: DA I. 10
Comments: The sub tokens could be placed in any order. And. as seen in example 2, the channel
value can be reset without having to reenter the slope and offset. The digital to analog channels
can assume any integer value in the range 0 to 4095 therefore the slope multiplied by the channel
should not exceed 4095.
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C om m and:
SYSTEM
SETTING THE HARDWARE) SPECIFICS
Purpose: Sets the hardware specifies such as the instrument board number, the multi function
board channel number or the analog to digital offset.
Sub tokens:
Parameters:
INST_BRD_NUM
MFB_CHN_NUM
AD_OFFSET
an integer (0-16)
an integer (0-1 ft)
an integer (0-4095) <= depending on the
number bit A/D used.
Syntax: SYSTEM INST_BRD_NUM X MFB__CHN_NUM Y AD_OFFSET Z
Example: SYSTEM INST_BRD_NUM 2. MFB_CHN_NUM 2 AD_OFFSET 204S
Comments: The sub tokens could be placed in any order. These parameters arc specific to the
Truiogic Systems Inc. data acquisition boards.
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Com m and:
GOSUB
JUMPING WITHIN THE PROCRAM
Purpose: Saves the next position in the program and goes to a specific position. It can then
return to the saved position and continue operation (using the command RETURN).
Syntax: GOSUB X
where X is a label.
Example 1:
PROCESS
R 1 OFF
GOSUB PROCESS
Example 2:
R 1 ON
GOSUB PROCESS
R 2 OFF
EXIT "End of main program
PROCESS
R 2 ON
RETURN
Comments: In the first example it can be seen that an infinite loop is created. The second
example will work without causing an infinite loop since the label 'PROCESS' is located after the
end of the main program (This is a recommended way of using GOSUB such that infinite loops
can be avoided). Used in conjunction with the command token 'RETURN'.
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C om m and:
RETURN
r e t u r n in g ; f r o m a j u m p r o u t i n e
Purpose: Used in conjunction with the GOSUB command. Returns execution to the token
immediately after the GOSUB command.
Svntax:
GOSUB X
RETURN
where X is a label.
Example:
R 1 ON
GOSUB RELAY
TIME DELAY 5.0
EXIT "End of main program
RELAY
TIME DELAY 1.0
R 9 0FF
RETURN
Comments:
In the example, the GOSUB command causes the execution to go to the line
denoted by the label 'RELAY'. The program then waits I second, turns relay 9 off and then
returns to the time delay of 5 seconds.
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Com m and:
CONTROLLING PROGRAM FLOW
Purpose: Compares Variable or Channel using logical operators to determine destination.
Sub tokens:
Parameters:
GOTOLABEL
a label
Syntax: IF X Y GOTOLABEL Z
X
variable name
Y
is a logical operator. LT(less than), ELT(equal less than).
EQ(equal).EGT(equal greater than). GT(greatcr than)
Z
variable or channel name
Example:
IF HighProc EQ 0 GOTOLABEL Continue^ Level
Comments: In the example, the variable HighProc is tested to see if it is zero. This is done in
this ease to sec if the program has passed through a certain part of the program that would change
this variable.
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C o m m a nd:
TIME
CONTROLLING AND USINC THE TIME
Purpose: Uses (and controls) die lime by setting. resetting and delaying the execution of the
program.
Sub tokens:
Parameters:
DELAY
RESET
IS
Syntax:
a real
none
a real
TIME DELAY X
or
TIME RESET
or
TIME IS X
Example:
TIME RESET
R 1 ON
TIME DELAY 1.0
R I OFF
TIME IS 3.0
R 2 ON
Comments: In the example, the time is reset to zero (this lime corresponds to the time that
belongs to this program and not to the system), relay I is turned on. the execution of the program
is delayed for I second, then relay 1 is turned off and the execution of the program waits until the
time reaches 3 seconds and then turns relay 2 on.
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Com m and:
SAVE
SAVING PERTINENT INFORMATION
Purpose: Saves data in a predetermined output tile.
