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A comparison of microwave assisted and conventional leaching using EPA method 3050B

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A COMPARISON OF MICROWAVE ASSISTED AND
CONVENTIONAL LEACHING USING EPA METHOD 3050B
A Thesis Presented to the Bayer School of Natural and
Environmental Sciences of Duquesne University
As partial fulfillment of the requirements for the degree of
Master of Science
Dr. Elke M. L. Lorentzen
December 1996
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UMI Number: 1383653
Copyright 1997 by
Lorentzen, Elke Martha L.
All rights reserved.
UMI Microform 1383653
Copyright 1997, by UMI Company. All rights reserved.
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SIGNATURE PAGE
Name
Dr. Elke M. L. Lorentzen
Thesis Title
A COMPARISON OF MICROWAVE ASSISTED
AND CONVENHQNAL LEACHING
USING EPA METHOD 3Q5QP
Degree
Master of Science_________________________
Date
Decemt>€Ll7,18%_______________________
APPROVED
/-/.A.
Dr. H. M. Skip^Cingston, Professor of Chemistry
APPROVED
Dr. Mitchell E. Johnson, Professor of Chemistry
APPROVED
Dr. Dan K. Donnelly, Director, Environmental
f Science and Management Program
ACCEPTED
Dr. Heinz W. Machatzke, Dean,
School of Natural and Environmental Sciences
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ABSTRACT
A microwave heated EPA method 3050B for the leaching of
key elements (cadmium, chromium, copper, lead, nickel, and zinc)
of environmental
importance was tested and compared to
conventional
hot-plate
heated
EPA
commercially
available
temperature
Method
and
3050B.
power
All
controlled
atmospheric pressure microwave systems were used for the
adaptation of EPA method 3050B.
Three temperature feedback
control systems were evaluated for regulating the temperature of
the leachate outside (IR sensors) and inside the sample flask (gas
bulb thermometer).
Results
showing
the
efficiency
and
effectiveness of the microwave sample preparation method are
discussed for the leaching of three NIST Standard Reference
Materials (SRMs): 2704, 2710, and 2711. The elements cadmium,
chromium, copper, lead, nickel, and zinc were determined either ty
using ICP-MS, ET-AAS or F-AAS.
This study demonstrates that
microwave control is an efficient and effective alternative to
conventional heating sources for EPA Method 3050B. Controlling
temperature rather than microwave power is better for m aintaining
specific sample preparation temperatures. Temperature feedback
control
microwave
systems are capable of controlling
the
temperature at the required 95°C with an accuracy of ± 1.1-20C
which is not achievable by hot-plate.
Moreover, precision is
achieved in reduced time and reagent addition is automated in a
system that can be fully automated.
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ACKNOWLEDGMENTS
I would like to thank my advisor Prof. H. M. Skip Kingston
with the deepest gratitude for his exceptional guidance, his valuable
time, and indispensable discussions.
I would like to express my thanks to Dr. Peter J. Walter for his
time and many helpful discussions. In addition, I would like to
thank him and Dengwei Huo for providing the ICP-MS data.
Moreover, I would like to thank all members of the Kingston
research group. Special thanks to Mary Ann, Marlene, Dirk, Peter,
and former group members Stuart, Leo, Wenchun, and Karen.
Also, I would like to thank the Department of Chemistry and
Biochemistry for their support, especially Kathy de Rose, Ian, Dan,
and Dave.
I would like to express my gratitude to Dr. Heinz. W.
Machatzke, Dean of the School of Natural and Environmental
Sciences, for his support. In addition, I would like to thank Kathy
Whitfield for her friendly assistance.
Furthermore, I would like to thank the CEM Corporation and
the Prolabo Corporation for their support of the project.
Finally, I would like to thank my husband Jens for his loving
support and patience.
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TABLE OF CONTENTS
ABSTRACT
ii
Acknowledgments
iii
Table of Contents
iv
List of Tables
viii
List of Figures
ix
Page
1. INTRODUCTION
1.1. Sample Preparation
1
1.2. Research Goal
3
1.3. Atmospheric Pressure Microwave Sample Preparation
5
1.3.1. Brief History of Microwave Sample Preparation
9
1.3.2. Single-Mode Microwave Sample Preparation
12
1.3.3. Multi-Mode Microwave Sample Preparation
18
1.3.4. Applications of Atmospheric Pressure Microwave
Sample Preparation
1.4. Comparison to Alternative Microwave Instrumentation
18
20
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Page
2. EPA METHOD 3050B
2.1. Leaching, Extraction, and Digestion
24
22. Add Leaching
26
2.3. Mineral A dd Dissolution
27
2.3.1. Nitric A dd
27
2.3.2. Hydrochloric A dd
28
2.3.3. Hydrogen Peroxide
29
2.3.4 Additional Reagents
30
2.4. Scope and Application of Method 3050B
31
2.5. Key Parameters for Leaching Procedures
31
2.5.1. Leach Medium
31
2.5.2. Temperature and Leach Procedures
33
2.6. Comparison of EPA's Methods 3050B and 3051
34
2.7. Microwave Adaptation of EPA Method 3050B
35
3. EXPERIMENTAL SECTION
3.1. Reagents
37
3.2. Equipment
37
3.3. Procedure for Conventional Hot-Plate EPA Method 3050B
41
v
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Page
3.4. Procedure for Power Control Microwave Implementation of
Method 3050B
42
3.5. Procedure for Temperature Feedback Controlled Microwave
Implementation of Method 3050B
42
4. RESULTS and DISCUSSION
4.1. Power Controlled Microwave Assisted Leaching
44
4.2. Temperature Feedback Controlled Microwave Assisted
Leaching
46
4.2.1. Temperature Measurement in a Microwave Field
46
4.2.2. Gas Bulb Thermometer
47
4.2.3. IR Sensor
50
4.3. Temperature Control on a Hot-Plate
57
4.4. Leachable Concentrations of the Analytes
60
4.4.1. Elemental Analysis
60
4.4.2. Leachable Recoveries of Certified Values for Total
Digestion
64
5. SUMMARY
76
6. REFERENCES
78
vi
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Page
Appendix A:
88
EPA Method 3050: Acid Digestion of Sediments,
Sludges, and Soils In Test Methods for Evaluating
Solid Waste-SW846, September, 1986.
Appendix B:
96
EPA Method 3050B: Acid Digestion of Sediments,
Sludges, and Soils In Test Methods for Evaluating
Solid Waste-SW846, Revision 2, December, 1996.
Appendix C:
104
Publication: Lorentzen, E. M. L.; Kingston, H. M. S.
"A Comparison of Microwave Assisted and
Conventional Leaching Using EPA Method 3050B”,
Analytical Chemistry, 1996, 68, 4316-4320.
vii
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LIST of TABLES
Pa{
Table 1:
Microwave Characteristics of Laboratory Materials (at 3000
MHz, 25°C)
7
Table 2:
Laboratory Microwave Equipment and Application Evolution
11
Table 3:
Comparative Procedural Outlines of EPA Method 3050B and
Modified Microwave Assisted 3050B
36
Table 4:
Instrumentation for Conventional and Microwave Assisted
Method 3050B
38
Table 5:
Standard Conditions for F-AAS Determination with AAS
Instrument PE 1100
40
Table 6:
Temperature Accuracy Test of Temperature Control 3
(Set Temperature 93°C, Solvent 40 mL HzO)
51
Table 7:
Results of the Analysis of NIST Standard Reference Material
2704 Using Method 3050B
61
Table 8:
Results of the Analysis of NIST Standard Reference Material
2710 Using Method 3050B
62
Table 9:
Results of the Analysis of NIST Standard Reference Material
2711 Using Method 3050B
63
v
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LIST of FIGURES
Page
Figure 1:
Schematic of Single-Mode Focused Microwave Digestor
with Gas Bulb Thermometer (Prolabo)
13
Figure 2:
Single-Mode Microwave Digestor with Temperature Feedback
Control
16
Figure 3:
Atmospheric Pressure Microwave Systems: a) A301, Prolabo
and b) Star System 2, CEM
17
Figure 4:
CEM Star System 2 with Microwave Slot Technology
19
Figure 5:
Multi-Mode Cavity Type Microwave System
22
Figure 6:
Temperature vs. Time Profile of MW Assisted 3050B with
Power Control
45
Figure 7:
MW Heating Program (45 mL F^O) Using Temperature
Control (Gas Bulb Thermometer) and Temperature Accuracy
Acquisition with a Fiber Optic Sensor
49
Figure 8:
Temperature vs. Time Profile of MW Assisted 3050B with
Temperature Control 2 (Gas Bulb Thermometer)
52
Figure 9:
Temperature vs. Time Profile of MW Assisted 3050B with
Temperature Control 1 (IR Sensor)
53
ix
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Page
Figure 10:
Temperature vs. Time Profile of MW Assisted 3050B with
Temperature Control 3 (IR Sensor)
54
Figure 11:
Temperature vs. Time Profile at 95° (Temperature Control 3, IR
Sensor)
56
Figure 12:
Temperature vs. Time Profile (Temperature Control 3,
IR Sensor)
56
Figure 13:
Temperature Distribution on a Hot-Plate
58
Figure 14:
Mean Temperature of 4 Runs at 95°C (45 mL HzO) with
Temperature Control 1 (IR Sensor)
59
Figure 15:
Percentage Recoveries of Elemental Concentrations Using
EPA Method 3050B (SRM 2704)
65
Figure 16:
Percentage Recoveries of Elemental Concentrations Using
EPA Method 3050B (SRM 2710)
66
Figure 17:
Percentage Recoveries of Elemental Concentrations Using
EPA Method 3050B (SRM 2711)
67
Figure 18:
Copper, A dd Leached, SRM 2710
69
Figure 19:
Copper, Add Leached, SRM 2711
69
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Page
Figure 20:
Lead, A dd Leached, SRM 2710
70
Figure 21:
Lead, A dd Leached, SRM 2711
70
Figure 22:
Zinc, A dd Leached, SRM 2710
71
Figure 23:
Zinc, Add Leached, SRM 2711
71
Figure 24:
Cadmium, Add Leached, SRM 2710
72
Figure 25:
Cadmium, Add Leached, SRM 2711
72
Figure 26:
Chromium, A dd Leached, SRM 2710
73
Figure 27:
Chromium, A dd Leached, SRM 2711
73
Figure 28:
Nickel, A dd Leached, SRM 2710
74
Figure 29:
Nickel, A dd Leached, SRM 2711
74
Figure 30:
Comparison of Leachable Recoveries (Ranges) of Six Analytes
on SRM 2710 in Percentage of Certified Values, MW 3050B with
Temperature Control 1 versus NIST Leachable Values
75
xi
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1. INTRODUCTION
1.1. Sample Preparation
In the last 25 years the number of environmental samples
requiring analysis has increased due to growing environmental
pollution and thus, the number of regulations, such as RCRA (the
Resource Conservation and Recovery Act) and CERCLA (the
Comprehensive Environmental Response Compensation, and Liability
Act) also has increased. Sample preparation methods which are
commonly used to prepare RCRA wastes for analysis for metals or
other elements can require hours or days. New analytical techniques
and instruments have been introduced to analytical laboratories.
These newer instruments require only a few minutes to measure
components in a sample, but their efficiency is limited by time
consuming sample preparation procedures. Inefficiency of sample
preparation methods may reduce sample throughput, increase errors,
increase costs, and slow down effective use of these analytical data.
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Since most analytical measurements are performed on solutions
of the analyte, decomposition, dissolution, and extraction of samples
is essential. Some samples may dissolve easily in water or aqueous
dilutions of common adds or bases; others require powerful reagents
and rigorous treatment. Conventional sample dissolution is not only
the most time consuming procedure of the entire analysis but also the
step in which the most errors occur. In 1991 an analytical chemist
spent two-thirds of his time on sample preparation. Moreover,
sample preparation had the highest percentage of error (30%) in the
distribution of errors generated during a sample analysis (1).
Using microwave energy as the heating source significantly
improved many sample preparation methods. Microwave instrument
technology coupled with dosed Teflon™ PFA [(Perfluoro alkoxy)
ethylene] vessels has become a standard for EPA methods 3015 and
3051 (2, 3). To date, the range of applications for microwave sample
preparation extends to "dosed-vessel", "open-vessel", and "flowthrough” systems (3-5). Espedally, the introduction of temperature
feedback controlled microwave digestors allows implementation of
this technology for all kinds of traditional sample preparation
methods, as this study demonstrates.
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1.2. Research Goal
Previously most leach methods have been accomplished on hot
plates in open vessels or using heating mantles and these methods
have not changed in decades. The sample preparation method 3050
(Appendix A), included in Test Methods for Evaluating Solid Waste SW846, has traditionally been a leach test performed on a hot-plate (2).
In recent years, the EPA has proposed to revise and update
certain testing methods used to comply with die requirements of
subtitle C of RCRA of 1976. Method 3050B (Revision 1 of Method
3050) is one of several methods included in the list of draft revised
methods reviewed in SW-846 Update HI in die U.S. Federal Register
(6). This method has been made more broadly applicable by adapting
the prescription-based method to create a performance-based method
(7). By adding the words "or equivalent" to the electric hot-plate
designation, and specifying that the heating device be "adjustable and
capable of maintaining a temperature of 90 - 95°C", die use of heating
blocks and microwave energy are permitted as acceptable heating
alternatives within the structure of the method. In addition, with the
incorporation of feedback control of the leachate temperature,
increasing the reproducibility of the measurement while automating
the process is easier.
However, in dedicated atmospheric pressure microwave
systems, only power control has been available until recently.
3
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Therefore, two new commercial systems of focused microwave power
with temperature feedback control were evaluated for EPA method
3050B. Configurations of single-mode and multi-mode microwave
equipment were used for adapting EPA method 3050B to a
microwave-assisted method. These temperature feedback controlled
and power controlled microwave digestors were compared to
conventional heating means. Their ability to accurately reproduce the
method's specified 95°C temperature, a key element in the leaching
protocol, was evaluated. Due to the homogeneity of the material,
standard reference materials (SRM 2704,2710, and 2711) were chosen
for monitoring the reproducibility of method 3050B.
The
environmentally important key elements - cadmium, chromium,
copper, lead, nickel, and zinc - were determined either using
inductively coupled mass spectrometry (ICP-MS) or atomic
absorption spectrometry (electrothermal (ET) - and flame (F) - AAS)
depending on the detection limits of the analytical instruments and
sample concentration. Elemental concentrations of samples leached
with controlled microwave heating were compared to elemental
concentrations of samples leached w ith conventional hot-plate
heating. This study evaluates the precision, accuracy, and efficiency
of the microwave adapted method 3050B. In addition, the evaluation
of this study was submitted to Analytical Chemistry and the manuscript
was accepted for publication (Appendix C) (8).
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13. Atmospheric Pressure Microwave Sample Preparation
Microwave sample preparation can be accomplished under
atmospheric pressure or under low or moderate pressure. Both
applications offer several advantages which will be discussed. The
terms "open" and "closed" vessel microwave digestions are
commonly used in the literature to indicate the difference in pressure
during sample digestion (3, 4, 9).
Focused single-mode or
atmospheric pressure microwave digestion is a better term to verify
the difference from closed vessel digestions in multi-mode microwave
oven. Multi-mode microwave digestor with slot technology is another
new development in microwave equipment which allows operation
under atmospheric pressure (10). The difference of both modes will
be summarized in an extra section!
Hot-plate and conventional open vessel dissolution systems are
limited to the boiling point(s) of the used acid(s) or azeotropic
mixtures. Atmospheric pressure microwave digestors are not limited
by the heating mechanisms of convection or conduction.
Superheating has been observed with temperatures from 5 to 38°C
above the boiling points of solvents (11,12). Refluxing is important in
open vessel dissolution procedures.
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For a better understanding of microwave heating, microwave
energy and the two main mechanisms for microwave heating are
briefly reviewed.
In the electromagnetic spectrum the microwave region lies
between infra-red (IR) radiation and radio frequencies with
corresponding wavelengths of 1 cm to 1 m and frequencies of 300,000
MHz to 300 Hz. Due to requirements by the Federal Communications
Commission and International Radio Regulations adopted at Geneva
in 1959, 2,450 MHz is the most common applied frequency for
industrial, medical, and scientific use (12).
Microwave energy is a non-ionizing radiation. The energy of
microwave radiation, 0.0016 eV at 2,450 MHz is low compared to
other radiation, such as X-ray radiation (1.25 x 105 eV at 3.0 x 1013
MHz) (3).
Dielectric loss and ionic conduction are the two main
mechanisms for microwave absorption and its conversion into heat.
The sample converts microwave energy into thermal energy at a rate
dependent on its dissipation factor, tan 5. The dissipation factor (or
loss tangent 8) is defined as the ratio between the sample's dielectric
loss e " to its dielectric constant e ' at a certain frequency and
temperature (12).
tan 8 = e " / e '
(Eq. 1)
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The dielectric loss, e ", is a measure of the samples' efficiency in
converting microwave energy into heat energy, and the dielectric
constant, e ', is defined as the ability of a molecule to be polarized by
an electric field. Table 1 shows dissipation factors for specific
materials used in a laboratory.
Table 1: Microwave Characteristics of Laboratory Materials (at 3000
MHz, 250O (3)
Material
tan 8 (xlO4)
Water
Aqueous Sodium
Chloride (0.1M)
Borosilicate Glass «
1570
2400
12-75
Quartz, Fused
0.6
Teflon™ PFA
1.5
Microwave energy causes molecular motion by ion
conductance and dipole rotation (13). Exposing a solution of ions to
an electromagnetic field causes the ions to migrate. This phenomena
is called ion conductance. The disturbance of free flow produces
friction and, as a result, heating of the solution. Ion conductance is
less frequency dependent than dipole rotation.
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Dipole rotation occurs in an electric field where molecules with
permanent or induced dipole moments align with the field. An
increase in the electric field enlarges the order in solution by aligning
the dipoles with die field. Conversely, a decrease in die field enlarges
the disorder. At microwave frequencies, the time in which the field
changes is almost equal to the response time of the dipoles. The
dipoles rotate but the resulting polarization lags behind the changes of
the electric field. As a result, the solution is heated due to a release of
thermal energy. At the oscillating microwave field of 2,450 MHz
frequency molecular dipoles align and disorder 4.9 x 109 times a
second. The rapid movement, forced by an electromagnetic field,
causes molecular friction (14).
Material in a microwave field can be characterized as reflective
(metals), absorptive (aqueous salt solutions) or transparent (Teflon™
PFA). Reflective material is used for the cavity walls, wave guide, and
mode stirrers (3). For highly conductive liquids containing large
amounts of salts at one point the conductive loss effects are larger than
the dipolar relaxation effects. The dissipation factor increases with an
increase in ion concentration and thus, the heating time will decrease
(3,12). Therefore, aqueous add solutions, ionized, charged and polar,
are an ideal medium for microwave absorption.
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13.1. Brief History of Microwave Sample Preparation
Microwave sample preparation has now been practiced over
two decades and has become a standard for sample dissolution (14).
A brief development of microwave digestions as a sample preparation
tool is given in Table 2 (15).
First acid digestions, in an open vessel with microwave
radiation as a heating source, were accomplished by using home
appliances (16). Biological samples in a nitric-perchloric acid mixture
placed in an Erlenmeyer flask were digested in a domestic microwave
oven. These microwave ovens were modified and equipped with
strong acid scrubbers to prevent corrosion and allow open vessel
digestions. Acid fumes were more or less successfully trapped or
vented. Domestic microwave ovens require modifications allowing
safe operation to overcome corrosion and damage to the magnetron or
electronics (17).
Moreover, the controlled energy input and
temperature measurements are lacking. These drawbacks in safety
features consequently led to the development of specially designed
equipment for analytical use focusing on digestions in closed vessels.
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A broad spectrum of sample matrices/ such as geological (1820)/ metallurgical (21), biological (9/ 22,23), and botanical samples (24)
can be digested using microwave energy. The number of applications
is numerous, including drying (25-27), ashing (28, 29), organic
extraction (30), and organic synthesis (12, 31). Several reviews
summarize different aspects of microwave digestion (32-37). Mingos
published a review about applications of microwave dielectric heating
effects to synthetic problems in chemistry (12).
Today, the analytical chemist can choose between microwave
equipment operating under atmospheric, low or moderate pressure,
or using a flow-through system (3-5). The microwave process can be
controlled by pressure or temperature feedback (38).
10
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Table 2: Laboratory Microwave Equipment and Application
Evolution (15)
1975
1979-84
1984
1985
1986
1986
1986
1986
1987
1987
1988
1988
1989
1989-90
1989-92
1990
1991
1993
1992
1994
1995
1995
1995
1995-96
Domestic microwave oven open beaker heating of adds/samples
Gosed polycarbonate tube and dosed Teflon™ PFA vessel add digestions
Temperature and pressure measurement in situ demonstrated
Commercial dosed vessel microwave system introduced
Commercial focused atmospheric pressure microwave introduced
Flow through microwave digestion demonstrated
Microwave extraction demonstrated
Fundamental dosed vessel sample decomposition paper
Measurement of oxidation temperatures of organics in microwave systems
Sample Preparation IR-100 award for microwave instrumentation
First book on microwave sample preparation
Draft methods 3051 and 3015 presented to EPA
Commercial pressure feedback control introduced
Validation of EPA method 3051, adoption by CERCLA
Automation of EPA microwave methods
Simultaneous temperature and pressure feedback control first introduced
Demonstration of solvent superheating in microwave systems
Commercial temperature feedback control introduced
Commercial flow through microwave instrumentation introduced
Development of environmental microwave leach standards
Total microwave sample processing
Approval of EPA RCRA methods 3051 and 3015
Development and proposed of EPA method 3052
Conversion of EPA methods 3050B, 3031,3060A to microwave heating
11
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(16)
(39,40)
(22)
(41))
(42)
(43)
(44)
(22)
(45)
(41)
(3)
(46)
(47)
(48)
(38,49-51)
(38)
(52,53)
(47)
(5)
(54)
(55)
(6)
(6)
(49-52)
1.3.2. Single-Mode Focused Microwave Sample Preparation
In 1987 tiie Prolabo Corporation developed the first commercial
version of a single-mode microwave digestor designed for operation
under atmospheric pressure (42). A single-mode digestor allows a
sample to be placed at positions of much higher electric field strengths
than can be obtained in a multimode oven. In these so-called focused
microwave digestors, one vessel is directly placed in a special
designed microwave guide which by its geometry produces a
standing wave where the microwave intensity is very high in a small
(1-2 mm) region (4).
Therefore, only the lower part of the vessel is exposed to
microwave energy. The upper part of the vessel serves as a reflux
system and can prevent the loss of volatile elements. The exact
positioning of the wave antipode is important. Microwave radiation,
which is not absorbed by the sample, will be annulled by the
geometry of the waveguide or the attenuator through which the vessel
is placed. The arrangement of the proportion for aperture and
dimension of the waveguide is adjusted by using a tuning device.
Through this design, the volume power density to which the sample is
exposed is generally ten times that of conventional multimode
cavities. This technique is different from a multi-mode cavity oven,
where the energy is distributed among several vessels and
consequently, a high homogeneity of applied energy results.
12
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Figure 1 : Schematic of Single-Mode Focused Microwave Digestor
with Gas Bulb Thermometer (Prolabo)
I
Sample.
Vessel
[
Wavelength
Attenuator
Magnetron
O
.Wayg Guide
,
h.
Gas Bulb
Thermometer
13
Focused microwave digestor applies continuously adjustable
percentage of maximum power to a sample. This differs from multimode cavity ovens, which are usually controlled by pulsed power.
The instruments are calibrated by the manufacturer, using a
continuous flow calorimeter. During two calibration runs at low
(1 0 %) and high power settings (1 0 0 %), a thermister measures the inlet
and outlet temperature of water. A precision flow meter ensures an
unchanging flow rate, since a decrease in the flow rate leads to an
increase in temperature due to a longer residence time.
The microwave energy applied to the sample can be either
controlled by power settings or by temperature feedback control.
Heating programs for power controlled instruments are edited by
applying a specified percentage of the maximum power output for a
certain predetermined time perioci. Generally, digestions or leaching
protocols require a certain temperature. Microwave digestors with
temperature feedback can precisely control reaction conditions.
Focused single-mode microwave digestors are commercially available
having different temperature feedback controls to measure the
temperature either inside (gas bulb thermometer) or outside (IR
sensor) the sample vessel (Figures 1,2). The temperature can be used
to control the power or to monitor temperature conditions. The
procedure, using non-invasive temperature control, can easily be fully
automated. Figure 3 shows two atmospheric pressure microwave
systems used in this study (A301, Prolabo Corporation and Star
System 2, CEM Corporation).
14
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Atmospheric pressure microwave digestion offers several
advantages as compared to pressurized digestions. Some of these
advantages are:
•
safe handling of samples with high carbon content due to
operation under atmospheric pressure,
• effective handling of gas forming matrices or reagents,
•
sequential and automated incremental addition of reagents
during digestion,
• working at high temperatures >300°C, for example, with
boiling sulfuric add,
•
handling of larger and dynamic sample sizes,
• performing of catalyst-free digestions, and
•
using glass, quartz, or Teflon™ vessels.
On the other hand, atmospheric pressure microwave digestion
has some disadvantages in comparison to pressurized digestions.
These disadvantages indude:
•
limitation of digestion temperature by boiling points of adds or
solvents,
•
requirement of higher add or solvent volume, and
•
longer digestion time.
15
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Figure 2 : Single-Mode Microwave Digestor with Temperature
Feedback Control
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Figure 3: Atmospheric Pressure Microwave Systems:
a) A 301, Prolabo, and b) Star System 2, CEM
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1.3.3. Multi-Mode Microwave Digestor with Slot-Technology
In 1996 a newly designed atmospheric pressure microwave
digestor operating with a special slot-technology was presented at die
Pittsburgh Conference in Chicago (10) (Figure 4). A magnetron
produces microwave energy for two or six single cavities, which are
positioned along the length of a waveguide. Each cavity has a
microwave slot controlling the amount of microwave energy entering
the cavity.
The vessels are strategically positioned to achieve
maximum coupling of microwaves with the sample. Each cavity can
be operated independently. Temperature feedback is obtained
simultaneously from both cavities by non-invasive vessel control. IR
sensors measure the radiation emitted through a hole in the bottom of
the cavity under the flask. Both cavities can be individually calibrated
by using a low and a high boiling point of solvents, usually
concentrated nitric and sulfuric add.
1.3.4. Applications of Atmospheric Pressure Microwave Sample
Preparation
Applications are increasing for open vessel atmospheric
pressure sample preparation. For example, common applications are
phosphorus and Kjeldahl nitrogen determination (56,57), digestion of
oils, polymers (4), and food-samples (58), and flow-injection on-line
digestions of arsine in blood (59).
Kohlrausch vessels and specially designed Teflon™ reflux
18
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Figure 4: CBA Star System 2 with
Microwave Slot T echnology
Sample
Tubes
Glass
Liner
Magnetron
Waveguide
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vessels were used by White for biological sample digestions in nitric
add/hydrogen peroxide mixtures. An eighty percent reduction of
digestion time was reported (17). Krushevska et al. reported on zinc
determination in milk comparing different digestion procedures with
or without microwave assistance (58). Feinberg et al. described
microwave digestion of food samples with sulfuric ad d followed by
hydrogen peroxide without the addition of a catalyst. Results of a
collaborative study on the determination of nitrogen in flour, milk and
casein are reported. Sample preparation times were considerably
reduced as compared to conventional methods.
Furthermore,
Feinberg et al. developed a fully automated open vessel focused
microwave digestion expert system (60).
A tested procedures
database was constructed from -800 digestion procedures validated
by several application laboratories. The system is specific to the
Kjeldahl nitrogen determination in foods.
Other applications of single-mode microwave digestion are
extractions of elemental species, such as chromium (VI), selenium or
tin (61, 62). Organic compounds, for example polycyclic aromatic
hydrocarbons (PAHs), and polychlorobiphenyls (PCBs), can be
extracted with low power settings and an average recovery of 83% in
reduced time (63,64).
1.4. Comparison to Alternative Instrumentation
The typical multi-mode microwave oven consists of a
magnetron (microwave generator), a wave guide, a microwave cavity,
20
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a mode stirrer, a circulator and a turntable (Figure 5). A wave guide
transports the microwave radiation produced by die magnetron to the
cavity. A mode stirrer distributes the incoming energy. Vessels are
placed on a turntable, which alternately rotates back and forth 180° to
360° to obtain uniform heating of all vessels. A terminal circulator
directs reflected energy to a dummy load to prevent magnetron
damage. The power output of a magnetron is generally controlled by
'cycling' to achieve an average level of power. Most magnetrons in
microwave ovens for analytical use operate at a time base of Is. The
magnetron has to be on and off for 0.5 s to obtain 50 % power output
Vessels are commonly made of Teflon™ PFA, microwave
transparent material, including a vessel body and vessel cap with a
safety relief valve incorporated. Depending on die vessel design they
can be used for digestions under low
(< 1 0
atm) or moderate pressure
(10-80 atm). One or more vessels can be monitored and feedback
controlled by temperature and/or pressure.
Pressurized microwave sample preparation offers several
advantages. In microwave transparent vessels, reaction solutions
absorb the energy directly. Compared to atmospheric pressure
microwave heating, higher temperatures are achieved because the
boiling point increases by the pressure produced in the vessel.
Consequently, digestion time decreases with higher temperature.
The amount of add used for the digestion can be reduced since
no evaporation occurs in a closed vessel. Volatile elements are
contained in the closed vessel and, in addition, so are the fumes
21
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Figure 5: Multi-Mode Cavity Type Microwave System (3)
Wave Guide
M agnetron
A ntenna
*
i
Mode
Stirrer
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produced by the digest. The source of airborne contamination is
minimized. Heat-up and cool down times are reduced as compared to
conventional closed vessel methods.
On the other hand, closed vessel digestion has some
disadvantages.
In contrast to atmospheric pressure microwave
heating, concentrated sulfuric ad d cannot be used in dosed vessel
microwave units, since the commonly used vessel material, Teflon™
PFA, melts at -306°C temperature. Pressurized digestions can be
dangerous, espedally digestions of unfamiliar samples with high
contents of organic material, metallic components, peroxides, or
flammable solvents.
Microwave absorption by a liquid or by a gas phase is different.
In a gas phase, discrete ions are lacking. Therefore, only dipole
rotation is effective as a heating mechanism. Transfer of heat energy
through collisions of molecules in the gas phase does not occur as
readily as in liquid phase. Heat loss must be considered due to
thermal loss of the vapor phase through the vessel walls.
A
temperature gradient results from the warmest region of the vessel, in
contact with the liquid phase, to the coolest zone near the top of the
vessel. This temperature gradient causes condensing and refluxing
inside the vessel. A so-called "sustained, dynamic thermal non­
equilibrium" exists in a microwave system due to this spedal heating
mechanisms (70). In conventional heating systems, liquid, gas phase,
and vessel walls are in a thermal equilibrium.
23
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2. EPA METHOD 3050B
2.1. Digestion, Extraction, and Leaching
EPA method 3050B has been written for acid digestion of
sediments, sludges, and soils. In general, solid samples need to be
dissolved in order to determine elemental concentrations of interest by
using analytical instruments. Digestions of solid samples are usually
total, otherwise, the term sample dissolution should be used. Due to
insoluble silicate fractions in soil and sediment samples, and the
required acid mixture used for digestion, recoveries of leached
elements will be lower than the values for total digestion. Recoveries
can only achieve total values if an element is completely soluble in the
leaching solvent (6 8 ).
In the literature, all three terms - digestion, leaching, and metal
extraction - are used to describe the same process. Since these terms
differ in meaning, the definitions for extraction and leaching will be
briefly discussed.
Extraction is a method of separating the constituents of a
mixture using preferential solubility of one or more components in a
second phase. Usually this added phase is a liquid while the mixture
to be separated may be either solid or liquid. Liquid/solid extraction
may be defined as the dissolving of one or more components in a solid
matrix by simple solution, or by the formation of a soluble form
through chemical reaction. The field may be subdivided into the
24
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following categories: leaching, washing extraction, and diffusional
extraction.
Leaching involves the contacting of a liquid and a solid (usually
an ore) and the imposing of a chemical reaction on one or more
substances in the solid matrix so as to render them soluble (65). Thus,
leaching can be described as the removal of a soluble fraction, in the
form of a solution, from an insoluble, permeable solid with which it is
associated. Leaching is closely related to solvent extraction, in which
a soluble substance is dissolved from one liquid by a second liquid
immiscible with the first. Both leaching and solvent extraction are
often called extraction. In addition, the following terms are used for
leaching: solid-liquid extraction, lixiviation, percolation, infusion,
washing, and decantation-settling (6 6 ).
«
An environmental encyclopedia defines leaching as the process
by which soluble substances are dissolved out of a material (67).
Leaching is generally used in soil chemistry and refers to the process
by which nutrients in the upper layers of soil are dissolved and
carried into lower layers where they can be valuable nutrients for
plant roots. In the environmental field, the term is used to describe
the possible leaching of toxic chemicals stored in underground
containers. Leaching will be the common form of sample preparation
if the mobility of toxic metals from contaminated sites to adjacent
areas has to be monitored. In laboratories, leaching procedures are
simulated by using rigorous conditions to speed up the procedure
(67).
25
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Leaching is a term that has been applicable to the extraction of
metals from environmental samples and has entered the common
terminology of the EPA and the environmental analytical field. In
summary, the subtitle of EPA Method 3050B "Acid Digestion of
Sediments, Sludges, and Soils" is incorrect and should be changed to
"Acid Leaching". Therefore, in this study only the term leaching is
used to describe EPA method 3050B.
2.2. Acid Leaching
Acid leaching is a common form of sample preparation
releasing elemental species from the matrix as ions into solution for
analysis.
These leaching procedures simulate a w orst case
environmental leaching scenario, representing the most probable
pathway for human exposure. On the other hand, high concentration
of hydronium ions can be found in mine-tailings and acid-rain,
increasing the leachability and mobility of elements in the
environment.
For several purposes, labile and leachable
concentrations of analytes provide efficient information for an
environmental evaluation.
Method 3050B has traditionally been a leach test, performed on
a hot-plate (2). Leach studies using conventional EPA method 3050B
have been evaluated or compared to closed vessel microwave
digestions by Kammin, Binstock, and Kingston (48,69,70).
26
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23. Mineral A dd Dissolution
Add dissolution or wet ashing is the most frequent procedure
for dissolving samples for elemental analysis (71). The mineral adds/
nitric, hydrochloric, sulfuric, and perchloric ad d are commonly used
for sample dissolution. Sample and add(s) are generally heated in an
open vessel or beaker on a hot-plate for certain periods of time.
Depending on the sample matrix and the use of ad d or ad d mixture,
samples may be partly or totally decomposed.
Method 30506 requires leaching of the sample in nitric add,
hydrogen peroxide, and hydrochloric add. These reagents, applied in
method 3050B, will be discussed in detail. Additional adds, used for
sample dissolution, are briefly mentioned.
2.3.1. Nitric Add
Because of its oxidizing potential, the ability of dissolving most
metals, and the excellent solubility of nitrates, nitric a d d is most
frequently used for wet ashing. Some metals, such as aluminum, iron,
and chromium, are resistant to nitric add due to the formation of an
oxide film on the surface (72). Containers made of these "passive"
metals can be used for storing and transporting of nitric add.
Diluted nitric add has only a weak oxidizing potential. The
oxidizing power increases with acid concentration and temperature.
Under pressure, substantial increase in oxidizing power is achievable
at temperatures above the boiling point. Many organic compounds
can be decomposed depending on the molecular structure and the
27
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temperature used for dissolution (3). Nevertheless, residues of
incomplete oxidation can interfere with analytical techniques such as
electro-thermal atomic absorption spectroscopy, polarography, and
voltammetry (73).
Concentrated nitric add, an azeotropic mixture of
68%
nitric
add and 32% water, boils at 120°C and can easily be purified by sub­
boiling distillation. The add is light-sensitive, forming nitrogen
dioxide and coloring the add yellow (72).
Nitric add is often used in combination with hydrochloric add.
The mixture of concentrated nitric and hydrochloric add (1:3 v/v) is
known as aqua regia and nitric add oxidizes hydrochloric add to
nitrosyl chloride and chlorine.
HNO3 + 3 HC1 -> NOCI + CI2 + 2 H 2O
Aqua regia oxidizes many materials more effidently than nitric
add alone. Gold, insoluble in nitric add, can be dissolved in aqua
regia (72). For safety reasons, only freshly prepared aqua regia can be
used in a microwave oven (3).
23.2. Hydrochloric Add
Hydrochloric acid, a non-oxidizing acid, has a strong
complexing capadty. The concentrated add can dissolve certain metal
oxides and other metals that are more easily oxidized than hydrogen
(71). The add prevents predpitation of certain metal ions by forming
complexes with, for example, antimony or iron. In addition, many
chlorides are soluble.
28
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Hydrochloric acid is not capable of oxidizing organic
compounds, but is capable of dissolving basic compounds such as
amines or alkaloids in aqueous solutions. Another application of
hydrochloric ad d is the hydrolysis of amino adds and carbohydrates
(3).
As mentioned above, hydrochloric a d d is often used in
combination with nitric add.
Z3.3. Hydrogen Peroxide
Hydrogen peroxide is the first reduction product of oxygen and
hydrogen. Traces can be determined with titanyl sulfate, forming the
orange red compound peroxotitanylsulfate, Ti0 2 S0 4 . Pure hydrogen
peroxide is a viscous liquid. In water hydrogen peroxide is soluble in
all concentrations. Aqueous solutions of 3 to 30% hydrogen peroxide
are commerdally available, and are commonly stabilized with
phosphoric add or stannate. The dielectric constant of H20 2 is 93 at
25°C and 120, if diluted with 35% water (72).
Hydrogen peroxide autocatalytically decomposes into water
and molecular oxygen, releasing thermal energy.
H2O 2 -+ H20 + 1/2 O2 +23 kcal
The reagent has a strong oxidizing power in addic as well as in
alkaline solutions. In addic solutions, the oxidation is slower than in
alkaline solution.
Hydrogen peroxide is produced by autooxidation of 2-ethylanthrahydroquinone to the corresponding quinone, which can be
continuously reduced to hydroquinone in the presence of hydrogen
29
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and a palladium catalyst After extraction with water, the product is
separated from the organic phase and can be concentrated or purified
by distillation (72).