Suh tokens:
Second sub tokens:
DATA
HEADER
MACROFILE.
none
TIME, DOSTIME. DATE. INSTFILE,
"name of A/D channel” (255 byte limited)
Svntax:
SAVE HEADER TIME DATE DOSTIME INSTFILE.Y:
where X is the name of an A/D channel
SAVE DATA
Example:
SAVE HEADER TIME DATE TUBE_TEMP :
SAVE DATA
Comments: The second sub tokens could be placed in any order. A semicolon has to be placed at
the end of the save header procedure to indicate the termination of this procedure. To save data,
this command must be preceded by the SAVE HEADER command. In the example, in order to
save the time, date and the temperature of the tube, the header is saved first and then the
corresponding data.
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Com m and:
TRIGGER
SETTING TRIGGERS
Purpose: Triggers an action once a specific condition has been met.
Sub tokens:
PRIORITY
SET
ON
OFF
Svntax:
TRIGGER SET X Y Z GOTOLABEL L
X is the name of the input channel.
Y is a logical operator, LT{less than). ELT(equai less than). EQ(equaS).
EGT(cqual greater than). GT(greatcr than)
Z is a number or. variable
L is a label.
TRIGGER PRIORITY X - Y
X channel name. Y priority level 1-32
or
TRIGGER ON X
or
TRIGGER OFF X
where X is the name of the input channel
Example:
Minirmim_T emp = 8 5
Maxim uxn_Temp = 90
M axim um _Press = 130
V ent_Press = 5
Vent_Temp = 7 0
Time_Value = 2
Trigger Set Time EGT Time_Value GoToLabel SaveJD ata
T rigger S et Tube_P EGT M aximum _Press GoToLabel High_Press
Trigger Set Tube_Temp_T EGT Maximum_Temp GoToLabel Temp_High
T rigger Set Tube_Temp_B ELT Minimum_Temp GoToLabel Temp_Low
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Trigger Priority
Trigger Priority
Trigger Priority
Trigger Priority
Time
=1
Tube_P = 2
Tube_Temp_T = 3
Tube_Temp_B = 4
Trigger On Time
Trigger On Tube_P
"END DIGESTION
Trigger Off Tube_Temp_T
Trigger OfFTube_Temp_B
Comments: More than one trigger can be set at a time. When a trigger is encountered the
execution of the program continues and does not wait for the condition to be fu! til led. The
example above is from a digestion program. Triggers are set. prioritized, turned on and turned off
during the course of the digestion. Triggers can be reprogrammed within the program. If
variables arc used it is possible to reset triggers according to certain conditions. The '‘END
TRIGGER” statement is used to return from a trigger, all trigger routines must have the end
statement at the end.
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Com m and:
LOOP
REPKATlNt; IN'STKl ifm ONS USIXC: A LOOP
P u r p o s e : R e p e a ts a s e rie s o f in stru c tio n s a sp e c ific n u m b e r o f tim e s.
Syntax:
LOOP X
} instructions
END
Example:
LOOP 3
R 1 ON
T I M E D E L A Y 1.0
R 1 OFF
END
Comments: LOOP is always used in conjunction with END ( to be discussed next). In the
example, relay 1 is turned on. delays lor one second and then the relay is turned off; this series of
instructions is repeated three limes.
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END
Com m and:
TERMINATING LOOPS AND TRIGGERS
P u r p o s e : U s e d in c o n ju n c ti o n w ith a L O O P c o m m a n d o r u s e d to t e r m in a te lo o p s o r trig g e rs .
S u b to k e n s :
LOOPING
TRIGGER
Syntax:
LOOP X ... END
or
END LOOPING
or
END TRIGGER
Example 1: LOOP ... END
Example 2:
TRIGGER SET TEMP GT 100 GOTOLABEL RELAY
TRIGGER ON TEMP
LOOP 60
R 1 ON
TIME DELAY 1.0
R 1 OFF
END
RELAY
R 1 OFF
END LOOPING
Example 3:
TRIGGER SET TIME EGT 200 GOTOLABEL RELAY
TRIGGER ON TIME
LOOP 60
R 1 ON
TIME DELAY 1.0
R 1 OFF
END
EXIT
RELAY
RESET TIME
END TRIGGER
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C o m m e n ts: In e x a m p l e 2, the te m p e r a tu re tr ig g e r is set and th e n the p r o g r a m e n t e r s a loop. If the
tri" tie r c o n d it io n is m et e x e c u tio n g o e s to the label R E L A Y w h e r e rela y I is tu r n e d o f f a n d the
lo o p is te r m in a te d . W h e r e a s in e x a m p l e 3. w h e n the trig g e r c o n d itio n is m e t. the e x e c u ti o n g o e s
to R I ;L A Y w h e r e th e lim e is reset a n d the E N D T R I G G E R te r m in a te s th e tr ig g e r a n d c o n ti n u e s
e x e c u tio n fro m w h e r e the p r o g r a m w a s he lo r e th e t r ig g e r c o n d itio n w a s m et.