For sample dissolution, hydrogen peroxide is often used in
combination with nitric or sulfuric add.
2.3.4. Additional Reagents
The boiling point of sulfuric ad d (340°C) is high compared to
nitric acid (120°C).
Concentrated sulfuric a d d is a powerful
dehydrating reagent and decomposes most organic compounds. Also,
most metals and many alloys are attacked by the hot add (71), but
many sulfates are insoluble. Since sulfates can cause interference with
several analytical techniques, this add has a limited use.
Hydrofluoric acid is a non-oxidizing acid with good
complexing capability. It has to be added to an add mixture to
decompose siliceous materials in soils and sediments. The silicon is
then evolved as the tetrafluoride (71).
Hot concentrated perchloric acid has the most powerful
oxidizing potential. Care must be taken in using this reagent since
violent explosion can occur in contact with organic or easily oxidized
inorganic material (71).
30
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2.4. Scope and Application of Method 3050B
The method is written for the analysis of 23 elements (Ag, Al,
As, Ba, Be, Ca, Cd, Co, Cr, Cu, Fe, K, Mg, Mn, Mo, Na, Ni, Pb, Sb, Se,
Tl, V, and Zn) in sediments, sludges, and soils (Appendix A). Two
leaching procedures of the same material are required to analyze all 23
elements. Samples leached with hydrochloric arid can be determined
by F-AAS or ICP-OES. Without the addition of hydrochloric arid, the
leachate is prepared for analysis by ET-AAS or ICP-MS to reduce
interferences w ith the analytical spectrometer.
To improve the
recoveries of Sb, Ba, Pb, and Ag an optional procedure for these
elements is included in method 3050B, but is not required on a routine
basis.
2.5. Key Elements for Leaching Procedures
Important key elements for leaching are sample .matrix,
occurrence of elemental species, leach medium, leach time and
temperature conditions, and the pH of the sample-leach medium
mixture. The influence of temperature and leach medium will be
discussed in detail.
2.5.1. Leach Medium
Method 3050B requires arid leaching, operating at a low pH.
The leach medium consists of nitric arid, hydrochloric acid, and
hydrogen peroxide. The oxidizing nitric arid is usually used for arid
digestions and leaching due to the good solubility of most elements as
31
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nitrates. Under atmospheric pressure the oxidation potential of nitric
add is not strong enough to digest the sample in a short time period.
Therefore, another powerful oxidizer, hydrogen peroxide, has to be
added to increase the rate of leaching and to allow finishing the
method within a few hours. Hydrogen peroxide autocatalytically
decomposes into water, heat, and molecular oxygen. An additional
heating step with the non-oxidizing hydrochloric ad d is required to
enhance the recovery of certain elements by using this strong
complexer.
The reaction conditions and reagents have an influence on the
results of the leach analysis, with a large number of different spedes
being involved. During the leach procedure, the following chemical
spedes can possibly be formed in the reaction mixture:
HNO3 , H 2 O2, HC1, H 20 , H3 O+ NO3 -, NO2 , 0 2 , Cl", NOC1, CI2,
HOz-, 022", O2 ' and digest products (70).
The primary difference in results between 3050B and 3051, an all
nitric ad d method, is the lack of the complexing chloride ion. For
example, the addition of hydrochloric acid, a complexing add,
prevents the predpitation of Al and Fe by forming chloride complexes
with both elements.
The use of hydrogen peroxide complexes
vanadium and increases the results of V analysis. The differences
between both methods will be outlined in more detail in Section 2 .6 .
32
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2.5.2. Temperature and Leach Procedures
In sample preparation, temperature is a key parameter for all
leaching procedures, as well as for digestions and extractions.
Therefore, controlling the temperature is paramount to reproducing
leaching of elements. Leach studies are an assessment of worst-case
environmental scenarios where components of the sample become
soluble and mobile. Temperature is a primary parameter used to
increase the rate of leaching and to bring these tests into appropriate
duration for laboratory evaluation. Previously, most leach methods
have been accomplished in beakers on a hot-plate. These methods are
traditional, time consuming, fairly inefficient, and, in general,
imprecise.
During the early 1980's, fundamental research established
temperature control as the most 'significant contributor to leach test
error (40,74). These experiments focused on the analysis of simulated
nuclear waste glass materials. Temperature was determined to be the
dominant parameter in leaching uncertainty and imprecision. The
control of temperature to within ± 0.04%, instead of ± 1% over a 28day leach period, changed the inter-element leaching uncertainty from
50% to 3%. By substituting microwave control for conventional
heating devices, advantages are gained in efficiency and in reduced
waste.
These advantages are due to direct microwave energy
inductance into digest and leachate solutions.
33
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With the required leach medium and the required low
temperature (95°C) in method 3050B, only labile elements can be
extracted horn soil, sediment or sludge samples.
2.6. Comparison to EPA Method 3051
Closed vessel microwave digestions have become a standard
for EPA methods 3015,3051, and 3052 (2,3). The EPA method 3051 is
briefly discussed since this method was provided as an alternative to
method 3050 by the EPA (2). Method 3051 is specifically written for
microwave acid digestion of sludges, soils, sediments, and oils. In this
method samples are digested and leached using only concentrated
nitric acid as a leach solvent. The digestate is suitable for the
determination by atomic absorption and inductive coupled plasma
spectrometry. Most of the organic material can be digested under
moderate pressure due to the higher oxidation potential of nitric acid
with increasing pressure and temperature (175 ± 5°C). Residual
carbon has been reported and determined to be nitrobenzoic acid
isomers (75). Method 3050B requires a digestion time of 2 - 6 hours.
In method 3051, this time is reduced and optimized at 15.5 minutes.
Small sample sizes (0.25-0.5g) can be digested in less reagent volume
(10 mL nitric add) as compared with method 3050B (l-2g sample in 45
mL reagent mixture). Method 3051 is a generic method with excellent
effidency and reproducibility.
Nevertheless, the use of nitric add without hydrogen peroxide
or hydrochloric add changes the chemistry of the leach medium. As
34
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described in Section 2.5.1., the primary difference between 3050B and
3051 is the deficiency of the complexing chloride ion and thus, less
chemical species formed in the reaction mixture. Therefore, die results
of several elemental concentrations, such as aluminum or vanadium,
differ between method 3050 and 3051 (48, 70). The last revision of
method 3051 from June 1996 includes an optional procedure allowing
an alternative addition of hydrochloric add.
2.7. Microwave Adaptation
In comparison w ith indirect heating by convection and
conduction in hot-plate digestions, microwave radiation is directly
absorbed by the mechanisms of ionic conductance and dipole rotation.
Since the sample is heated directly, equilibrium conditions are
obtained much more rapidly. In 3050B, the sample preparation time
can be reduced from over 2.5 hours to one hour by using microwave
technology. As well as allowing greater throughput, this method may
also improve the reprodudbility, and decrease contamination as the
sample is exposed to the reagents and the atmosphere for a shorter
period of time. A comparative procedural outline of method 3050B
and modified microwave assisted 3050B is shown in Table 3. The
equipment used in this study is listed in Table 4.
35
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Table 3: Comparative Procedural Outlines of EPA Method 3050B
and Modified Microwave Assisted 3050B
Vessel:
Sample Size:
Reagents:
Time:
Procedure:
Microwave Assisted Method
3050B
Open microwave vessel at
atmospheric pressure
1-2 g
15 mL concentrated HNQj
17 mL HzO
5 mL concentrated HQ
10 mL 30% HzOz
60 minutes
Weigh sample into vessel
and add 10 mL 1:1 HN03.
Reflux at 95°C for 5 min.
Cool, then add 5 mL
concentrated HNQj.
Reflux at 95°C for 5 min.
Cool, then add 5 mL*
concentrated HN03.
Reflux at 95°C for 5-10 min.
Cool, add max. 10 mL 30%
HjOj.
Heat until effervescence is
minimal.
Reflux at 95°C for 5-10 min.
Cool, add 15 mL 1:2 HQ,
reflux at 95°C for 5 min.
Hot-plate
Method 30S0B
Usually a covered beaker or
flask at atmospheric pressure
1-2 g
15 mL concentrated HNQj
10 mL HzO
10 mL concentrated HC1
10 mL 30% H20 2
2-6 hours
Weigh sample into vessel and
add 10mLl:lHNOj.
Reflux at 95°C for 10-15 min.
Cool, then add 5 mL
concentrated HNQj.
Reflux at 95°C for 30 min.
Cool, then add 5 mL
concentrated HN03.
Reflux at 95°C for 30 min,
evaporate to 5 mL,
or heat for 2 hours.
Cool, add max. 10 mL 30%
Heat until effervescence is
minimal.
Evaporate to 5 mL or heat for
2 hours.
Cool, add 10 mL concentrated
HC1, reflux at 95°C for 15
min.
36
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3. EXPERIMENTAL SECTION
3.1. Reagents
Solutions were made up in 18 Mflcm water from a Bamstead
NanoPure system in all experiments (Bamstead, Dubuque, IA).
Concentrated nitric acid, concentrated hydrochloric acid, and
hydrogen peroxide 30% were obtained from Fisher Chemical (A.C.S.
Reagent Grade, Fisher, Pittsburgh, PA). The adds were sub-boiled
distilled before use, using either a quartz still (Milestone s.r.l., Sorisole,
Italy) or an all-PFA Teflon™ still, built in-house from Teflon™
components (Savillex Corporation, Minnetenka, MN). The Standard
Reference Materials 2704 (Buffalo river sediment), 2710 (Montana soil,
highly elevated trace element concentrations), and 2711 (Montana soil,
moderately elevated trace element concentrations) were obtained from
NIST (the National Institute for Standards and Technology,
Gaithersburg, MD).
3.2. Equipment
The atmospheric pressure microwave procedures were
performed using equipment from Prolabo Corporation (Paris, France)
and CEM Corporation (Matthews, NC). The sample leaching and
reaction control equipment used in this study are summarized in
Table 4. All microwave digestors, used in the study, operate at a
frequency of 2,450 MHz and have a maximum power output of 200 W
(A301), 300 W (401), 350 W (MX350) and 500 W (Star System 2).
37
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 4:
Instrum entation for Conventional and Microwave Assisted Method 3050B
Atmospheric pressure microwave assisted methods
Hot-plate
Temperature
control 1
Temperature
control 2
Temperature
control 3
Calibrated
power control
Heating
device
Microwave system
MX 350* with
temperature control
Microwave system
401* with
temperature control
Microwave system
Star System 2**with
Temperature control
Microwave system
A301* with
power control
Hot-plate with
power settings***
Temperature
measurement
1R sensor
(M 402*)
Gas bulb
thermometer
(Megal 500*)
IR sensor
Fiber optic sensorx
Thermocouple
XX
thermometer
Vessel
Quartz glass
250 mL
Dorosilicate glass
250 mL
Dorosilicate glass
250 mL
Dorosilicate glass
250 mL
Dorosilicate glass
250 mL
(* Prolabo, France)
(** CEM Corporation. Matthews, NC)
(*** Fisher Scientific, Pittsburgh, PA)
(x Luxtron 750, Santa Clara, CA)
(’“ Fluke 52, Paramus, NJ)
I
38
The microwave leaching with power control was accomplished
using an automated version of the Microdigester A301 consisting of a
TX32 programmer, an exhaust system (ASPIVAP) to evaporate and
neutralize the ad d fumes, a pump unit for the automatically addition
of three reagents, and a sample carousel for up to 16 samples. The
whole instrument was set up in an independent hood connected via
ventilation fan to a hood exhaust The temperature feedback control
unit Megal 500 (gas bulb thermometer) with a 401 Microdigester and a
MX350 Maxidigester with an IR temperature sensor M402 were used
for temperature controlled, microwave assisted leaching.
In addition, a CEM microwave instrument Star System 2 with
non-invasive temperature control (IR sensors) was tested.
The
instrument has a vapor containment system to scrub vapors produced
during the leaching process. Four reagents can automatically be
added in increments of 0.5 to 5.0 mL.
A Luxtron Model 750 with a fluoroptic temperature probe
(Luxtron, Santa Clara, CA) was used for temperature acquisition in
the microwave digestor with power control, for temperature accuracy
tests, and for calibration.
A Perkin Elmer atomic absorption
spectrometer PE1100A (Norwalk, CT) with both flame (F-AAS) (Table
5) and electrothermal modes (ET-AAS, HGA 300), and an inductively
coupled plasma mass spectrometer (ICP-MS, VG Plasma Quad 2STE,
Fisons Instruments, Beverly, MA) were used under standard
conditions in a class 1 0 0 0 , 1 0 0 , and
10
combination clean room facility
for the analysis of elemental concentrations.
39
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Table 5: Standard Conditions for F-AAS Determination with AAS Instrument PE 1100
Element Wavelength
BGC Slit
(nm)
Lamp
LampCurrent
Flame
Type
(mA)
Gases
Replicates/
Measurement
Integration
Time (sec)
Copper
325
D
0.7
HCL
5
C2H2/Air
5
1.0
Lead
217
D
0.7
HCL
5
C2H2/Air
5
1.0
Zinc
214.4
D
0.7
HCL
7
C2H2/Air
5
1.0
(BGC = Background Correction)
( D = Deuterium Lamp)
(HCL = Hollow Cathode Lamp)
40
The SRMs were dried in a drying-oven (GS-Blue M Electric/
Lindberg, Asheville, NC) for 2 hours at 110°C as specified in the SRM
certificates. For the conventional procedure, a Fisher Scientific hot­
plate was used (Fisher Scientific, Pittsburgh, PA). Glassware and
plastic containers were cleaned with detergents and soaked in diluted
hydrochloric and nitric arid before use.
3.3. Procedure for Conventional Hot-Plate EPA Method 3050B
A 1.0 sample, known to ± O.Olg, was weighed in an Erlenmeyer
flask and 10 mL HNO3 1:1 (v/v) was added. The solution was heated
to ~95°C without boiling on a hot-plate and this temperature was
maintained for 15 minutes. After cooling to less than 70°C, 5 mL
concentrated HNO 3 was added and the sample was refluxed for 30
minutes at ~95°C without boiling. This step was repeated a second
time. The sample was evaporated to ~5 mL without boiling. After
cooling to less than 70°C, 2 mL 18 MQ water was added, followed by
the slow addition of 10 mL H 2Q2 (30%). Care was taken to ensure that
losses did not occur due to excessively vigorous effervescence caused
by rapidly adding the strong oxidizer hydrogen peroxide. The
solution was then heated until effervescence subsided. After cooling
to less than 70°C, 5 mL concentrated HC1 and 10 mL 18 MQ water
were added and the sample was refluxed for 15 minutes without
boiling. After cooling to room temperature, the sample was filtered
and diluted to 100.0 mL using 18 MQcm water.
41
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
3.4. Procedure for Power Control Microwave Implementation of
Method 3050B
A 1.0 sample, known to ± O.Olg, was weighed in a borosilicate
glass vessel and 10 mL HNO 3 1:1 (v/v) was added. A microwave
digestion program consisting of 80 W for 2 minutes and 30 W for 5
minutes was applied by a Microdigester A301. After cooling to 70 ±
10°C, 5 mL concentrated HNO 3 was automatically added and a
second power program of 80 W for 2 minutes and 30 W for 5 minutes
was applied. This step was repeated a second time. After cooling to
70 ± 10°C, 3 mL H 2O2 (30%) was added slowly (2mL/minute) and a
third power program of 40 W for 5 minutes was applied. This step
was repeated twice. After cooling to 70 ± 10°C, 5 mL concentrated
HC1 in 10 mL 18 Mfl water was added and a fourth power program of
80 W for 2 minutes and 30 W for 5 minutes was applied. After cooling
to room temperature, the sample was filtered and diluted to 100.0 mL
using 18 Mflcm water.
3.5. Procedure for Temperature Feedback Controlled Microwave
Implementation of Method 3050B
A 1.0 sample, known to ± O.Olg, was weighed in a borosilicate
vessel or quartz glass vessel (Table 4) and 10 mL HNO 3 1:1 (v/v) was
added. A microwave temperature program consisting of heating the
solution to 95 ± 2°C in 2 minutes and maintaining the temperature for
5 minutes was applied. After cooling to 70 ± 5°C, 5 mL concentrated
HNO 3 was automatically added and a second temperature program of
42
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
heating the solution to 95 ± 2°C in 2 minutes and maintaining the
temperature for 5 minutes was applied. This step was repeated a
second time. After cooling to 70 ± 5°C 10 mL H 2 O2 (30%) was added
slowly (2 mL/minute addition by temperature control 1 and 2, and in
0.5 mL aliquots by temperature control 3). The solution was then
heated to 95 ± 2°C in 6 minutes and the temperature was maintained
for 5 minutes. After cooling to 70 ± 5°C, 5 mL concentrated HC1 in 10
mL 18
water was added and die solution was heated to 95 ± 2°C
in 2 minutes. This temperature was maintained for 5 minutes. After
cooling to room temperature, the sample was filtered and diluted to
100.0 mL using 18 MQcm water.
For the analysis by ET-AAS and ICP-MS, digestion procedures
as described above were performed without hydrochloric acid
addition, as is possible in the EPA procedure.
43
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4. RESULTS and DISCUSSION
41. Microwave Assisted Leaching with Power Control
For EPA Method 3050B, the procedure requires heating the
sample solution to 95°C and holding this approximate temperature
without boiling. At a temperature of 95°C, concentrated nitric add
should not boil. Since maintaining exactly 95°C on a hot-plate is
difficult, using a microwave digestor should make the procedure
easier. However, the temperature profile for the microwave assisted
modified method 3050B using power control, obtained by
simultaneous temperature measurement with a fiberoptic temperature
sensor, was more like that of hot-plate (Figure 6 ). A power program
of 80 W for 2 minutes and 30W for 5 minutes was applied for the
several reflux steps. The resultant temperature of 105 to 110°C was
found to be above the required 95°C. Several other power settings
have been tested with the result being either above or below the
required temperature of 95°C. Programming the power to maintain a
certain temperature is limited since microwave digestors with power
control are restricted in control due to the power increments (10 W
increments) and time settings (1 minute increments) available. The
dissipation factor of the sample solution is dependent on temperature
and ion concentration.
As ions are released, the microwave
absorption changes and the same power settings have an increased
absorption causing an increase in the dissipation factor. In addition,
heat loss and evaporation of reagents must be considered.
44
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
O
Q.
C
&
CD
Q_
I
I
I
with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 6: Temperature vs. Time Profile of MW Assisted 3050B
with Power Control
HNO
120
U
(U
M
100
t
80
2
80 W, 2 min
30 W, 5 min
60
6
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80 W, 2 min
30 W, 5 min
40
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11
20
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1 1 1
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30 40 50
Time (Min.)
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60
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70
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80
The alternative to this method is to control the temperature
rather than power during microwave leaching. Until 1994, power
control was the only form of control in dedicated atmospheric
pressure microwave systems. This factor was the single most limiting
feature of these early microwave systems, as also demonstrated in this
study.
4.2. Microwave Assisted Leaching with Temperature Feedback
Control
4.2.1 Temperature Measurement in a Microwave Field
Temperature feedback control makes atmospheric pressure
microwave systems much more capable.
Feedback control
compensates for changes in power absorption due to temperature,
ionic strength, and sample size. Since temperature measurement in a
microwave field is limited to devices which are transparent to the
field, classic temperature measurement devices such as mercury
thermometers or unshielded thermocouples cannot be used.
Thermocouples have to be shielded to prevent the interaction of the
metal with the RF field. The shielding has to be grounded to the
microwave cavity's wall; otherwise microwave energy can be
transmitted out of the cavity into the laboratory (22). The use of
shielded thermocouples is inappropriate as compared with its use in
closed vessel systems because the microwave field will be conducted
from the vessel on the surface of the shielding.
46
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Fiber optic thermometry is commonly used in microwave
cavity systems due to the absence of interaction with microwave
energy, and is compatible with open vessel systems (76). Typical
fiberoptic sensors operate in the visible or near visible spectral range.
They are suitable for measurements in the low to moderate
temperature range. A temperature sensitive rare-earth phosphor is
attached to the end of an optical fiber which is connected with the
instrument. Blue-violet light pulses are sent down the fiber causing
the phosphor to glow red. The decay of the fluorescence after each
pulse varies precisely with the temperature. The fluorescent decay
time is measured by a multipoint digital integration of the decay
curve. The same optical fiber transmits the excitation pulses and
returns the fluorescent signal to the instrument. The drawbacks are
the high costs of the equipment and the fragility of fiber optic probes.
Gas bulb thermometer and IR sensors are two alternatives
currently developed for the use in microwave digestors.
4.2.2. Gas Bulb Thermometer
The gas bulb thermometer is based on gas-law principles with
the temperature being proportional to the internal gas pressure. A
pressure transducer sends the acquired data to a RS 232C serial port.
The fragile gas bulb thermometer, made of borosilicate glass, is 50 cm
long and it has to be handled carefully. After calibration by the
manufacturer, the thermometer is suitable for temperatures from 2 0 to
500°C. This device measures the actual temperature inside the sample
47
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
flask. The gas bulb has to be fully covered by solution to assure an
accurate temperature measurement. The accuracy of temperature
measurement was checked by simultaneous temperature acquisition
with a fiber optic probe as shown in Figure 7. A short microwave
heating program was edited heating 45 mL water in two steps via
50°C to 95°C. In addition, the temperature versus time profile shows
the power applied during the short heating program of 45 mL H 2O.
Less than 30 Watts (10%) are needed to maintain 45 mL water at 95°C.
48
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 7: MW Heating Program (45 mL HzO) Using Temperature
Control (Gas Bulb Thermometer) and Temperature Accuracy
Acquisition with a Fiber Optic Sensor
C^as Bulb Thermometer
100
80
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3
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2
£
h
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Fiber Optic Sensor
60
40
Set Temperature
20
Power
0
0
49
5
10
15
20
Time (Min.)
25
4.23. IR Sensor
IR sensors were used for the non-invasive vessel control. The
intensity of IR-radiation emitted from the vessel base is measured
through a hole in the bottom of the cavity under the flask. In one
configuration (temperature control 1, Prolabo), the IR-radiation
emitted is reflected by a mirror at an angle of 90° towards the IR
detector. The radiation is converted by an energy transducer into an
electrical signal. The detector measures the spectral range between 8
and 14 jim. IR sensor calibration was accomplished by simultaneous
temperature acquisition with a fiber optic sensor or the gas bulb
thermometer. This technique allows the emissivity of the quartz glass
vessel used during the experiments to be corrected by an emissivity
factor (range 0 .1 0 - 1 .0 0 ± 0 .0 1 ) within the controlling software.
In the CEM microwave unit (temperature control 3), the IR
sensors are placed several centimeters below each vessel. These IR
sensors were individually calibrated by a low (concentrated nitric
add, b.p. 121°C) and high solvent boiling point (concentrated sulfuric
acid, b.p. 330°C) and verified by an independent fiber optic
measurement system. The low and high boiling points should differ
by at least 50°C.
A major advantage of the IR sensor is that no cross
contamination can occur between samples from this source since the
probe has no contact with the sample. Therefore, no deaning step is
required after each digestion as compared, for example, to a gas bulb
thermometer. Both IR sensors require frequent checking of the
50
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
calibration. The IR sensors also measure energy reflected or emitted
by any surface directly behind the vessel base, and therefore, the
sensors have a response that changes slightly with solution volume.
Table 6 shows a temperature accuracy test of the CEM unit
with an independent fiber optic temperature measurement system.
Table 6 : Temperature Accuracy Test of Temperature Control 3 (Set
Temperature 93°C, Solvent 40 mL H 20 )
Temperature
Cell One
Min/Max
Cell Two
Min/Max
Sensor
Temperature °C
IR Sensor
93.2 ± 0.88
91/95
91.7 ± 1.55
90/98
FO Sensor
92.6 ±2.06
92.6/101.1
96.6 ± 2.86
90.8/102.5
Temperature °C Temperature ®C Temperature °C
Figures 8-10 show temperature versus time profiles with either
temperature feedback control by gas bulb thermometer or IR sensors.
Temperature spikes, seen as result of overshooting the goal
temperature, are mainly observed with temperature control 2 and 3,
primarily due to deficiencies in the control algorithms (Figure 9-10).
Adjustments of the feedback algorithm could improve their
temperature control. Less major thermal and power spikes are
observed with temperature control 1. Except for the spikes, the
temperature is maintained with much greater accuracy. The
maximum temperature is maintained with much higher accuracy as
shown in Figures 8 and 10. Figures 8 and 9 also show power versus
time profiles. Since the CEM microwave digestor, used in this study,
doesn't monitor applied power, this profile is not displayed in Figure
10.
51
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 8: Temperature vs. Time Profile of MW Assisted 3050B
with Temperature Control 2 (Gas Bulb Thermometer)
HNO
L
100
0
8 0
\
60
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Power
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Thermometer
Set
Temperature .
20
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0
0
52
10
l-
20
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30
40
Tim e (M in.)
50
60
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 9: Temperature vs. Time Profile of MW Assisted 3050B
with Temperature Control 1 (IR Sensor)
HNO,
100
r
I
HCl
\
80
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2 £
5 *
2
£
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IR Sensor
60
Set
Temperature
40
s *
01
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Power
20
0
0
53
10
20
30
40
Tim e (M in.)
50
60
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 10: Temperature vs Time Profile of MW Assisted 3050B
w ith Temperature Control 3 (IR Sensor)
HNO
u
0
V
2(0
1H
QJ
Ph
Cell Two
Cell O ne
601
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1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1 1
10
20
30
40
50
Time (Min.)
54
i
1 1 1 1 1 1 1 1 1 1
60
70
80
Figure 10 shows a temperature versus time profile of the CEM
unit with the same program for both cells started at die same time.
The graph shows that there is a lag time between cell one and two. A
vessel in cavity two needs longer to cool dow n to a certain
temperature, thus leading to a different finishing time. In addition,
microwave cavities one and two are not independent, and reflected
microwave energy can enter die next cell. In Figure 11, a microwave
program was only applied to cell one of the instrument.
The
temperature versus time profile shows that the temperature in cell two
raised to 160°C with a vessel containing sulfuric acid.
This
phenomenon doesn't always occur to this extreme (Figure 12). In
general, the temperature would raise to 50°C or 60°C. This increase
has no effect on temperature, if both cells are in the heating mode. But
programs with cooling steps may interfere when one cell ,starts a
heating mode and the other cell pauses to cool down sample solution.
Power settings can be chosen to minimize the effect (here
30%/60%power). For safety reasons, if only one cell was used, the
instrument was always equipped with two vessels, filled with
solvents to absorb reflected power.
55
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 11: Temperature vs Time Profile at 95°
(Temperature Control 3, IR Sensor)
Temperature Recorded
in Cell Two ( H S O ) /
150
U
o
2v
Heating Program
40 mL H O
100
a,
£
H
Cell One
Cell Two
0
2
4
6
10
8
Time (Min.)
12
14
Figure 12: Tem perature vs Time Profile
(Tem perature Control 3, IR Sensor)
110
100
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3
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90
Set Tem perature 95°C
40 mL H O
80
70
60
Cell One
Cell Two
50
40
0
2
4
6
8
10
12
14
Time (Min.)
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
4.3. Temperature Control on a Hot-Plate
The regulation of a certain temperature on a hot-plate was
found to be much more difficult than with the use of microwave
digesters. Calibration of a hot-plate to produce 95°C in a single flask
resulted in the other flasks temperatures ranging from 85 to 118°C
(Figure 13).
Generally depending on the flask measured, the
distribution pattern, number of flasks, surface temperature, and air
movement, the temperature may vary by 35°C. This variation agrees
with results found by other researchers (77). The advantage of
preparing several samples at the same time (hot-plate) is counteracted
by the problem of regulating and controlling a certain temperature for
all sample flasks.
In comparison, the microwave IR sensor
(temperature control 1) is able to regulate and control 95°C with a
mean temperature of 95.4 ± 1.1°C (4 replicates with 300 data points in
5 minutes and standard deviation) (Figure 14).
Since hot-plate procedures are seldom automated, reagents
have to added manually.
On the other hand, reagents were
programmed to be added automatically on all microwave systems.
Temperatures and reagent speed of addition were coordinated
through sensor and software control. This coordination contrasts with
the manual hot-plate implementation. The microwave digestors can
automatically add up to three or four reagents with selectable reagent
speed of 0.5 mL to 10 mL per minute or selectable increments, starting
with 0.5 mL.
57
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Set Temperature 95 °C
Figure 13: Temperature Distribution on a Hot-Plate
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
44. Leachable Concentrations of the Analytes
4.4.1. Elemental Analysis
The results of metal analysis using a modified microwaveassisted 3050B (temperature control 1-3 and power control) and
conventional method 3050B are listed for SRMs 2704, 2710 and 2711
(8 ) in Tables 7-9. F-AAS analyses were performed for the elements
copper, lead and zinc.
Cadmium, chromium and nickel were
determined either by ICP-MS (temperature control) or ET-AAS
(power control and conventional method). Results are compared to
NIST leachable concentrations using method 3050 (6 8 ) and certified
values for total digestion are included for convenience.
Leach
concentrations for SRM 2704 are not provided by NIST. All data are
in the range of the NIST or other published leachable concentrations
(48) with minor exceptions. The NIST leach values are not certified
but are a compilation of a 17 laboratory collaborative leach study. In
the NIST study, all laboratories used conventional hot-plate
equipment
60
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 7: Results of the Analysis of NIST Standard Reference Material 2704 Using Method 3050B (8)
Element
Cu
n>
Zn
Cd
Cr
Ni
Atmospheric pressure microwave assisted methods
Temperature
Temperature
Power control
control 1
control 2
(p g g 'iS D )
(p
g g 'lS D )
(Pgg'±SD)
(% Recovery)
(% Recovery)
(% Recovery)
Temperature
control 3
(Pg g *1 SD)
(% Recovery)
(pgg’ iSD)
(% Recovery)
NIST* certified
values for total
digestion
(Pg g 195% Cl)
98.615.0
101 ±7
89 ± 1
9811.4
101.614.8
10012
102
90
99
103
101
160 ±2
14516
14517
134 + 5
14611
99
90
90
83
91
427 ±2
41113
405114
407 1 7.4
427 1 5
97
94
92
93
97
NA
3.510.66
3.710.9
3.0510.65
NA
3.4510.22
101
107
88
82±3
7912
8514
8218
8911
13515
61
58
63
61
66
42 ± 1
3611
3814
35.413
4412
95
61
Hot-plate
86
80
82
100
* 3050 and 3051 leach data are presented in Reference 48, NA - Not available
Temperature Control 1 - IR Sensor (Prolabo)
Temperature Control 2 - Gas Bulb Thermometer (Prolabo)
Temperature Control 3 - IR Sensor (CEM)
161117
438112
44.113.0
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 8: Results of the Analysis of NIST Standard Reference Material 2710 Using Method 3050B (8)
Element Atmospheric pressure microwave assisted methods
Temperature
Power control
Temperature
control
2
control 1
(Pg g ' ± SD)
(Pgg’ ± SD)
(Pg g *1 SD)
(% Recovery)
(% Recovery)
(% Recovery)
Gi
Pb
Zn
Cd
Cr
Ni
Hot-plate
Temperature
control 3
(Pg g ' * SD)
(% Recovery)
NIST teachable NIST certified
concentrations values for total
digestion
(Pg g *1 SD) using method
(% Recovery) 3050*(jig g 1) (Pg g ** 95% Cl)
(Range)
2640 1 60
2790 1 41
2480133
3080122
2910159
2700
89
95
82
104
99
(2400 - 3400)
56401117
5430172
5170134
5065 1 89
57201280
5100
102
98
93
92
103
(4300 - 7000)
6410174
5810134
6130127
6212184
62301115
5900
92
84
88
89
90
(5200 - 6900)
NA
20.311.4
20.210.4
17.810.8
NA
20
93
93
82
2011.6
1912
1812.4
20.910.5
23 10.5
19
51
49
46
54
59
(15 - 23)
7.810.29
1011
9.111.1
10.210.55
710.44
10.1
55
70
64
71
49
(8.8 -15)
5532 1 80
6952 1 91
21.810.2
(13 - 26)
Reference 68, *Non-certified values - for information only. NA - Not available
Temperature Control 1 - IR Sensor (Prolabo)
Temperature Control 2 - Gas Bulb Thermometer (Prolabo)
Temperature Control 3 - IR Sensor (CEM)
62
29501130
39*
14.311.0
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 9: Results of the Analysis of NIST Standard Reference Material 2711 Using Method 3050B (8)
Element Atmospheric pressure microwave assisted methods
Temperature
Temperature
Power control
control 1
control 2
(pgg'±SD)
(Pg
g ' ± SD)
(Pg
g
*
±
SD)
(% Recovery)
(% Recovery)
(% Recovery)
Cu
Pb
Zn
ca
Cr
Ni
Hot-plate
Temperature
control 3
(P gg'iS D )
(% Recovery)
NIST teachable NIST certified
concentrations values for total
digestion
(Pg g ' ± SD) using method
3050*
(pgg1
)
(Pg
8 ± 95% Cl)
(% Recovery)
(Range)
107 ±4.6
98±5
98 ±3.8
113 ±8.1
111 ±6.4
100
94
86
86
99
97
(91 -110)
1240 ±68
1130 ±20
1120 ±29
1119 ±60
1240 ±38
1100
107
97
96
96
107
(930 -1500)
330 ±17
312 ±2
307 ± 12
326 ±3.7
340± 13
310
94
89
88
93
97
(290 - 340)
39.6 ±3.9
40.9 ±1.9
39.4 ± 1.2
NA
40
95
98
94
22 ±0.35
21 ± 1
15 ±1.1
17.3 ± 1.3
23 ±0.9
20
47
45
32
37
49
(15 - 25)
15 ±0.2
17 ±2
15 ±1.6
15.5 ±0.75
16 ±0.4
16
73
83
73
75
78
(14 - 20)
NA .
1162 ±31
350.4 ±4.8
41.7 ±0.25
(32 - 46)
Reference 68, *Non-certified values - for information only. NA - Not available
Temperature Control 1 - IR Sensor (Prolabo)
Temperature Control 2 - Gas Bulb Thermometer (Prolabo)
Temperature Control 3 - IR Sensor (CEM)
63
114 ±2
47*
20.6 ±1.1
4.4.2. Leachable Recoveries of C ertified Values for Total
Concentrations
The certified values for total digestion of SRM's are the best
estimates for the true concentrations and elemental concentrations as
have been determ ined by two or more independent analytical
methods. Leachable recoveries of analytes are generally lower than
total concentrations, and recoveries can only be total if an element is
completely leachable.
The recoveries of leachable concentrations of the analytes as a
percentage of certified values for total digestions are listed in Tables 79 and are graphically displayed in Figures 15-17. Leachable recoveries
of the following elements, Cd, Cu, Pb, and Zn, are generally high
(>90%). Recoveries of the elements Ni (49-100%) and Cr (32-66%) are
generally lower, which is consistent with NI5T leachable values (6 8 ) or
other published data (48).
The recoveries obtained w ith tem perature control (gas bulb
thermometer and IR sensor) are slightly lower than the corresponding
values of power control and hot-plate. The lower values obtained by
using microwave-assisted modified temperature controlled 3050B can
be explained by more accurately controlled but relative lower
temperature during the leaching procedures, since the temperature
with power control and on the hot-plate were higher than the required
95°C.
64
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Fiigure 15: Percentage Recoveries of Elemental Concentrations
Using EPA Method 3050B (Table 7)
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F iigu re 16: P ercentage R ecoveries of E lem ental C oncentrations
U sing EPA M ethod 3050B (Table 8 )
SRM 2710
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Fiigure 17: Percentage Recoveries of Elemental C oncentrations
Using EPA Method 3050B (Table 9 )
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Figures 18 to 29 show examples for the precision of addleached elements (SRM 2710 and 2711) obtained with a microwaveassisted method 3050B (power control and temperature control 1-3, in
comparison to conventional Method 3050B and leach values by NIST.
The ranges of 95% confidence lim its for six analyzed elements
obtained w ith a microwave-assisted 3050B are frequently better, or
equivalent to the ranges of leach data provided by NIST, due to the
increased precision achievable with microwave-assisted leaching over
heat control by convection or conduction. Exemplary, Figure 30
shows recovery ranges on SRM 2710. The recoveries of six analytes
obtained by a microwave-assisted modified 3050B with temperature
control 1 (IR sensor) are compared to NIST leachable values. The
graph shows better precision for all analytes, obtained w ith
temperature-controlled microwave leaching.
Both designs of atmospheric pressure microwave equipm ent
produced sim ilar results, dem onstrating the appropriateness of
microwave induced heating and its ability to be implemented in very
different manners while producing consistent results.
68
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Copper
Add-Leached
SRM 2710
Figure 18:
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Figure 19:
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69
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Figure 20:
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Value
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Zinc
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SRM 2710
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Figure 24:
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Figure 26:
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Acid Leached
SRM 2710
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Nickel
Acid-Leached
SRM 2710
Figure 28:
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with permission of the copyright owner. Further reproduction prohibited without permission.
Figure 30: C om parison of L eachable R ecoveries (R anges) of
Six A nalytes on SRM 2710 in P ercentage of C ertified V alues,
M W 3050B w ith T em p eratu re C on trol 1 vs. N IST L eachable V alues
MW-IR sensor
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75
5. SUMMARY
This study demonstrates that microwave control is a very
efficient and effective alternative to conventional heating sources for
EPA Method 3050B. Moreover, the study shows the required leachtem perature of 95°C cannot be obtained and m aintained by
conventional hot-plates w ith an acceptable accuracy. In comparison
to microwave power control microwave temperature feedback control
is better at maintaining specific sample preparation temperatures.
Temperature feedback control microwave systems are capable of
controlling the temperature at the required 95°C with an accuracy of ±
2°C that is not achievable either by hot-plate or microwave power
control. In summary, precision is improved, leach time is reduced by
60%, and reagent addition is automated with the microwave-adapted
Method 3050B. As result, revision 2 of Method 3050B alternatively
includes the use of direct energy coupling devices. In addition, the
required leach temperature is now specified by a range of ± 5°C.
Furthermore, the data of this study are included in 3050B as reference
leach data (Appendix B).