179
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Com m and:
HALT
PAUSING THIC KXKCUTION
P u r p o s e : P a u s e s th e e x e c u t i o n o f a p r o g r a m until th e u s e r d e c id e s to c o n ti n u e o r e x it the
program .
Svntax: HALT
E x am p le:
LOOP 10
R 1 ON
HALT
R 1 OFF
END
Comments: In the example, relay 1 is turned on and then the execution of the program pauses
until the user presses a key (except 'e ‘ or 'E ') or the user types V or *E* to exit the program.
180
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Com m and:
GOTOLABEL
■lUMPINt: TO A I.ABKI. WITHOUT RETURNING
Purpose: Rcdireels execution to the specified label
Syntax: GOTOLABEL*
where * is a label defined in the program
E x a m p l e 1:
TRIGGER SET TIME C.T 10 GOTOLABEL RELAY
TRIGGER ON TIME
RELAY
R I ON
R 2 OFF
EXIT
Example 2:
RELAY
R I ON
R 2 OFF
R I OFF
GOTOLABEL RELAY
Comments: Example 1 depicts that when a trigger condition is met. execution goes to the label
RELAY. In example 2. it can be seen that an infinite loop is created since the three instructions
relay 1 is turned on. relay 2 is turned on and relay I is turned o ff. are continuously repeated.
181
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Com m and:
EXITING THK PROGRAM
P u r p o s e : T e r m i n a t e s th e p r o g r a m e x e c u t io n . It is th e o n ly n e c e s s a r y to k e n in th e w h o l e p r o g r a m .
It m u s t b e p r e s e n t (in o r d e r to g o b a c k to D O S )
Syntax: EXIT
Example 1:
R 1 ON
R 2 OFF
EXIT
Comments: In the example, relay 1 is turned on. relay 2 is turned off and the program then
terminates. Every instruction file is required to have an EXIT command so that the program
execution can terminate.
182
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission.
11.3.2 Implementation of Macros
P u r p o s e : U s e s a se rie s o f c o m m a n d s b y a call to o n e nam e.
Example: MyHeader
Save header date dostime;
M yileaderParms ins!file date dostime ;
MyHeader
save header MyHeaderParms
Comments: Macros facilitate the programming. Macros are listed in a file which can then be
called within the program. This file could be entered at the command line as follows:
> Micro2 <fdename>
eg.
Micro2 micro2.mac
All macros in the instruction file have to be defined in the macro file.
183
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11.3.3 Hotkey Function
FI = >
E x its a lo o p in progress and c o n tin u e s the program im m ed iate after the loop .
F7 = >
E x its the m ain program
184
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11.3.4
Example Program :
"1)1 l.SKQ
"DIGESTION ROUTINE TO DISSOLVE ORGANICS
"COOLING TO A LOW TEMPERATURE
"THEN VENTING TILL LOW PRESSURE
"BEFORE CONTINUING DIGESTION
Time reset
System inst_brd_num 2, mfb_chn_num 2
ad 1, Tube_T, Gain 1000, SQR -7.6563 Slope 79.7298. Offset 21.S4
ad 1, Tube_Temp_T, Gain 1000, SQR -7.6563 Slope 79.729S, Offset 21.S4
ad 1, Tube_Temp_B, Gain 1000, SQR -7.6563 Slope 79.729S, Offset 21.S4
ad 2, Mag_T, Gain 1000, Slope 3S.46, Offset IS. 15
ad 3, Tube_P, Gain 1000, Slope 103.6, Offset -29.3
ad 4, W aterJT, Gain 1000, Slope 52.05, Offset 14.4S
ad 15, Power,
Gain 1, Slope 1, Offset 204S
Shut_Relays
MyHeader
Save d ata
Save header Tube_T Power Tube_P W ater_T Mag T Time ;
Save d a ta
mag_Fan_on
Fil_On
Time reset
Mi nim um_Temp = 8 5
Maximum_Temp = 90
M axim um JPress = 130
Vent_Press = 5
Vent_Temp - 70
Time_Value = 2
Trigger S et Time EGT Time_Value GoToLabel SaveJD ata
Trigger S et Tube_P EGT Maximum_Press GoToLabel High_Press
Trigger S et Tube_Temp_T EGT Maximnm_Temp GoToLabel Temp_High
Trigger S e t Tube_Temp_B ELT Minimum_Temp GoToLabel Temp_Low
Trigger Priority Time
=1
Trigger Priority Tube_P = 2
Trigger Priority Tube_Temp_T = 3
185
with permission of the copyright owner. Further reproduction prohibited without permission.