Moreover, sample preparation, usually the time consuming
step in analytical chemistry, can be performed on a time scale more
convenient for analysis. These procedures may be automated or semiautomated and newer instrumentation w ith 2, 4, or
6
samples will
reduce the limitation in throughput. These newer instrumentations
need further testing and if neccessary improvements to guarantee
simultaneous processing of samples.
76
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
This application dem onstrates atm ospheric pressure
microwave sam ple preparation, applying tem perature feedback
control, is an appropriate alternative to traditionally implemented
convection and conduction heating on hot-plates. Today, additional
environmental methods, such as total decompositions of oils and
polymers in EPA m ethod 3031, or species extractions, such as
chromium (VI) in method 3060, are also being implemented using this
newer technology w ith similar improvements. Other traditional
sample preparation methods requiring control of reaction temperature
will find advantages in microwave tem perature feedback control
implementation.
77
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
6. REFERENCES
(1)
Majors, R. E. "An Overview of Sample Preparation", LC-GC
1991,9,16-20.
(2)
Test Methods for Evaluating Solid Waste-SW846, Update ID, 3. Ed.,
Washington, D.C., 1995.
(3)
Kingston, H. M.; Jassie, L. B., Eds., Introduction to Microwave
Sample Preparation: Theory and Practice; American Chemical Society:
Washington, D.C., 1988.
(4)
Grillo, A. C. "Microwave Digestion by Means of a Focused
Open-Vessel System", Spectroscopy 1989,4,16-21.
(5)
Haswell, S. J.; Barclay, D. A. "On-line Microwave Digestion of
Slurry Samples w ith Direct Flame Atomic Absorption Spectrometric
Elemental Detection", Analyst 1992,117,117-120.
(6 )
Federal Register, 1995; Vol. 60, No. 142,37974-37978.
(7)
Friedman, D. "Debating Performance Based Methods", Environ.
Lab. 1993, April/May, 37-39.
(8 )
Lorentzen, E. M. L.; Kingston, H. M. S. "A Comparison of
Microwave Assisted and Conventional Leaching Using EPA Method
3050B”, Analytical Chemistry, 1996,68,4316-4320.
(9)
Matusiewicz, H. "Add Vapor-Phase Pressure Decomposition
for the Determination of Elements in Biological Materials by Flame
Atomic Emission Spectrometry", J. Anal. At. Spectrom. 1989,4,265-269.
(10)
King, E. E.; "New Wave for Sample Preparation" presented at
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(11)
Bond, G.; Moyes, R. B.; Pollington, S. D.; Whan, D. A. "The
Superheating of Liquids by Microweave Radiation", Chem. Ind. 1991,
686-687.
(12)
Mingos, D. M. P.; Baghurst, D. R. "Applications of Microwave
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(13)
Copson, D. A. Microwave Heating; The Avi Publishing Cobalt.,
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(14)
Kingston, H. M.; Haswell, S. J. Microwave Enhanced Chemistry;
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(15)
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A ssisted Environm ental M easurement" in Microwave Enhanced
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Society: Washington, D.C., in press 1997.
(16)
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(17)
White, R. T.; "Open Reflex Vessels for Microwave Digestion:
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(18)
Hewitt, A. D.; Reynolds, C. M. "Dissolution of Metals from Soils
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At. Spectrosc. 1990,11,187-192.
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(19)
Alexander, W. R.; Shimmield, T. M. "Microwave Oven
D issolution of Geological Samples: Novel A pplication in the
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Alvarado, J.; Petrola, A. "D eterm ination of cadm ium ,
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(21)
M atthes, S. A.; "Guidelines for Developing Microwave
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L. B., Kingston, H. M., Eds.; ACS: Washington, D.C., 1988, pp 33-52.
(22)
Kingston, H. M.; Jassie, L. B. "Microwave Energy for A dd
Decomposition at Elevated Tem peratures and Pressures Using
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(23)
Lachica, M. "Use of microwave oven for the determination of
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Report D 4643 - 93; "Standard Test Method for Determination of
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American Sodety for Testing and Materials ASTM; September 1993.
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Beary, E. S. "Comparison of M icrowave D rying and
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Thompson, R. Q.; G hadiali, M. "Microwave Drying of
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Budke, C. C.; McFadden, D. G. "Rapid-Ash Procedure", Plast.
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Dry Ashing Procedures" presented at Pittsburgh Conference and
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Loupy, A.; Petit, A.; Ramdani, M.; Yvanaeff, C.; Majdoub, M.;
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Irradiation Using Dry-Media Conditions", Can. J. Chem. 1993, 71, 909S.
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Matusiewicz, H.; Sturgeon, R. E. "Present Status of Microwave
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M atusiewicz, H. "A Review of Acid Vapor-Phase Sample
Digestion of Inorganic and Organic Matrices for Elemental Analysis",
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Z ehr, B. D. "D evelopm ent of Inorganic M icrowave
Dissolutions", Am. Lab. 1992, December, 24-29.
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Kuss, H. M. "Applications of Microwave Digestion Technique
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Kingston, H. M.; W alter, P. J.; Settle, F. A.; Pleva, M. A.;
"Encapsulation and Transfer of Standard Methods Using Automated
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Report 120; "A Microwave System for the A dd Dissolution of
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(44)
Ganzler, K.; Bati, J.; Valko, K.; "A New Method for the
Extraction and H igh-Perform ance Liquid C hrom atographic
Determination of V idne and Convidne in Fababeans" presented at
Internation al E astern
E uropean-A m erican
Sym posium
of
Chromatography, 1984; 435-442.
(45)
Kingston, H. M.; Jassie, L. B.; "Monitoring and Predicting
Parameters in Microwave Dissolution" in Introduction to Microwave
Sample Preparation: Theory and Practice; Jassie, L. B., Kingston, H. M.,
Eds.; ACS: Washington, D.C., 1988, pp 93-154.
(46)
Report IAG# DWI-3993254-01-0; "Microwave M ethod
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Binstock, D. A.; Grohse, P. M.; Gaskill, A.; Sellers, C.; Kingston,
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Determining Elements in Solid Waste using Microwave Digestion", /.
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Settle, F. A.; Walter, P. J.; Kingston, H. M.; Pleva, M. A.; Snider,
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Microwave Dissolution. II. Electronic Transfer and Implementation of
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Dissolution. 1 . Selection of Analytical Descriptors", J. Chem. Inf.
Comput. Sci. 1989,29,11-17.
83
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Walter, P. J.; Kingston, H. M.; Settle, F. A.; Pleva, M. A.; Buote,
W.; Christo, J.; "Automated Intelligent Control of Microwave Sample
Preparation" in Advances in Laboratory Automation Robotics; Strimaitis, J.
R., Little, J. N., Eds.: Hopkinton, MA, 1990; Vol. 7, pp 405-416.
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of microwave heating in organic reactions" presented at American
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Majetich, G.; Neas, E.; Hooper, T.; "Proceedings of the First
World Congress on Microwave Chemistry" presented in Breukelen,
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(54)
Report "The Performance of Leaching Studies on Soil SRMs
2710 and 2711"; Duquesne University; April 5,1994.
(55)
Walter, P. J.; Kingston, H. M.; "Total Microwave Processing
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Collins, L. W.; Chalk, S. J.; Kingston, H M. S. "An Atmospheric
Pressure Microwave Sample Preparation Procedure for the Combined
Analysis of Total Phosphorus and Kjeldahl Nitrogen", Analytical
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Feinberg, M. H.; Ireland Ripert, J.; Mourel, R. M. "Optimization
Procedure of Open Vessel Microwave Digestion for Kjeldahl Nitrogen
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Krushevska, A.; Barnes, R. M.; Amarasiriwaradena, C. J.; Foner,
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for the Determination of Zinc in Milk by Inductively Coupled Plasma
84
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Atomic Emission Spectrometry", J. Anal. At. Spectrom. 1992, 7, 851 858.
(59)
Welz, B.; He, Y.; Sperling, M. "Flow Injection Online A dd
Digestion and Pre-Reduction of Arsenic for Hydride Generation
Atomic Absorption Spectrometry - A Feasibility Study", Taknta 1993,
40,1917-1926.
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Feinberg, M. H.; Suard, C.; Ireland Ripert, J. "Development of a
Fully Automated Open Vessel Focused Microwave Digestion System",
Chemom. Intell. Lab. Syst. 1994,22,37-47.
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Chalk, S. J.; Lorentzen, E. M. L.; Taylor, D.; Nogay, D.;
Kingston, H. M.; "Microwave Temperature Feedback Control for
Improved Leaching of Chromium(VI) in Draft EPA Method 3060A"
presented at Pittsburgh Conference, Chicago, IL 1996; No. 974.
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Donard, O. F. X.; Lalere, B.; Martin, F. M.; Munoz, R.; "Sample
Preparation for the Spedation of Tin, and Selenium Compounds
Using Microwaves Digestion Techniques" presented at Pittsburgh
Conferencce, Cchicago, IL 1994; No. 1147.
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Budzinski, H.; Garrigues, P.; Mathe, D.; "Microwave Assisted
Extraction of Polycyclic Aromatic Compounds from Standard
Reference M aterials and Sediments" presented at Pittsburgh
Conference, New Orleans, LA 1995; No. 1307.
(64)
Budzinski, H.; P ierard, C.; G arrigues, P.; M athe, D.;
"Microwave Assisted Extraction of Polychlorobiphenyls from
Standard Reference Materials and Sediments" presented at Pittsburgh
Conference, New Orleans, LA 1995; No. 1301.
85
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
(65)
Encyclopedia of Science and Technology, McGraw Hall Books
Company: New York, 1987; VoL 6.
(66)
Encyclopedia of Science and Technology, McGraw Hill Books
Company: New York, 1987; Vol. 9.
(67)
Cunningham, W. P., Ed., in Environmental Encyclopedia, Gale
Research Inc.: Detroit, 1994.
(68)
Report "Addendum to the Certificate of Analysis for SRM's
2709, 2710, 2711"; National Institute of Standards and Technology;
August 23,1993.
(69)
Kammin, W. R.; Brandt, M. J. "Simulation of EPA Method 3050
Using a High-Temperature and High-Pressure Microwave Bomb",
Spectroscopy 1989,4,22-24.
(70)
Kingston, H. M.; W alter, P. J. "Comparison of Microwave
versus Conventional Dissolution for Environmental Applications",
Spectroscopy 1992,7,20-27.
(71)
Skoog, D. A.; West, D. M.; Holler, F. J. Fundamentals of Analytical
Chemistry, 6. ed.; Saunders College Publishing: Philadelphia, PA, 1992.
(72)
Schmidt, M. Anorganische Chemie; BI Hochschultaschenbuecher:
Mannheim, 1967.
(73)
Jackwerth, E.; Gomiscek, S. "Acid Pressure Decomposition in
Trace Element Analysis", Pure & Appl. Chem. 1984,56,479-489.
(74)
Liggett, W. S.; W., I. K. G.; "Pilot Studies for Im proving
Sampling Protocols" in Principles of Environmental Sampling; Keith, L.
H., Ed.; American Chemical Society: Washington, D.C., 1996.
86
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(75)
Pratt, K. W.; Kingston, H. M.; MacCrehan, W. A.; Koch, W. F.
"Voltammetric and Liquid Chromatographic Identification of Organic
Products of Microwave-Assisted Wet Ashing of Biological Samples",
Anal. Chem. 1988,60,2024-2027.
(76)
Wickersheim, K. A.; Sun, M. H. "Fiberoptic Thermometry and
its Applications", /. Micro. Power 1987,22,85-93.
(77)
Kane, J. S. "Leach data v's total: what is relevant for SRM's",
Fresenius' J. Anal. Chem. 1995,352,209.
87
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Appendix A
EPA Method 3050:
Acid Digestion of Sediments, Sludges, and Soils
In
Test Methods for Evaluating Solid Waste-SW846f
September 1986.
88
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METHOO 3050
ACID DIGESTION OF SEDIMENTS. SLUOGES. AND SOILS
1.0
SCOPE AND APPLICATION
1.1
. This method I s an a d d d ig estio n procedure used to p repare s e d i­
ments, sludges, and s o i l samples f o r a n a ly s is by flame o r furnace atomic
absorption spectroscopy (FLAA and GFAA, re s p e c tiv e ly ) o r by Inductively
coupled argon plasma spectroscopy (ICP)J
Samples prepared by t h i s method may
be analyzed by ICP f o r a l l th e l i s t e d m etals, o r by FLAA o r GFAA as Indicated
below (see a lso Paragraph 2 . 1 ) :
FLAA__________-___
A1um1num
Barium
Beryllium
Cadmium
Calcium
Chromium
Cobalt
Copper
Iron
Lead
2.0
Magnesl um
Manganese
Molybdenum
Nickel
Potassium
Sodium
Thallium
Vanadium
Z1nc
GFAA
A rsenic
Beryl 11 um
* Cadmium
Chromium
Cobalt
Iron
Molybdenum
Selenium
Thallium
Vanadlum
SUMMARY OF METHOO
2.1
A r e p r e s e n ta tiv e 1- to 2-g (wet weight) sample i s d ig ested in n i t r i c
acid and hydrogen p ero x id e. The d lg e s ta te Is then re flu x e d with e i t h e r n i t r i c
acid o r hydrochloric a c id .
D ilu te hydrochloric a d d Is used as the fin al
re flu x acid fo r (1) th e ICP a n a ly s is of As and Se, and (2) th e flame AA o r ICP
a n a ly s is o f Al, Ba, 8 e , Ca, Cd, Cr, Co, Cu, Fe, Mo, Pb, N1, K, Na, Tl, V, and
Zn. D ilute n i t r i c a d d 1s employedas the fin a l d i l u t i o n a d d f o r the furnace
AA a n a ly sis o f As, Be, Cd, Cr, Co,
Pb, Mo, Se, Tl, and V. A se p a ra te sample
sh all be d ried f o r a t o t a l s o lid s determ ination.
3.0
INTERFERENCES
3.1
Sludge samples can contain diverse m atrix ty p e s , each o f which may
p resen t I t s own a n a l y t i c a l challenge.
Spiked samples and any relev an t
standard reference m a te ria l should be processed to a id In determ ining whether
Method 3050 Is a p p lic a b le to a given waste.
3050 - 1
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0
Date September 1986
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4.0
APPARATUS AND MATERIALS
4.1
4.2
4.3
4 .4
4.5
4.6
5.0
Conical P h i l l ip s beak ers; 250-mL.
Watch g l a s s e s .
Drying ovens: That can be maintained a t 30*C.
Thermometer: That covers range o f 0 to 200*C.
Whatman No. 41 f i l t e r paper (o r e q u iv a le n t) .
C entrifuge and c e n trifu g e tu b e s .
REAGENTS
5.1 ASTM Type II water
im p u ritie s.
(ASTM DU93):
Water
should be monitored fo r
5.2 Concentrated n l t r i e a c i d , reagent grade (HNO3 ) :
Acid should be
analyzed to determine level of im p u ritie s .
I f method blank is <M0l, the acid
can be used.
5.3 Concentrated hydrochloric a c i d , reagent grade (HC1): Acid should be
analyzed to determine level of im p u ritie s .
I f method blank 1s <M0L, the acid
can be used.
5.4 Hydrogen peroxide (30S)
determine level of im p u ritie s .
6.0
(H2 O2 ) :
Oxidant
should
be analyzed to
SAMPLE COLLECTION, PRESERVATION, ANO HAN0LING
6.1 All samples must have been c o lle c te d using a sampling plan th a t
addresses the co nsid eration s discussed in Chapter Nine o f t h i s manual.
6.2 All sample co ntain ers must be prewashed w ith d e te rg e n ts , acid s, and
Type II w ater. P l a s t i c and g la s s c o n tain ers are both s u i t a b l e . See Chapter
Three, Section 3 .1 .3 , f o r f u r th e r Inform ation.
6.3 Nonaqeuous samples sh a ll be
as soon as p o s s ib le .
7.0
re frig e ra te d
upon r e c e ip t and analyzed
PROCEDURE
7.1 Mix the sample thoroughly to
achieve homogeneity.
d ig e stio n procedure, weigh to th e n e a re st 0 . 0 1
g and t r a n s f e r to
beaker a 1 . 0 0 - to 2 . 0 0 -g po rtion o f sample.
For each
a conical
7.2 Add 10 mL o f 1:1 HNO3 , mix the s l u r r y , and cover with a watch g la s s .
Heat the sample to 95*C and r e flu x f o r 10 to 15 mln w ithout b o ilin g . Allow
the sample to cool, add 5 mL o f concentrated HNO3 , re p la c e the watch g la s s ,
and re flu x f o r 30 mln.
Repeat t h i s l a s t s te p to ensure complete o xidation.
3050 - 2
Revision
Date Seotember I5S6
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Using a ribbed watch g la s s , allow the s o lu tio n to evaporate to 5 mL without
b o ilin g , while maintaining a covering o f so lu tio n over the bottom of the
beaker.
7.3 A fte r Step 7.2 has been completed and th e sample has cooled, add 2
ml o f Type I I water and 3 mL o f 30S H2 O9 . Cover th e beaker with a watch g la s s
and re tu rn th e covered beaker to th e not p l a t e f o r warming and to s t a r t the
peroxide r e a c tio n . Care must be taken t o ensure t h a t lo sse s do not occur due
t o ex cessiv ely vigorous efferv escen ce.
Heat u n t i l effervescence subsides and
cool th e beaker.
7 .4 Continue to add 30S H9 O2 In 1 -ml a liq u o ts with warming u n t i l the
efferv escence I s minimal o r u n til th e general sample appearance Is unchanged.
NOTE: Do not add more than a t o t a l o f 10 mL 30X H2 O2 .
7.5 I f th e sample 1s being prepared f o r (a) th e ICP a n a ly s is o f As and
Se, o r (b) th e flame AA o r ICP a n a ly s is o f A1, Ba, Be, Ca, Cd, Cr, Co, Cu, Fe,
Pb, Mg, Mn, Ho, Nl, K, Na, Tl, V, and Zn, then add 5 mL o f concentrated HC1
and 10 mL o f Type II water, re tu rn th e covered beaker to the hot p l a t e , and
r e f lu x f o r an additional 15 mln w ithout b o ilin g .
A fte r cooling, d i l u t e to
100 mL with Type II w ater.
P a r ti c u l a t e s 1n the d l g e s t a t e t h a t may clog the
n e b u liz e r should be removed by f i l t r a t i o n , by c e n t r i f u g a t io n , o r by allowing
the sample to s e t t l e .
7 .5 .1 F i l t r a t i o n : F i l t e r through Whatman No. 41 f i l t e r paper (or
eq u iv alen t) and d i l u t e to 100 mL w ith Type I I w a te r.
1s
7 .5 .2 C entrifugation: C en trifu g a tio n a t 2,000-3,000 rpm f o r
u su ally s u f f i c i e n t to c l e a r th e su p ern atan t.
10
mln
7 .5 .3 The d ilu te d sample has an approximate acid co n centration of
5.05 (v/v) HC1 and 5 .OS (v/v) HNO3 .
The sample 1s now ready fo r
a n a ly s is .