T rigger P riority Tube_Tem p_B - 4
Trigger On Time
Trigger On Tube_P
"PULSE MICROWAVE TO BREAKUP CAPSULE GENTLY
Capsule_Breakup
H ighPressProc = 0
H V .O n
Time delay 0.75
KV_Off
Time delay 1
Time delay 1
Time delay 1
If Tube_T ELT Minimum_Temp GoToLabel Capsulc_Breakup
"RAMP TO DIGESTION TEMPERATURE
Trigger On Tube_Temp_T
T rigger On Tube_Temp_B
Start_R am p
H ighPressProc = 0
Loop 45
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
tim e delay 0.5
"WAS THE HIGH PRESS ROUTINE USED IF SO RESTART RAMP
If H ighPressProc EQ 0 GoToLabel Cont_Ramp_Loop
E nd Looping
M inimum_Temp = 85
Maximum_Temp = 90
GoToLabel Start_Ram p
Cont_Ramp_Loop
M inimum_Temp = Minimnm_T emp + 2
Maximum_Temp = MaximumJTemp + 2
Trigger S et Tube_Temp_T EGT Maximum_Temp GoToLabel Temp_High
Trigger S et Tube_Temp_B ELT Minimum_Temp GoToLabel Temp_Low
186
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
end
"DIGESTION TEMPERATURE REACHED
Maximum_Press = 170
Loop 300
If HighPressProc EQ 0 GoToLabel Continue_LeveI
End Looping
Minimum_Temp = 85
Maximum_Temp = 90
GoToLabel Start_Ram p
Continue_Level
Time delay 0.5
Time delay 0.5
end
"END DIGESTION
Trigger OfFTube_Temp_T
Trigger Off Tube_Temp_B
Microwave_Off
Cool_On
loop 120
Time delay 2
end
Loop 5
Vent_Closed
Time delay .3
Vent_Open
Time delay 1
Time delay 1
Time delay 1
end
Trigger Off Tube_P
Trigger Off Time
Cool.Off
Flush_Cool_On
tim e delay 20.0
Flush_Cool_Off
Mag_Fan_Off
Shut_Relays
ex it
187
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
sa v e d a ta
T im e_ V a lu e = T im e_V alue + 2
T rig g er S e t T im e EGT T im eJV aluc GoToLabel S a v e D ata
T rig g er On T im e
en d T rigger
Tem p_L o\v
H V _O n
Trigger On Tube_Temp_B
end Trigger
Temp_High
HV_Off
T rigger On Tube_Temp_T
end Trigger
"DEAL W ITH HIGH PRESS
High_Press
HV_Off
Time delay .1
IF Tube_P ELT M axim um JPress GoToLabel Pass_Over_Trigger
Cool_On
"LOOP TILL TEMPERATURE LOW ENOUGH TO VENT
Temp_Still_High
IF TubeJT ELT Vent_Temp GoToLabel Temp_No\v_Low
GoToLabel Temp_Still„High
Temp_Now_Low
"LOOP TILL PRESS LOW ENOUGH TO CONTINUE DIGESTION
Press_Still_High
IF Tube_T EGT Vent_Temp GoToLabel Temp_Still_High
Vent_Open
Time delay .25
Vent_Closed
Tim e delay 1
IF Tube_P ELT Vent_Press GoToLabel Press_Now_Low
GoToLabel Press_Still_High
Press_Now_Low
Vent_Closed
"FLUSH THE COOLING TUBE
Cool.Off
Flush_Cool_On
loop 10
188
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
tim e d elay 2
end
Flush_CooI_Ofl*
M inim um _T em p = 85
M axim um _Tem p = 90
H igh P ressP roc = 1
M axim u m _P ress = 170
T rigger S e t T ube_P EG T M axim u m _P ress GoToLabel H ig h _ P ress
T rigger On T ube_P
P ass_O vcr_T rigger
end T rigger
189
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11.4 APPENDIX D
Drawings
190
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
c /2 . -
S !\
C .5 C 5
/
- ---..Q.CCC"
-o .c o i
60?