7.6 I f th e sample Is being prepared f o r the furnace a n a ly sis o f As, Be,
Cd, Cr, Co, Pb, Mo, Se, Tl, and V, cover th e sample with a ribbed watch g lass
and continue heating the ac 1 d-perox 1 de d l g e s t a t e u n t i l the volume has been
reduced to approximately 5 mL. A fte r cooling, d i l u t e to 100 mL with Type II
w ater. P a r tic u la te s 1n the d l g e s t a t e should then be removed by f i l t r a t i o n , by
c e n tr if u g a tio n , o r by allowing th e sample to s e t t l e .
7 .6 .1 F i l t r a t i o n : F i l t e r through Whatman No. 41 f i l t e r paper (or
eq u iv alen t) and d i l u t e to 100 mL with Type I I w ater.
7 .6 .2 C entrifug atio n: C e n trifu g a tio n a t
u su ally s u f f i c i e n t to c l e a r th e -su p e rn a ta n t.
2,000-3,000 f o r
10
mln 1s
7 .6 .3 The d ilu te d d l g e s t a t e s o lu tio n c o n ta in s approximately 5S
(v/v) HNO3 . For a n a ly s is , withdraw a liq u o ts o f a p p ro p riate volume and
add any required reagent o r m atrix m odifier. The sample Is now ready fo r
a n a ly s is .
3050 - 3
Revision
Oate September 1986
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
7. 7
Ca l c u l a t i o n s ;
7.7.1 The co n cen tratio n s determined are to be reported on the basis
o f the actual weight o f the sample. I f a dry weight a n a ly s is i s d esired,
then the percent s o lid s o f th e sample must also be provided.
7 .7 .2 I f percent s o li d s 1s d e sire d , a sep arate determ ination of
p ercent s o lid s must be performed on a homogeneous a liq u o t o f the sample.
8 .0
QUALITY CONTROL
8.1 For each group o f samples processed, prep aratio n blanks (Type II
w ater and reagents) should be c a r r i e d throughout the e n t i r e sample p rep aratio n
and a n a ly tic a l p rocess. These blanks w ill be useful in determ ining i f samples
are being contaminated.
8 .2 Duplicate samples should be processed on a ro u tin e b a s is . Ouplicate
samples w ill be used to determine p r e c is io n . The sample load w ill d i c t a t e the
frequency, but 2 0 X 1 s recommended.
8.3 Spiked samples o r stand ard reference m a te ria ls must be employed to
determine accuracy. A spiked sample should be included with each group of
samples processed and whenever a new sample m atrix i s being analyzed.
8 .4 The concentration o f a l l c a l ib r a t io n standards should be v e r if ie d
a g a in s t a q u a lity control check sample obtained from an o u tsid e source.
9.0
METHOD PERFORMANCE
9.1
No data provided.
10.0 REFERENCES
10.1 None req u ired .
3050 - 4
Revision
0
Oate September 1986
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
MfiTHOO 3030
*CIO OICCSTXOn Of SCOXneNTS. suuooes. AHQ SOICS
I
■
Mis
• •mo l a . t l k (
1 -2 a a o r tio n
f o r aacn
d ia a s e i o n
7 .2
Add h n O j
• no r a f l u a :
r a f l u a m ie n
e a n e a n c ra e cd
m n Oj ,' r i a a i i
>
C w a e a ra c a
•a lu e ia n ta
7.3
i
Add
Tyoa
It
- • e a r ana Mf Ot;
■ a ra f a r
o a r o a ld a r a a e t
7 .4
*d o h »o
a n d warm u n e i
a ffa rv a a e a n e a
to a i n l a a l
)
3050 - 5
Revision
0
Date September 1986
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
tC* a n a l y s t s of as ana
an Mana aa o r xc*
a n a l y s t s a * a i . aa. 6a.
aa. Ca. Ca. Cr. Ca. Cu.
. r*. no. hq. m , mo. m .
?u**n«cc a n a l y s t * a t
as. a*. Ca, Cr. Co. *a.
ho. So. Tl. ana v
Tro* of
a n a ly sts ?
7.8
In
aao
can eancratca
h c l ana Tyco : :
- o e o r ; rcluo
7 .8
7 .5
Cael:
atlu c a
■ t t n Tyoc I Z
••to r;
rlite r
a s r t t e u l a t s s in
tno a t g e s t a t a
O llu ta « lt n
Tyoa 1 X u a t a r
F tlta r
aartteu lstts
tn a t g o s t a t o
ana
7 .5
Cantlnua
naactng ta
r t a u e a *olun*
7.8
*. ns. Tl. v.
? .7 .t
C o tam in o
aorconc
s o l t a s an
nanaganaeus
sanota sltouae
t a r calculatio n
?.7.a
OatorMina
c a n c a n t r a t tana:
r a o o r t oarcant
a o l t a a of
s an o la
3050 - 6
Revision
Date Seotemfcer 1985
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3.3
METHOOS FOR DETERMINATION OF METALS
This manual c o n ta in s
six
a n a ly tic a l
techniques fo r tr a c e metal
d e te rm in a tio n s : In d u ctiv e ly coupled argon plasma em ission spectrometry (ICP),
d l r e c t - a s p l r a t i o n o r flame atomic ab so rp tio n spectrom etry (FAA), g ra p h ite furnace atomic ab sorption spectrometry (GFAA), h y d rld e-g eneratio n atomic
ab so rp tio n spectrom etry (HGAA), cold-vapor atomic absorption spectrometry
(CVAA), and several procedures f o r hexavalent chromium a n a l y s is . Each o f
th e se 1 s b r i e f l y d iscussed below In terms o f advantages, disadvantages, .and
cau tio n s f o r a n a ly s is o f w astes.
ICP's primary advantage 1s t h a t
I t allow s simultaneous o r rapid
s e q u e n tia l determ ination o f many elements in a s h o r t time.
The primary
disadvantage of ICP I s background r a d ia tio n from o t h e r elements and th e plasma
g a se s. Although a l l ICP Instruments u t i l i z e h lg h - r e s o lu tlo n o p tic s and back­
ground c o r re c tio n to minimize these I n te r f e r e n c e s , a n a ly s is f o r tr a c e s o f
m etals 1 n th e presence o f a larg e excess o f a s in g le metal i s d i f f i c u l t .
Examples would be t r a c e s o f metals 1 n an a llo y o r t r a c e s o f metals In a limed
(high calcium) waste.
ICP and Flame AA have comparable d e te c tio n lim its
(w ithin a f a c to r o f 4) except t h a t ICP e x h i b i t s g r e a t e r s e n s i t i v i t y fo r
r e f r a c t o r i e s (Al, Ba, e t c . ) .
Furnace AA, In g e n e ra l, w ill e x h i b i t lower
d e te c tio n l i m it s than e i t h e r ICP o r FLAA.
FIame AAS (FLAA) determ inations, as opposed to ICP, a re normally
completed as s in g le element analyses and a re r e l a t i v e l y fre e o f Interelem ent
s p e c tra l In te rf e re n c e s .
E ither a n ltro u s -o x ld e /a c e ty le n e o r a i r /a c e ty l e n e
flame 1 s used as an energy source f o r d i s s o c i a t i n g th e a s p ira te d sample into
th e f r e e atomic s t a t e making analyte atoms a v a i l a b l e f o r absorption o f l i g h t.
In th e a n a ly s is of some elements the tem perature o r type o f flame used 1s
c ritic a l.
I f th e p ro p er flame and a n a l y t ic a l co n ditio ns a re not used,
chemical and Io n iz atio n In te rfe re n c e s can o ccu r.
G raphlte Furnace AAS (GFAA) rep laces t h e flame with an e l e c t r i c a l l y
heated g ra p h ite furnace. The furnace allows f o r gradual heating o f the sample
a li q u o t In several s ta g e s .
Thus, th e p ro cesses o f d eso lv a tio n , drying,
decomposition o f organic and Inorganic molecules and s a l t s , and formation of
atoms which must occur 1n a flame o r ICP 1n a few m illiseco n d s may be allowed
to occur over a much longer time period and a t c o n tr o lle d temperatures 1 n the
fu rn ace.
This allows an experienced a n a l y s t to remove unwanted matrix
components by using tem perature programming a n d /o r m atrix m o d ifiers. The
major advantage o f t h i s technique Is t h a t 1 t a f f o r d s extremely low detectio n
l i m i t s . I t i s the e a s i e s t to perform on r e l a t i v e l y clean samples. Because
t h i s technique 1 s so s e n s i t i v e , In te rfe re n c e s can be a re a l problem; finding
th e optimum combination o f d ig e stio n , h e a tin g tim es and tem peratures, and
m atrix m odifiers can be a challenge f o r complex m a tric e s .
THREE - 6
Revision
Date September 1986
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Hydride AA u t i l i z e s a chemical reduction to reduce and sep arate arsenic
o r selenium s e l e c t i v e l y from a sample d i g e s t a t e . The technique th e re fo re has
the advantage o f being able to i s o l a t e th ese two elements from complex samples
which may cause In te rf e re n c e s f o r o th e r a n a ly tic a l procedures. S ig n ific a n t
in te rfe re n c e s have been reported when any o f th e following Is p re se n t: 1)
e a s ily reduced m etals (Cu, Ag, Hg): 2) high co n c e n tra tio n s o f t r a n s i t i o n
metals (>200 mg/L); 3) oxidizing agents (oxides o f nitrogen) remaining
following sample d ig e s tio n .
Cold-Vaoor AA uses a chemical red uction to reduce mercury s e le c t iv e ly .
The procedure 1s extremely s e n s itiv e b u t i s su b je c t to in te rf e r e n c e s from some
v o l a t i l e o rg an ics, c h lo r in e , and s u lf u r compounds.
THREE - 7
Revision
Oate Seotember 1986
Reproduced with permission o f the copyright owner. Further reproduction prohibited without permission
Appendix B
EPA Method 3050B:
Add Digestion of Sediments, Sludges, and Soils
In
Test Methods for Evaluating Solid Waste-SW846,
Revision 2., December 1996.
96
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
METHOD 3050B
ACID DIGESTION OF SEDIMENTS. SLUDGES. AND SOILS
1.0 SCOPE AND APPLICATION
1.1
This method has been written to provide two separate digestion procedures, one for
the preparation of sediments, sludges, and soil samples for analysis by flame atomic absorption
spectroscopy (FLAA) or inductively coupled plasma atomic emission spectroscopy (ICP-AES) and
one for the preparation of sediments, sludges, and soil samples for analysis of samples by Graphite
Furnace AA (GFAA) or inductively coupled plasma m ass spectrometry (ICP-MS). The extracts from
these two procedures are not interchangeable and should only be used with the analytical
determinations outlined in this section. Samples prepared by this method may be analyzed by ICPAES or GFAA for all the listed metals as long as the detecion limits are adequate for the required
end-use of the data. Alternative determinative techniques may be used if they are scientifically valid
and the QC criteria of the method, including those dealing with interferences, can be achieved.
Other elements and matrices may be analyzed by this method if performance is demonstrated for
the analytes of interest, in the matrices of interest, at the concentration levels of interest (See
Section 8.0). The recommended determinative techniques for each element are listed below:
FLAA/ICP-AES
Aluminum
Antimony
Barium
Beryllium
Cadmium
Caldum
Chromium
Cobalt
Copper
Iron
Lead
Vanadium
GFAA/ICP-MS
Magnesium
Manganese
Molybdenum
Nickel
Potassium
Silver
Sodium
Thallium
Vanadium
Zinc
Arsenic
Beryllium
Cadmium
Chromium
Cobalt
Iron
Lead
Molybdenum
Selenium
Thallium
1.2
This method is not a total digestion technique for most samples. It is a very strong
add digestion that will dissolve almost all elements that could become “environmentally available.”
By design, elements bound in silicate structures are not normally dissolved by this procedure as they
are not usually mobile in the environment. If absolute total digestion is required use Method 3052.
2.0 SUMMARY OF METHOD
2.1
For the digestion of samples, a representative 1-2 gram (wet weight) or 1 gram (dry
weight) sample is digested with repeated additions of nitric add (HNOO and hydrogen peroxide
(H A ).
2.2
For GFAA or ICP-MS analysis, the resultant digestate is reduced in volume while
heating and then diluted to a final volume of 100 mL
2.3
For ICP-AES or FLAA analyses, hydrochloric add (HCI) is added to the initial
digestate and the sample is refluxed. In an optional step to increase the solubility of some metals
(see Section 7.3.1: NOTE), this digestate is filtered and the filter paper and residues are rinsed, first
3050B -1
Revision 2
December 1996
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with hot HC1 and then hot reagent water. Filter paper and residue are returned to the digestion flask,
refluxed with additional HCI and then filtered again. The digestate is then diluted to a final volume
of 100 mL
2.4
determination.
If required, a separate sample aliquot shall be dried for a total percent solids
3.0 INTERFERENCES
3.1
Sludge samples can contain diverse matrix types, each of which may present its own
analytical challenge. Spiked samples and any relevant standard reference material should be
processed in accordance with the quality control requirements given in Sec. 8.0 to aid in determining
whether Method 3050B is applicable to a given waste.
4.0 APPARATUS AND MATERIALS
4.1
Digestion Vessels - 250-mL
4.2
Vapor recovery device (e.g., ribbed watch glasses, appropriate reftuxing device,
appropriate solvent handling system).
4.3
Drying ovens - able to maintain 30°C + 4°C.
4.4
Temperature measurement device capable of measuring to at least 125°C with
suitable precision and accuracy (e.g., thermometer, IR sensor, thermocouple, thermister, etc.)
4.5
Filter paper - Whatman No. 41 or equivalent.
4.6
Centrifuge and centrifuge tubes.
4.7
Analytical balance - capable of accurate weighings to 0.01 g.
4.8
Heating source - Adjustable and able to maintain a temperature of 90-95°C. (e.g., hot
plate, block digestor, microwave, etc.)
4.9
Funnel or equivalent.
4.10
Graduated cylinder or equivalent volume measuring device.
4.11
Volumetric Flasks - 1 00-mL.
5.0 REAGENTS
5.1
Reagent grade chemicals shall be used in all tests. Unless otherwise indicated, it is
intended that all reagents shall conform to the specifications of the Committee on Analytical
Reagents of the American Chemical Society, where such specifications are available. Other grades
may be used, provided it is first ascertained that the reagent is of sufficiently high purity to permit its
use without lessening the accuracy of the determination. If the purity of a reagent is questionable,
analyze the reagent to determine the level of impurities. The reagent blank must be less than the
MDL in order to be used.
3050B - 2
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5.2
Reagent Water. Reagent water will be interference free. All references to water in
the method refer to reagent water unless otherwise specified. Refer to Chapter One for a definition
of reagent water.
5.3
Nitric acid (concentrated), HN03. Add should be analyzed to determine level of
impurities. If method blank is < MDL, the add can be used.
5.4
Hydrochloric add (concentrated), HCI. Add should be analyzed to determine level
of impurities. If method blank is < MOL, the add can be used.
5.5
Hydrogen peroxide (30%), H2 O2 . Oxidant should be analyzed to determine level of
impurities. If method blank is < MOL, the add can be used.
6.0 SAMPLE COLLECTION, PRESERVATION, AND HANDLING
6.1
All samples must have been collected using a sampling plan that addresses the
considerations discussed in Chapter Nine of this manual.
6.2
All sample containers must be demonstrated to be free of contamination at or below
the reporting limit Plastic and glass containers are both suitable. See Chapter Three, Step 3.1.3,
for further information.
6.3
possible.
Nonaqueous samples should be refrigerated upon receipt and analyzed a s soon as
6.4
It can be difficult to obtain a representative sample with wet or damp materials. Wet
samples may be dried, crushed, and ground to reduce subsample variability as long as drying does
not affed the extraction of the analytes of interest in the sample.
7.0 PROCEDURE
7.1
Mix the sample thoroughly to achieve homogeneity and sieve, if appropriate and
necessary, using a USS #10 sieve. All equipment used for homogenization should be cleaned
according to the guidance in Sec. 6.0 to minimize the potential of cross-contamination. For each
digestion procedure, weigh to the nearest 0.01 g and transfer a 1-2 g sample (wet weight) or 1 g
sample (dry weight) to a digestion vessel. For samples with high liquid content, a larger sample size
may be used as long as digestion is completed.
NOTE: All steps requiring the use of adds should be conducted under a fume hood by
property trained personnel using appropriate laboratory safety equipment. The use of an add
vapor scrubber system for waste minimization is encouraged.
7.2
For the digestion of samples for analysis by GFAA or ICP-MS, add 10 mL of 1:1
HN03i mix the slurry, and cover with a watch glass or vapor recovery device. Heat the sample to
95°C ± 5°C and reflux for 10 to 15 minutes without boiling. Allow the sample to cool, add 5 mL of
concentrated HN03, replace the cover, and reflux for 30 minutes. If brown fumes are generated,
indicating oxidation of the sample by HN03, repeat this step (addition of 5 mL of conc. HNO^ over
and over until no brown fumes are given off by the sample indicating the complete reaction with
HN03. Using a ribbed watch glass or vapor recovery system, either allow the solution to evaporate
to approximately 5 mL without boiling or heat at 95°C ± 5°C without boiling for two hours. Maintain
a covering of solution over the bottom of the vessel at all times.
3050B - 3
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NOTE: Alternatively, for direct energy coupling devices, such as a microwave, digest
samples for analysis by GFAA or ICP-MS by adding 10 mL of 1:1 HN03, mixing the slurry
and then covering with a vapor recovery device. Heat the sample to 95°C ± 5°C and reflux
for 5 minutes at 95°C ± 5°C without boiling. Allow the sample to cool for 5 minutes, add 5
mL of concentrated HN03, heat the sample to 95°C ± 5°C and reflux for 5 minutes at 95°C
± 5°C. If brown fumes are generated, indicating oxidation of the sample by HN03, repeat
this step (addition of 5 mL concnetrated HN03) until no brown fumes are given off by the
sample indicating the complete reaction with HN03. Using a vapor recovery system, heat
the sample to 95°C ± 5°C and reflux for 10 minutes at 95°C ± 5°C without boiling.
7.2.1 After Step 7.2 has been completed and the sample has cooled, add 2 mL of
water and 3 mL of 30%
Cover the vessel with a watch glass or vapor recovery device
and return the covered vessel to the heat source for warming and to start the peroxide
reaction. Care must be taken to ensure that losses do not occur due to excessively vigorous
effervescence. Heat until effervescence subsides and cool the vessel.
NOTE*Alternatively, for direct energy coupled devices: After the Step 7.2 NOTE: has
been completed and the sample has cooled for 5 minutes, add slowly 10 mL of 30%
H20 2. Care must be taken to ensure that losses do not occur due to excessive
vigorous effervesence. Go to Step 7.2.3.
7.2.2 Continue to add 30% H20 2 in 1-mL aliquots with warming until the
effervescence is minimal or until the general sample appearance is unchanged.
NOTE: Do not add more than a total of 10 mL 30% H20 2.
7.2.3 Cover the sample with a ribbed watch glass or vapor recovery device and
continue heating the acid-peroxide digestate until the volume has been reduced to
approximately 5 mL or heat at 95°C ± 5°C without boiling for two hours. Maintain a covering
of solution over the bottom of the vessel at all times.
NOTE: Alternatively, for direct energy coupled devices: Heat the acid-peroxide
digestate to 95°C ± 5°C in 6 minutes and remain at 95°C ± 5°C without boiling for
10 minutes.