1M" R..
A
0 . 9 2 l ”0 .!D .
A
MATERIAL: STAINLESS STEEL
1 8 -0 3 -9 4
N o.
DATE:
CUSTOMER:
ADDED DIM ENSIONS 09 21 ’ O J>. AND 0 8 9 0 ”
REVISIONS
GUYLEGERE
BY
DATE:
M ARCH 12.
1995
FLANGE VALVE NUT
SCALE:
not to scale
DRAWING No.
A-1001
191
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REV.
1
1/8" D IA . DRILL"
NOTE: USE STAINLESS STEEL
HEX CAP SCREW
1A T -28 X 1/2" L O N G A
FULLY THREADED
1 /6 4 .- R . A T E A C H E N D
CUSTOMER:
N o. I
D ATS:
REVISIONS
guyleg ere
BY
IDA.TB:
M AKCH12.
1/8” I.D . FLANG E BOLT
I IW
ISCALE:
not to scale
192
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REV.
DRILL- A N D TAJ? lM "-2 8
T O A D E P T H O F 0.3 0 "
A
X
j
/
A
A
0 .< 9 HO .D .
0.02" CHAMFER
1/16" D IA . DRILL.
BOTTOM MUST BE FLAT
MATERIAL: KEL-F
1 8 -0 3 -9 4 .
N o.
DATS:
IM --3S.090 * O JD . 0 4 9 " O J3. AND 020*
WBXS 1M*-3Q. 092* OX>_ 0 J0 " O JX AND030*
REVISIONS
CUSTOMER:
GUYLEGERE
BY
DATE:
MARCR12.
PRESSURE ADAPTER
DRAWING No.
REV.
ITO
SCALE:
not to scale
A -1006
193
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
1
0 .7 5 0 "
ACROSS FLATS
0 .0 5 " D I A . X 0 .1 4 " L O N (
STAINLESS STEEL PIN
PRESS F IT INTO FAR SIDE
0.02" CHAM FER
T Y P . 3 PL A C E S
1 .1 6 9 " D IA .
MATERIAL: STAINLESS STEEL
CUSTOMER;
GUYLEGERE
DATE:
Digestion Tube Holder
<n.
STAIR
Dot to scale
DRAWING No.
REV.
A-1011
o
194
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
CUSTOMER:
GUYLEGERE
DATE:
PRESSURE TRANSDUCER ADAPTER AND COMPONENTS
SCALE:
not to scale
DRAWDTONo.
REV.
A-1013
o
195
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
0 . 9 0 “ O .S D .
TOTAL LENGTH OF TUBE FROM
E N D T O E N D IS 2 8 .1 1 “
5” R.
M A T E R IA L .: 3 /8 " IJ D . X 1 /2 " O .D . P F A T U B E
ICUSTOMER:
GUYLEGERE
DA.TE:
M ARCH Z7.
F L A N G E D P F A T U B E - 1 /2 " O -D .
DRAWING No.
REV.
W*
SCALE:
not to scale
A-1014
197
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
o
cn
o
^1
0
ffl
-©
3
-i
—r
ii i !
14 9 / 1 6
as
is
\
-----
\
r
'
A
_
1L u
*
i
198
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
:
I
II
!
b.
3
o
/
t
Q
VI
o
t^J3awwswrt
a
O
1
n
•
B
D
6
S r e i» S r \O ^ T i
JC (APPROX.)
1m
tl^
) „
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1
VI
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ri
-* * * '
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fs
VI
[illsglj§fl
I^Sl
^ v O \\
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a
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/Cl -TATI
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09
o
1
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- I
199
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
//
:%
n
CH A M FER O F F
SH ARPHD GES
5 0 .8
o
.- d .
1 l/2”-20 THREAD
NOTES:M ATERIAL IS STAINLESS STEEL
DIMENSIONS ARE IN Tim. UNLESS NOTED OTHERWISE
M
CUSTOMER:
GUYLEGERE
STOP DOW N RING
DATE:
1 3 -0 2 -9 5
SCALE:
not to s c a le
DRAWING No.
A -I0 2 1
200
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
REV.
o
3/** X I
s
201
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
RUBBER RETAINER
r
D R I L L 1 /8 “ D IA . T
A D EPT H O F 2 4
TA PER ED
1 2 .7 !
C A P P IN
P R IL L 1/8" D IA . T O
A DEPTH OF 24,0
TAPERED
BO DY PIN
12.7
50.8
NOTE: UNLESS OTHERWISE NOTED, ALL DIMENSIONS ARE IN mm.
B O D Y A N D C A P PIN S
AND RUBBER RETAINER
202
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4 HOLES I t V IA ,
A S SHOW N ON A PXLD. O F 141
I
COUTfTERBORB 10 DIA.
7 D IA .
I
'S
5 / l 6 ' - l l X 1 'L O N O
AUflflXWM THUMB SCXBW
(4 REQUIRED)
I
1
DRILL THRU AND TAP 5/16*-11 TYPICAL 4 rU vCZS ON CEICUMFZRENC3
M]
NOTES: MATERIAL IS 11/16“ THICK PLEXIGLASS
(M Y *
UNLESS OTHERWISE NOTED, DIMENSIONS ARB IN i m
MAIL 10.
OUY LEO [IRE
1995
n o t to ( ca!&
a
10-01*95
K * W M LA W 9M I
No.
DATE
REvmora
41
it
CAPSULE PIN WHEEL
cajknom
i*y
-1020
0
£ 3 -1 0 2 1
not to l o l e '
Vi-'
o
_
| 4>
CAPSULE
PIN RACK
5
to-o>-93
KnrwAWTw
£9
204
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
205
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11.5 APPENDIX E: Microwave Control Hardware
The microwave used for this work is a domestic type. Eaton Viking Model RE77STC, equipped with a 720W m agnetron a t 2540 Mhz, a 1 ft ’ cavity, and mode
stirring. It is not fitted with a rotating carousel. Two identical ovens were bought.
An oven w ith a mode stirrer (instead of a rotating carousel) was used because the
fixed tube cannot be rotated in the cavity. A single transform er powers the high
voltage and filam ent current to generate microwave power. A delay mechanism
ensures th a t the filam ent current has tim e to heat the electron source in the
m agnetron before the high voltage is turned on. This delay, which is a t least two
seconds long even if the m agnetron is turned off only momentarily, does not allow
th e microwave power to respond rapidly enough for our needs.
The delay w as elim inated by placing a second identical transform er in the oven.
One transform er supplies the filam ent current, the other high voltage; each has its
own 120VAC supply th a t can be independently switched on or off.
The 120VAC supplied to both transform ers is controlled by separate relays. The
filam ent current uses a n onboard I2VDC relay. The 720 w atts of power are
tran sfe rred through th e high voltage transform er a t ten am peres, well above w hat
a n onboard relay can control. Therefore, an onboard relay controls a power relay,
which, in tu rn , is used to control th e 120VAC to the high voltage transform er.
The delay in production of microwave energy when th e high voltage is applied to th e
m agnetron is elim inated by leaving th e filament current powered for th e duration of
th e digestion. This delay gives th e tube tim e to warm up before the high voltage is
applied, b u t th e microwave power still cannot be cycled on and off faster th a n every
th ree seconds. Once th e filam ent current has been turned on for more th a n three
seconds, an d th en left on, sw itching th e high voltage on to th e m agnetron will
supply microwave energy w ithout delay.