7.2.4 After cooling, dilute to 100 mL with water. Particulates in the digestate should
then be removed by filtration, by centrifugation, or by allowing the sample to settle. The
sample is now ready for analysis by GFAA or ICP-MS.
7.2.4.1
equivalent).
Filtration - Filter through Whatman No. 41 filter paper (or
7.2.4.2
Centrifugation - Centrifugation at 2,000-3,000 rpm for
10 minutes is usually sufficient to clear the supernatant.
7.2.4.3
The diluted digestate solution contains approximately 5% (v/v)
HN03. For analysis, withdraw aliquots of appropriate volume and add any required
reagent or matrix modifier.
7.3
For the analysis of samples for FLAA or ICP-AES, add 10 mL conc. HQ to the sample
digest from 7.2.3 and cover with a watch glass or vapor recovery device. Place the sample on/in
the heating source and reflux at 95°C ± 5°C for 15 minutes.
3050B - 4
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December 1996
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NOTE: Alternatively, for direct energy coupling devices, such as a microwave, digest
samples for analysis by FLAA and ICP-AES by adding 5 mL HCI and 10 mL H20 to the
sample digest from 7.2.3 and heat the sample to 95°C ± 5°C, Reflux at 95°C ± 5°C without
boiling for 5 minutes.
7.4
Filter the digestate through Whatman No. 41 filter paper (or equivalent) and collect
filtrate in a 100-mL volumetric flask. Make to volume and analyze by FLAA or ICP-AES.
NOTE: Section 7.5 may be used to improve the solubilities and recoveries of antimony,
barium, lead, and silver when necessary . These steps are optional and are not
required on a routine basis.
7.5
Add 2.5 mL conc. HN03 and 10 mL conc. HCI to a 1-2 g sample (wet weight) or 1 g
sample (dry weight) and cover with a watchglass or vapor recovery device. Place the sample on/in
the heating source and reflux for 15 minutes.
7.5.1 Filter the digestate through Whatman No. 41 filter paper (or equivalent) and
collect filtrate in a 100-mL volumetric flask. Wash the filter paper, while still in the funnel,
with no more than 5 mL of hot (-95°C) HCI, then with 20 mL of hot (-95°C) reagent water.
Collect washings in the same 100-mL volumetric flask.
7.5.2 Remove the filter and residue from the funnel, and place them back in the
vessel. Add 5 mL of conc. HCI, place the vessel back on the heating source, and heat at
95 °C ± 5°C until the filter paper dissolves. Remove the vessel from the heating source and
wash the cover and sides with reagent water. Filter the residue and collect the filtrate in the
same 100-mL volumetric flask. Allow filtrate to cool, then dilute to volume.
NOTE- High concentrations of metal salts with temperature-sensitive solubilities can
result in the formation of precipitates upon cooling of primary and/or secondary
filtrates. If precipitation occurs in the flask upon cooling, do not dilute to volume.
7.5.3 If a precipitate forms on the bottom of a flask, add up to 10 mL of
concentrated HCI to dissolve the precipitate. After precipitate is dissolved, dilute to volume
with reagent water. Analyze by FLAA or ICP-AES.
7.6
Calculations
7.6.1 The concentrations determined are to be reported on the basis of the actual
weight of the sample. If a dry weight analysis is desired, then the percent solids of the
sample must also be provided.
7.6.2 If percent solids is desired, a separate determination of percent solids must
be performed on a homogeneous aliquot of the sample.
8.0 QUALITY CONTROL
8.1
All quality control measures described in Chapter One should be followed.
8.2
For each batch of samples processed, a method blank should be earned throughout
the entire sample preparation and analytical process according to the frequency described in Chapter
One. These blanks will be useful in determining if samples are being contaminated. Refer to
Chapter One for the proper protocol when analyzing method blanks.
3050B - 5
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8.3
Spiked duplicate samples should be processed on a routine basis and whenever a
new sample matrix is being analyzed. Spiked duplicate samples will be used to determine precision.
The criteria of the determinative method will dictate frequency, but 5% (one per batch) is
recommended or whenever a new sample matrix is being analyzed. Refer to Chapter One for the
proper protocol when analyzing spiked replicates.
8.4
Limitations for the FLAA and ICP-AES optional digestion procedure. Analysts should
be aware that the upper linear range for silver, barium, lead, and antimony may be exceeded with
some samples. If there is a reasonable possibility that this range may be exceeded, or if a sample’s
analytical result exceeds this upper limit, a smaller sample size should be taken through the entire
procedure and re-analyzed to determine of the linear range has been exceeded. The approximate
linear upper ranges for a 2.00-g sample size:
Ag
2,000 mg/kg
As 1,000,000 mg/kg
Ba
2,500 mg/kg
Be 1,000,000 mg/kg
Cd 1,000,000 mg/kg
Co 1,000,000 mg/kg
Cr 1,000,000 mg/kg
Cu 1,000,000 mg/kg
Mo 1,000,000 mg/kg
Ni 1,000,000 mg/kg
Pb 200,000 mg/kg
Sb 200,000 mg/kg
Se 1,000,000 mg/kg
Tl 1,000,000 mg/kg
V 1,000,000 mg/kg
Zn 1,000,000 mg/kg
NOTE: These ranges will vary with sample matrix, molecular form, and size.
9.0
METHOD PERFORMANCE
9.1
In a single laboratory, the recoveries of the three matrices presented in Table 2 were
obtained using the digestion procedure outlined for samples prior to analysis by FLAA and ICP-AES.
The spiked samples were analyzed in duplicate. Tables 3-5 represents results of analysis of NIST
Standard Reference Materials that were obtained using both atmospheric pressure microwave
digestion techniques and hot-plate digestion procedures.
10.0
REFERENCES
1.
Rohrbough, W.G.; et al. Reagent Chemicals. American Chemical Society Specifications. 7th
ed.; American Chemical Society: Washington, DC, 1986.
2.
1985 Annual Book of ASTM Standards. Vol. 11.01; "Standard Specification for Reagent
Water"; ASTM: Philadelphia, PA, 1985; D1193-77.
3.
Edgell, K.; USEPA Method Study 37 - SW-846 Method 3050 Add Digestion of Sediments.
Sludges, and Soils. EPA Contract No. 68-03-3254. November 1988.
3050B - 6
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December 1996
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4.
Kimbrough, David E., and Wakakuwa, Janice R. Add Digestion for Sediments. Sludoes.
Soils, and Solid Wastes. A Proposed Alternative to EPA SW 846 Method 3050. Environmental
Sdence and Technology, Vol. 23, Page 898, July 1989.
5.
Kimbrough, David E., and Wakakuwa, Janice R. Report of an Interlaboratorv Study
Comparing EPA SW 846 Method 3050 and an Alternative Method from the California Department
of Health Services. Fifth Annual Waste Testing and Quality Assurance Symposium, Volume I, July
1989. Reprinted in Solid Waste Testing and Quality Assurance: Third Volume, ASTM STP 1075,
Page 231, C.E. Tatsch, Ed., American Sodety for Testing and Materials, Philadelphia, 1991.
6.
Kimbrough, David E., and Wakakuwa, Janice R. A Study of the Linear Ranges of Several
Acid Digestion Procedures. Environmental Sdence and Technology, Vol. 26, Page 173, January
1992. Presented Sixth Annual Waste Testing and Quality Assurance Symposium, July 1990.
7.
Kimbrough, David E., and Wakakuwa, Janice R. A Study of the Linear Ranges of Several
Acid Digestion Procedures. Sixth Annual Waste Testing and Quality Assurance Symposium,
Reprinted in Solid Waste Testing and Quality Assurance: Fourth Volume, ASTM STP 1076, Ed.,
American Sodety for Testing and Materials, Philadelphia, 1992.
8.
NIST published leachable concentrations. Found in addendum to certificate of analysis for
SRMs 2709, 2710, 2711 - August 23,1993.
9.
Kingston, H.M. Haswell, S.J. ed., Microwave Enhanced Chemistry. ACS Symposium Series,
American Chemical Sodety, Washington, D.C., Chapter 3, in press.
3050B - 7
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TABLE 1
STANDARD RECOVERY*
Percent Recovery
a
Analyte
3050A
Ag
As
Ba
Be
Cd
Co
Cr
Cu
Mo
Ni
Pb
Sb
Se
Ti
V
Zn
9.5
86
97
96
101
99
98
87
97
98
97
87
94
96
93
99
3050B w/option
98
102
103
102
99
105
94
94
96
92
95
88
91
96
103
95
All values are percent recovery. Samples: 4 mL of 100 mg/mL multistandard; n = 3.
3050B - 8
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December 1996
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TABLE 2
Percent Recovery*-*
Ag
As
Ba
Be
Cd
Co
Cr
Cu
Mo
Ni
Pb
Sb
Se
Tl
V
Zn
Samole 4435
Samole 4766
Samole HJ
3050A 3050B
3050A 3050B
3050A 3050B
3050A 3050B
56
83
b
99
95
89
72
70
87
87
77
46
99
66
90
b
27
77
81
99
93
89
83
77
83
92
81
32
85
74
87
87
9.8
70
85
94
92
90
90
81
79
88
82
28
84
88
84
96
103
102
94
102
88
94
95
88
92
93
92
84
89
87
97
106
15
80
78
108
91
87
89
85
83
93
80
23
81
69
86
78
89
95
95
98
95
95
94
87
98
100
91
77
96
95
96
75
93
102
b
94
97
93
101
106
103
101
91
76
96
67
88
b
Averaae
95
100
94
97
94
94
97
94
98
98
91
79
94
83
93
99
a - Samples: 4 mL of 100 mg/mL multi-standard in 2 g of sample. Each value is percent recovery
and is the average of duplicate spikes.
b - Unable to accurately quantitate due to high background values,
c - Method 3050B using optional section
3050B - 9
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December 1996
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
Table 3: Results of Analysis of NIST Standard Reference Material 2704
"River Sediment” Using Method 3050B (pg/g ± SD)
Element
Atm. Pressure Microwave
Assisted Method with
Power Control
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (gas-bulb)
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (IR-sensor)
Hot-Plate
NIST Certified Values for
Total Digestion
(uglg *95% Cl)
Cu
101*7
89*1
96*1.4
100*2
96.6*5.0
Pb
160*2
145*6
145*7
146*1
161 * 17
Zn
427*2
411*3
405*14
427 * 5
438*12
Cd
NA
3.5*0.66
3.7 4 0.9
NA
3.45*0.22
Cr
82*3
79*2
85*4
89*1
135*5
Nl
42*1
36*1
38*4
44*2
44.1 * 3.0
NA- Not Available
Table 4: Results of Analysis of NIST Standard Reference Material 2710
"Montana Soil (Highly elevated trace element concentrations)” Using Method 3050B
(Mg/g ± SD)
Element
Atm. Pressure
Microwave
Assisted Method
with Power Control
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (oas-bulb)
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (IR-sensor)
Hot-Plate
NIST teachable
Concentrations Using
Method 3060
NIST CertMsd Values for
Total Digestion
(ug/g *95% Cl)
Cu
2640 * 60
2790*41
2480*33
2910*59
2700
2960*130
Pb
5640*117
5430 * 72
5170*34
5720 * 280
5100
5532 * 80
Zn
6410*74
5810*34
6130*27
6230*115
5900
0952 * 91
Cd
NA
20.3*1.4
20.2*0.4
NA
20
21.8*0.2
Cr
20*1.6
19*2
18*2.4
23*0.5
19
39*
Nl
7.8*0.29
10*1
9.1 * 1.1
7*0.44
10.1
14.3*1.0
NA - Not Available
* Non-ceitified values, for Information only.
3050B • 10
Revision 2
December 1996
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Table 5: Results of Analysis of NIST Standard Reference Material 2711
“Montana Soil (Moderately elevated trace element concentrations)” Using Method 3050B
(MQ/Q ± SD)
Element
Atm. Pressure
Microwave
Assisted Method
with Power Control
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (gas-bulb)
Atm. Pressure
Microwave
Assisted Method
with Temperature
Control (IR-sensor)
Hot-Plate
NIST Leachable
Concentrations Using
Method 3050
NIST Certified Values for
Total Digestion
(ug/g ±95% Cl)
Cu
1071 4.6
98 ± 5
98 ±3.6
111 ±8.4
100
114±2
Pb
1240 ±68
1130 ±20
1120±29
1240 ±38
1100
1162 ±31
Zn
330 ±17
312 ±2
307 ± 12
340 ± 13
310
350.4 ±4.8
Cd
NA
39.6 ±3.9
40.9 ±1.9
NA
40
41.7 ±0.25
Cr
22 ±0.35
21 ±1
15± 1.1
23 ±0.9
20
47*
Nl
15 ± 0.2
17 ± 2
15 ±1.6
16 ±0.4
16
20.6 ±1.1
NA - Not Available
* Norvcertifled values, for information only.
3050B- 11
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METHOD 3050B
ACID DIGESTION OF SEDIMENTS, SLUDGES, AND SOILS
7 .1 M ix sam p le
to h o m o g e n e ity .
(in ly (o r S b . B a. Pl>. and Ac
If re q u ire d
______
G FAA or
IC P -M S
7 . 2 A dd 1 0 m l 1:1
H N O a and re flu x fo r
1 0 m in u te * .
7 .2 Add S m l c o n c .
H NO j an il ra llu x lo r
3 0 m in t.; re p e a t
u n til d ig . ie c o m p le te
e v a p o ra te to
5 m L ; cnnl.
7 .2 .1 - 7 . 2 . 2 A dd
2 m L w a te r and 3 m L
3 0 f t H jO j; co n tin u e
to add 1 m L aliq u o ts
o f Hi 0 2 u n til bubbling
e u b s id ts .
7 .3 A dd 1 0 mL c o n ­
c e n tr a te d H C I to th e
d ig e s t fro m 7 . 2 . 3 and
c o v e r re flu x (or
1 5 m in u te * .
FL A A e n d /
o r IC P -A E S
7 . 2 . 3 R ed uce volum e
to " 6 m l .
7 .4 Filter,
m a k e to v o lu m e .
7 . 2 . 4 F ilte r/c e n trifu g e .
II m tc e a s m y , d ilu te
to 1 0 0 m l w lrh w a te r .
7 .5 Add 7 .6 mL conc.
IIN O g and 10 mL eonc.
HCI to tam p la raflua
for
15 m in u ta a .
/ . b . i n it a r d ig e t ta te
and c o lle c t in
v o lu m e tric (le a k .
7 .5 .1 W a s h filte r p a p e r
w ith 5 m L h o t H C I and
th e n w ith 2 0 m L ho t
re a g e n t w a t e r . C ollect
in sa m e 1 0 0 m L flunk
as filtr a te .
f . h . 7 Itm im v e (lite r
and resid u es and p la c e
h e ck in v e s s e l. Add
5 m L H C L and h n a l
lillu r ; c u lle d In cam e
tla s k a * filtr a te .
7 5 . 3 II p r a o i p i l a r a
fo rm s add up (n
1 0 m L HC I to d is s o lv e .D ilu lc to vo lu m o .
7 .4 A n a lyze by
FI AA o r IC P -A E S .
I . 7 ..3 A nalyzo by
G F A A or IC P -M S .
7 . 5 . 3 A n a ly to by
FI. A A n r IC P -A C S .
7 .S C alculation*.
3050B -12
Revision 2
December 1996
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Appendix C
Publication: Lorentzen, E. M. L.; Kingston, H.M. S.,
"A Comparison of Microwave Assisted
and Conventional Leaching Using EPA Method
3050B",
Analytical Chemistry, 1996,6 8 , 4316-4320.
104
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
i r.a. Crsr~ 1996. f z
Tm bti
Comparison of M icrowave-Assisted and
Conventional Leaching Using EPA Method 3050B
heap:
dc
temp
nu
Elke M. L. Lorentzen and H. M. “Skip" K ingston*
ve>>»
Department of Chemistry & Biochemistry. Duquesne University. Mellon Hall. Pittsburgh. Pennsylvania 15282-1503
NJ
Amicrowave-heated EPAmethod 3050B for the leaching
of key elements (cadmium, chromium, copper. lead,
nickel, zinc) of environmental importance was tested and
compared to conventional hot plate-heated EPA method
3050B. AO commercially available temperature and
power-controlled atmospheric pressure microwave sys­
tems were used for the adaptation of EPAmethod 3050B.
Three temperature feedback control systems were evalu­
ated for regulating the temperature of the leachate includ­
ing outside (IR sensors) and inside the sample flask (gas
bulb thermometer). Results, which show the efficiency
and effectiveness of the microwave sample preparation
method, are discussed for the leaching of three NIST
Standard Reference Materials: 2704, 2710, and 2711.
The elements were determined either using ICPMS, ETA4S, or F-AAS. This study demonstrates that microwave
heating with enhanced reaction control leads to improved
precision compared to conventional heating sources.
Leaching is a term that has been applied to the extraction of
metals from environmental samples and has become common
terminology of the EPA and in the environmental analytical field.
Leaching is not a total decomposition, and teachable recoveries
of analytes are generally lower than total concentrations. Recover­
ies can only achieve total values if an element is completely soluble
in the leaching solvent. Leaching studies are an assessment of
worst case environmental scenarios where components of the
sample become soluble and mobile. Temperature is a key
parameter for all leaching sample preparation methods as well as
for extractions and digestions. The control of temperature is
paramount in achieving reproducible leaching of elements. Tem­
perature is a primary parameter used to increase the rate of
leaching and to bring these tests into appropriate duration for
laboratory evaluation. Previously, most leach methods have been
accomplished in beakers on a hot plate. These methods are
traditional, time consuming, fairly inefficient, and in general
imprecise. During the early 1980s. fundamental research estab­
lished temperature control as the most significant contributor to
leach test error.'- These experiments focused on the analysis of
simulated nuriear waste giass materials Temperature was found
to be the dominant parameter in leachmg uncertainty and
imprecision. The control of temperature to within =0.04%, instead
of =1% over a 28-day leach period, changed the interelement
leaching uncertainty from 50%to 3%. By substituting microwave
i t i Kingston. H . M e Cnxtm. D . J . . Epstein. M. S . S a d d r a t. Haste Menage.
19 8 4 . 5. 3 -15
:2) Liggett. W S.: Inn. K. G. W. Pilot Studies for bnprtmng Sampling Protocols.
It Pmctoies or Enrnvnmental Sampanc K rA . i. H . Ea.. Profession.1
Reterence 3ooic American Cbettuca: S corn- Washington. DC. 1996
.'hapter i’-
131$ Analytical C ham ofy VcL 68, No. 24, D ecem ber IS , 1i
heating for conventional heating devices, advantages are gained
in efficiency, precision, accuracy, and reduced waste. *l‘ This is
due to direct microwave energy inductance into leachate solutions.
The application of microwave energy as the heating source
significantly improved many sample preparation methods. Mi­
crowave instrument technology coupled with closed vessels has
become a standard for EPA methods 3015. 3051. and 3052/'"
To date, the range of applications for microwave sample prepara­
tion extends from “closed-vessel" to “open-vessel" to “flowthrough" systems."-11*v The control of temperature in leaching
is responsible for much of the precision of these procedures.
However, in dedicated atmospheric pressure microwave systems,
only power control has been available until recently. Two different
commercial systems of focused microwave power with tempera­
ture feedback control"-3 are now available and are evaluated for
EPA method 3050B.