T he logic lines th a t tu rn on th e filam ent and high voltage relays are fed to a NAND
g a te th a t tu rn s a counter on w hen the microwave is powered; a t the sam e tim e, a
206
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
second counter in the system runs continuously. A ratio of the two counters gives
the fraction of tim e the microwave has been powered.
The oven is equipped with a cooling fan for the m agnetron, and a light for the
microwave cavity, both of which are also controlled by relays.
207
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
11.6 APPENDIX F Suppliers
208
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
C om pany
A cutech P lastics
Aldrich C hem ical
C om pany Inc.
A m etek H aveg
Division
A llstate P lastics
B axter C orporation
C om m ents
T elep h o n e
Fax
PFA I'r o d
A ddress
City
Province Postal
Code
10A Me Arthur Road
Reading
PA
19605
2905 W est Hope Avenue
Milwaukee
Wisconsin
53216
source o( Into on PFA, sam ples of302-995 0491
tubing tor 1/8' flange
900 Greenback Road
Wilmington
DE
P F A 3 5 0 I'r o d , Kel-F I'rod
237 Raritan Street
South Amboy
NJ
plastic test tubes, polyethylene
bottles
6800 Route Trans Canada Pointe Claire
PO
535 N. Emerald Road
SC
glove bags, low powder Nilrile
gloves
414-273-3850
803-223-2270
6879
H9R 5L4
C apsugel, a division
of W arner-Lam bert
Co.
C hrom abec Inc.
capsule sam ples
Rhyeodyne valves, connectors In 514-335-6288
Kel-F
514-335-1837
6797 Place Metivier Suite Montreal
A
PO
C ole-P arm er
Instrum ent Co.
David Ja n d ro n
PFA tubing 1 /2 '. 1 /4 ', pressure 708-647-7600
g au ge (Burdon)
708-647-9660
7425 North Oak Park
Avenue
Chicago
IL
AutoCad drawings
514-653-1860
16 9 0 d e M onlesson
St. Bruno
Quebec
Electro insulation
C orporation
Electro Sonic Inc
3/8' Natglas g la ss sheath
708-632-1020
708-632-1089
3921 Venlura Drive
Arlington Heighls IL
Electronic com ponents, power
relays
416-494-1555
(order)
416-496-3030
1100 Gordon Baker Road Willowdale
Enllo C a n a d a Ltd.
F.C. Provalve
PTFE film
800-561*0050
506-473-2307
73 Industrial Rd.
NB
EOJ 1M0
Plastic ball valves, unions
514-695-5025
514-8630-5670
265 Hymus Blvd, unit 1000 Point Claire
Quebec
H9R IG6
Finnan E ngineered
P roducts Ltd.
F isher Scientific
Limited
Furon
two way teflon miniature solenoid 416-438-6070
valve
416-438-8739
1149 Bellamy Road, Unit
22
Scarborough
ON
M1H 1H7
8505 Devonshire Road
Montreal
Quebec
GM A sso c ia tes Inc
Pump Tubing, Bodies, Gloves
(nilrile), Capillary Tubing
Greenwood
Grand Falls
Ontario
609-423-8252
609-423-8162
1-295 and Harmony
Road,P.O. Box 71
Mickleton
NJ
Custom glassw are, sintered g!ass510-430 0806
filter, Tom Beady
510 562-9809
9803 Kitty bane
Oakland
CA
215-277-8200
215 277-3850
P.O. Box 61367
King of Prussia
PA
Bunnell Plastics, valves, lube,
molding, custom fiber glass
sheathing
G allagher Fluid S e a ls Tenon-Vik>n *Ka,fez O r,nos
Inc.
209
29646
H4K2N4
6064B
J3V 2W4
600047952
M2H 3B3
94603
194060857
Grasby Infrared
S737C P bSe delector assem bly
407*282-7700
407-273-9046
12151 R esearch Parkv/ay Orlando
FL
G ray Electric
Supplies Ltd.
Hoskin Scientific Ltd
big ground connectors in copper, 514*849-6081
Bruce
514-849-6622
4 46 Ste. Helena
Montreal
PO
H2Y2K7
Pressure Sensor, Front Surface, 514*735*5267
mating conneclor Mr. Kadoury
514-735*3454
8425 Devonshire
Montreal
Quebec
H4P2L1
ICN B iom edicals
C an a d a
IEE Service C entre
PPL Dept.
acrytamkte, ammonium
persulfate, TEMED
#1
ON
L5T 1L7
Microwave O vens book, J.PIatts,
Order from Jane Huber
445 HOES Lane, P.O. Box Piscataway
1331
N J.
108 Franklin A venue, P.O. Cheltenham
Box 159
PA
1 9 1 6 -3 2 E A v e .
Lachine
PO
H8T 3J7
Instrum ents for
R ese arch and
Industry
Jo h n sto n Industrial
Plastics Ltd.
glove bags
PlaClde MathieU &
215*379*3333
215-663-8847
Nylon rod, PFA f rod, 1/8 PTFE
rod S q u eeg ee, Leister hot air gun
Junior M achine &
Tool
distributor for Cole Parmer
Labcor V entes
T echniques, Inc.
Laurenlien Valve & Sw agelok, valves
Fitting
gotd pfate Thermocouples
M. Stanton
Electroplating Ltd,
PVC pump tubing
M andel Scientific
C om pany Ltd,
Patent copies
MicroMedia Ltd.,
Technical Information
C entre
Milllpore C a n a d a Ltd. 10"*™ finer,
O m ega Engineering Thermocouples, IfVTC, Heating
Inc
1800 Courtneypark Drive
088551331
19012
514-629-6266
514-629-6433
896 Berlier
Laval
PO
H7L 4K5
514*332-3651
514-332-4386
2425 Haipern
Vide St-L au ren t
Ouebc-c
H 4 S 1S3
905-438-5055
905-438-5091
1120 Bellamy Drive
Scarborough
ON
M1H 1H3
2 Admiral Pface, RR 6
Guelph
ON
N 1H 6J3
165 Hotel d e Viile
Hull
Q uebec
J8X 3X2
3688 Nashua Drive
M ississauga
ON
L4V 1M5
613-237-4251
800*268*4881
203-359*1660
203-359-7700
One Omega Drive. Box
4047
Stamford
CT
069070047
514-467-3565
514-467-2195
670 rue Picard
Bekreil
PO
J3G 5X9
Bflnd
various t o d ,
M ississauga
32826
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Precision G lass
Blowing of C olorado
U.S.A.
Provan Control
A sso ciates Inc.
R hyeodyne
C orporation
R ese arch &
D evelopm ent G lass
P roducts and
Equipm ent Inc.
S .P .E . Limited
S C P S cience
T echnel Engineering
Inc.
Truloglc S y stem s Inc
UMI
Unicorn Softw are
Z eu s Industrial
P rodu cts Inc.
Pyrex (rilled tube
303-693-7329
303-699-6815
14775 East Hinsdale Ave. Englewood
CO
Valves
514-332-3230
514-332-3552
2900Sabourin
St. Laurent
Quebec
Rotary valve with 1/8* tubing, type707-664*9050
60, Teflon rotor and stator
707-664-8739
P.O. Box 9 9 6 ,6 8 1 5
Redwood Drive
Cctati
CA
Pyrex U-lube, glass flange lube
510-547-6464
510-547-3620
1808 Harmon Street
Berkeley
CA
TFE ferrules, Tefzel Tee 1/8*
905-6607140800-2681056
514-337-6700
905-660-7138
161 Connie Cresent
Concord
Ouebec
L4K 1L3
514-745-2646
2367 Guenetlo
St. Laurent
Quebec
H 4R2E9
416-851-4244
416-851-5743
120 V/hilmore Rd., #8,
P.O. Box 15
Woodbridge
ON
L4L 1A9
O C19, QC7, David Lefson
Mediamate pressure sensor,
liquid transfer line sensor
6741 Columbus Road, Unit M ississauga
A/D, Relay, Instrumentation, ICP 905-85648417
interface
thesis (Jessie)
Ann Arbor
313-761-4700
803-531-2174
H4S 1M2
94703
L5T2G9
12
programming of windows In
MICR02
1/16* PFA for cooling lube
Onlario
80112
803-533-5694
211
Ml
#2- Gladstone Ave.
Ottawa
ON
501 Blvd., SE, P.O. Box
2167
Orangeburg
SC
48106
K1R5P4
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