Method 3050B. included in Test Methods for Evaluating Solid
Waste SW’-&#6. Update III. has traditionally been a leach test
performed on a hot plate.10 In recent years, the EPA has proposed
to revise and update certain testing methods used to comply with
the requirements of subtitle C of the Resource Conservation and
Recovery Act (RCRA) of 1976. Method 3050B is one of several
methods included in the list of draft revised methods reviewed in
SW-846. Update HI. in the U.S. Federal Register.16 This method
has become more broadly applicable by adapting the prescriptionbased method to create a performance-based method and to
permit newer technological implementations, such as microwave
heating.1" By adding the words “or equivalent" to the hot plate
designation, and specifying that the heating device be 'adjustable
and capable of maintaining a temperature of 90—95 cC". the use
O) Matusiewtcz. H.. Suszka. A.: Ciszewski. A. A d a Chim. Hung. 1 9 9 1 . 128.
849-859.
(4I Feinberg. M. H- Siarri. C_ Ireland Ripen. J. Otemom. InteU. Lab. Syst. 199 4 .
22. 37 -47.
(5) Barnes. R. M. Anai. Cktm. 1 9 9 0 . 62. 1023A-1033A.
16) Burguera. J L. Burguera. M /. A n a l A t Sptetmm. 1 9 9 3 . 6. 235-241
(71 Alvarado. J. S.: S e a l T. J.. Smith. L L: Erickson. M. D. Anai. Chim. A d a
1 9 9 6 . 322. 11-20.
(8) Kingston. H. M.. Walter. P J.: Chalk. S. J.: Lorentzen. E.. Link. D D
Environmental Microwave Sample Preparation: Fundamentals Methods,
and Appixanons. In Micrmoaoe Enhanced Chemistry. Kingston. H M..
HasweiL S_ Eds.. American Chemical Society- Washington. DC. in press
<9i Kingston. K M.. Jassie. L B_ Eds.. Introaucnon tc M icnw arr Sample
Preparation. Theory and Practice. American Chemical Society Washington.
DC. 1988.
1 10) Test Methods to- Evalaattng Solid Wajtf-S14S46 I'pdate til. 3rd ed. I S.
EPA: Washington. DC. 1995.
(11) Matusiewicz. H-. Sturgeon. R. E. Prog. A nai Sprrtrosc. 1 9 8 9 . 12. 2 1-3 9 .
1 12) Feinberg. M. H. Analusn 1 9 9 1 . 19. <7- 55.
(13) Haswefi. S J_ Barclay. D A. Analyst 1 9 9 2 . 117. : 17-120.
(14) Lorentzen. L M .L : Kingston. H. M. Pittsburgh Conference. New Orleans.
LA. 1995. Paper 1305.
1 15# King. E E.. Rttsourgfc Conference. Chicago. IL. 1996. Paper 1247.
I S , Fed R ep s 1995 60. (;42). 27974 -37978
■17. Friedman. D Enrrrvn Lab. 1993. (April TMavi 2"-*9
S O O D 3 -2 7 0 0 ( 9 6 ) 0 0 5 5 3 - 7 C C C . S J 2 . 0 0
—
- i - .': 2 „ /
- ." r .
C 1 9 9 6 A m e n c a r. C h e m ic a : S o c ie ty
s .o ..A t
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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Tjbi* 1. liw tiu n w iitiUoa for Conventional and W cn a i v i Au latad Method 3050S
atmospheric pressure mkrowav^essisied methods
temp control 1
-eating
device
em p
meas
.essei
temp control 2
xm p control 3
microwave system
microwave system 401*
microwave system Star
MX 250* with
with temperature control
System 2* with
temperature control
temperature control
IR sensor ^M 402*)
gas bulb thermometer
Hi sensor
<Megal500*)
quartz glass. 250 mL
borosincate glass. 250 mL
borosficare giass. 250 raL
calibrated power .ontrol
hot plate
microwave system .4301*
with power control
hot plate with power
settings1'
iber-opoc sensor*
thermocouple
thermometer*
borosiiicate glass. 250 mL
1 Prolabo. Paris. France.' CEM Corp~ Matthews. NC. ' Fisher Scientific. Pittsburgh. PA. * Luxtron 250. Sana Clara. C A .' Fluke 52. Paramus.
NJ.
of healing blocks and microwave energy is permitted as acceptable
heating alternatives within the structure of the method. In
addition, with the incorporation of feedback control of the leachate
temperature, it is easier to increase the reproducibility of the
measurement while automating the process.
In comparison to indirect heating by convection and conduction
in hot plate digestions, acids and polar solvents directly absorb
microwave radiation by the mechanisms of ionic conductance and
dipole rotation.1419 Since the sample is heated directly, equilibrium
conditions are obtained much more rapidly. In 3Q50B. the sample
preparation time can be reduced from over 2.5 to I h by using
microwave technology rather than conventional means. As well
as allowing greater throughput of samples, this may also improve
the reproducibility and reduce contamination by decreasing the
quantity of reagent and exposing the sample to a controlled
atmosphere for a shorter period of time.11* These procedures
may be automated or semiautomated using microwave instru­
mentation so the attendance by the chemist may be minimized.
Newer instrumentation with two. four. six. or more samples and
multiple instruments reduce the limitation in throughput Fur­
thermore. there are several advantages to atmospheric pressure
microwave digestion, such as effective handling of gas-forming
digestion products, sequential and automated incremental addition
of reagents during digestion, handling of larger and dynamic
sample sizes, and the use of quartz, glass, or Teflon vessels.*
The elements Ag. AL As. Ba. Be. Ca. Cd. Co. Cr. Cu. Fe. K.
Mg. Mn, Mo. Na. Ni. Pb. Sb. Se. TL V. and Zn can be determined
by EPA method 3050B in sediments, sludges, and soils. An
optional procedure for improved recoveries of the analytes 5b.
Ba. Pb. and Ag has been included in the revision. Two digestions
of 1.00 (dry weight) or 1.00—2.00 g (wet weight) of sample are
required to analyze all 23 elements.1" Elements determined by
F-AAS were leached with nitric acid, hydrogen peroxide, and
hydrochloric acid. For analysis with ET-.AAS-'—and ICPMS.-'
Copson. D A. Microwave rieatinf. ,\w Publishing Co. Inc.. '»Ves9 on. cT.
*.y> Mingos. M Microwave Theory :nr Chemistry In S fu 'w a r e £tmawed
Gtemuzry: Kingston. H. M . Hasweii. S
£d>.. American Chemical
SocieTv* Washington. DC. m press.
JV>> Gnllo. V C iprrrrosr-ipy 1989. ■/. .ri-Jl
;i) Kingston. H M . Waiter. P J Lurenuen. E M. L. UisruK. G ? Duquesne
Vniversity. Pittsburgh. PA. '.9*4
J2) Waiter. P J . Chalk. S J . Kingston. A. M L^rvntzm. E M L VReview- ot
Microwave .Afsjxte-c -amDte P~*-vanicon in
'• ' l v S r .
EXPERIMENTAL SECTION
Reagents. Concentrated nitric acid, concentrated hydrochlo­
ric add. and hydrogen peroxide (30%) were obtained from Fisher
Chemical (ACS Reagent Grade. Fisher. Pittsburgh. PA). The adds
were subboiled distilled before use. using either a quartz still
(Milestone s.r.L. Sorisole. Italy) or an all-PFA Teflon still, built
in-house from Teflon components (Savillex Corp.. Minnetenka.
MN). The Standard Reference Materials. SRM 2704 (Buffalo river
sediment). SRM 2710 (Montana soil highly elevated trace element
concentrations), and SRM 2711 (Montana soil, moderately el­
evated trace element concentrations) were obtained from NIST
(the National Institute for Standards and Technology. Gaithers­
burg. MD).
Equipm ent. The atmospheric pressure microwave proce­
dures were performed using equipment from Prolabo Corp. (Paris.
France) and CEM (Matthews. NO. The sample leaching and
reaction control equipment used in this study are summarized in
Table 1. A Perkin-Elmer atomic absorption spectrometer PE1100A
(Norwalk. CT) with both flame (F-AAS) and electrothermal modes
(ET-AAS. HGA 300) and an inductively coupled plasma mass
spectrometer (ICPMS. VG Plasma Quad 2STE. Ftsons Instru­
ments. Beverly. MA) were used under standard conditions in a
class 1000. 100. and 10 combination clean room facility for the
analysis of elemental concentrations.
Leach Procedures. The comparative procedural outlines of
method 3050B and modified microwave assisted 3050B are shown
in Table 2 and discussed below.
Procedure for Conventional Hot Plate EPA Method
3 0 5 0 B . A 1.0-g sample, known to 0.01 g. was weighed in an
Erienmever flask and 10 mL of HNO- I I iv/v) was added. The
solution was heated on a hot plate to ~05 C without boiling and
this temperature was maintained for 15 min. .After cooling to less
than 70 C. 5 mL ot concentrated HNO - was added and the sample
was refluxed tor 30 min at --05 C without boiling. This step was
fwjvrii dtmtsinr.
iCngsion. H M . Hu>*H. S.. Eds.. Vnercxn chemical i v i r t v -Visntrgton.
DC. ;r. press.
.ID IdT.is, K. £ .. G rav. A A. H o u k . R. ^
the samples were leached without hydrochloric acid. The reaction
conditions and reagents have an influence on the results of the
leach analysis, with a large number of different species being
involved.-3-5’
In this paper, we report the comparison of samples leached
with controlled microwave heating to conventional hot plate
heating and their ability to accurately reproduce the method's
specified 95 :C temperature.
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Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
r.. A V
T able 2. Comparative P rocedural O utlines of EPA M ethod 30500 and Modified M icrowave A ssisted 3050B
Tat
meir.' ic '■
> ;B
no: piatc
n U C T t 'W j V r 3 > < > l r C
r e a L rc r .t>
open m icrow ave
- ~- .c
15 m L n : cn n c HNi '
at jtm i» > p n crv p re s s u re
i'raL o: H.o
mL ot cone HC!
U'mLo: 3tM H.-O;
rit' min
weigh sample into vvsse. and add 1' mL <*: t:l HNO
reflux at H r :C tor 5 mm
cool, then add 5 mL ot cone HNO
5
time
p n iceriu re
reflux at 95 :C tor 5 mir.
cool, then add 5 mL ot cone HN( •
reflux at 95 ;C for 5 - If nun
cooL add max 10 mL of 30* H.-O.
heat until effervescence is minimal
reflux at 95 ;C for 5 -1 0 min
cooL add 15 mL of 1:2 HCL reflux at 95 :C for 5 min
csuaih" a covered ix-axer nr flask a: atmosohenc pressure
•”• c
15 mL »: cone HNt'
mL o: H.-O
I’-mL ot cone HC;
mL ot it's H.U.fl-e h
weigh sample mto vessel anc add 10 m l of 1:1 HNO
reflux a: 95 C tor lu -1 5 mir
cooL then add 5 mL of cone KN'O
reflux at 95 C ior 30 nun
cool then add 5 mL of cone HN'Oreflux a: 95 C for 3t’ min. evaporate to 5 mL or heat tor 2 h
cooL add max 10 mL of 30* H.-O;
hea: untii effervescence is minimal
evaporate to 5 mL or heat tor 2 h
cooL add U* mL of cone HCL reflux at 95 =C for 15 min
Tab
eien
repeated a second time. The sample was evaporated to ^5 mL
without boiling. After cooling to less than 70 =C. 2 mL of 18-MQ
water was added followed by the slow addition of 10 mL of H;0:
(30%) Care must be taken to ensure that losses do not occur
due to excessively vigorous effervescence caused by rapidly
adding the strong oxidizer, hydrogen peroxide. The solution was
then heated until effervescence subsided. After cooling to less
than 70 5C. 5 mL of concentrated HC1 and 10 mL of 18-MQ water
were added and the sample was refluxed for 15 min without
boiling. After cooling to room temperature, the sample was
filtered and diluted to 100.0 mL using 18-MQ water.
Procedure for Power Control Microwave Implementation
o f Method 3 0 50B . A 1.0-g sample, known to =0.01 g. was
weighed in a borosilicate glass vessel and 10 mL of HNO-. 1:1
(v/v) was added. A microwave digestion program consisting of
80 W for 2 min and 30 W for 5 min was applied by a Microdigester
A301. After cooling to 70 =10 ZC. 5 mL of concentrated HN’Owas automatically added and a second power program of 80 W
for 2 min and 30 W for 5 min was applied. This step was repeated
a second time. After cooling to 70 =10 5C. 3 mL of H;0 ; (30%)
was slowly added (2 mlv min) and a third power program of 40
W for 5 min was applied. This step was repeated twice. After
cooling to 70 =10 °C. 5 mL of concentrated HC1 in 10 mL of 18MQ water was added and a fourth-power program of 80 W for 2
min and 30 W for 5 min was applied After cooling to room
temperature, the sample was filtered and diluted to 100.0 mL using
18-MQ water.
Procedure for Temperature Feedback Controlled Micro­
wave Implementation of Method 3 0 5 0 B . A 1.0-g sampie.
known to =0.01 g, was weighed in a borosilicate vessel or quartz
glass vessei (see Table 1) and 10 mL of HNO-1:1 tv/v'i was added.
A microwave temperature program consisting of heating the
solution to 95 =2 SC in 2 min and maintaining the temperature
for 5 min was applied. After cooling to 70 =5 =C. 5 mL of
concentrated HNO; was automatically addec and a second
temperature program of heating the solution to 95 = 2 °C in 2
min and maintaining the temperature for 5 min was appBed. This
step was repeated a second time. After cooling to 70 = 5 3C. 10
mL of H;0; (30%) was slowly added (2 mL/min addition by
temperature controls 1 and 2. and in 0.5-mL aiiquots by temper­
ature controi 3). The solution was then heated to 95 =2 C in 6
min. and the temperature was maintained for 5 min. After cooling
to 70 =5 -C. 5 mL of concentrated HQ in 10 mL of 18-MQ water
was added. The solution was then heated to 95 = 2 °C in 2 min
and the temperature was maintained for 5 min. After cooling to
room temperature, the sample was filtered and diluted to 100.0
mL using 18-MQ water.
For analysis by ET-AAS and ICPMS. digestion procedures as
described above were performed without hydrochloric acid
addition, as possible in the EPA procedure
The results of metal analysis, using a modified microwave
assisted 3050B (temperature feedback control and power control)
and conventional method 3050B. are listed in Tables 3 - 5 for SRMs
2704.2710. and 2711. respectively. Results are compared to NIST
teachable concentrations using method 3050.r and certified values
for total digestion are inchided for convenience. Leach concentra­
tions for SRM 2704 are not provided by NIST. All data are in the
range of the NIST or other published leachable concentrationswith minor exceptions.
The majority of recoveries for six analytes, as a percentage of
certified values for total digestions, are in the same range or higher
than the leach reference data published by NIST.r These six
specific elements were chosen as being of universal interest in
environmental standards in consultation with NIST.- The NIST
leach values are not certified but are a compilation of a 17
laboratory collaborative leach study. In this study, all laboratories
used conventional hot plate equipment The recoveries obtained
with temperature control (TR sensor and gas bulb thermometer)
are slightly lower than the corresponding values of power control
and hot piate. The lower values obtained by using microwave
assisted modified temperaiure-controDed 3050B can be explained
by more accurately controlled, but relatively lower, temperatures
during the leaching procedures since the temperature with power
controi and on the hot piate was higher than the required 95 :C.
as will be described.
For EPA method 3050B. the procedure requires heating the
sample solution to 95 =C and holding this approximate temperature
ML
\u e u s
X
.9 rersora. curr.:"ur.!.^'jur. jear.
Z
c
C
N
a»
RESULTS AND DISCUSSION
A a a e n a u i r . to m e . - - m n c a t e o : A r a h - s i 5 i o r S R M >
C
F
9 . 3 “ 10. Z T II. N IS T
’. 9 9 3 .
NIST .
4310 Analytical Chem stry, VoL 68. No. 24. O ecomoer is , 1996
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
- for
Tabi
elerr
C
P1
Z:
C,
C:
N
*N
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Tabl* 3 . R esults a i th e A nalysis of NIST Standard R eference Material 2 7 0 4 U sing Method 3050B
ie-nent
<7u
?b
Zn
cd
er
Ni
acnosphenc pressure microwave-assisted methods' i;ig g~‘ = SD)
power controi
temp control 2
temp control 3
temp control I
101 = 7
-102)
160 = 2
(99)
427 = 2
-97)
na-'
82 = 3
>61)
42 = I
•95)
99 = 1
-90)
’.45 = 6
90)
411 = 3
(94)
2.5 = -1.66
101)
*9 = 2
(56)
36 = I
•82)
98 = 1.4
(99)
145 = ~
90)
405 = 14
(92)
3.7 = 0.9
107)
■15 = 4
63)
38 = 4
•86)
iOl.fi = 4.8
103)
.34 - 5
83)
407 = 74
93)
'. 6 = •) 65
38)
42 = 4
61)
35.4 = 3
•80)
^ pfcue(figg~l = SD)
s-[Sp certified values for total digesnon
M g g ' 1 =95% Cl)
1 0 0 -2
(101)
146 - 1
(91)
427 = 5
•97)
na-
98.6 = 5.0
89 = 1
•66)
44 = 2
(100)
135 = 5
161 = 17
438 = 12
2.45 r 0222
44.1 = 3.0
1 Numbers in parentheses are percent recoveries. ’ 3050 and 3051 leach data are presented in ret. 25. na. not available.
T able 4 . R esu lts of th e A n alysis o f NIST Standard R eferen ce Material 2 7 1 0 U sin g Method 3050B
element
Cu
Pb
Zn
cd
Cr
Ni
atmospheric pressure microwave-assisted methods' (Mg if'- = SD)
power controi
temp controi 2
temp control I temp control 3
2640 = 60
(89)
5610 = 117
(102)
6410 = 74
(92)
na*
20 = 1.6
(51)
78 = 0229
(35)
2790 = 41
•95)
>430 = 72
(98)
5810 = 34
■84)
20.3 = 1.4
(93)
19 = 2
(49)
10 = 1
(70)
2480 = 33
•82)
5170 = 34
•93)
6130 = 27
(88)
2022 = 0.4
•93)
18 = 2.4
(46)
9.1 = 1.1
(64)
3C80 = 22
■104)
5065 = 39
(92)
6212 = *4
(89)
17.5 = 0.8
(82)
20.9 = 0.5
(54)
1022 = 0.55
•71)
hot plate*
ifitf g - ‘ = SD)
2910 = 59
•99)
5720 r 280
(103)
6230= 115
•90)
na*
23 = 0.5
(59)
7 = 0.44
•49)
NIST teachable concns
usinif method 3050*
i/t g if :)f (Rangel
VIST certified values
for total digestion
<Mgg*‘ =95% CD
2700
(2400-3400)
5100
•4300-7000)
5900
•5200-6900)
20
(13-26)
19
(15-23)
10.1
(8.8-15)
2950 = 130
5532 = 80
6952 = 91
21.8 =0.2
39*
14.3 = 1.0
‘ Numbers in parentheses are percent recoveries. * Reference 27. Numbers in parentheses are ranges. ‘ na. not available.' Non-certified values
- tor intormauon only. NA - Not available
T able S. R esu lts of th e A nalysis o f NIST Standard R eferen ce Material 2711 U sin g Method 3 0 5 0 B
•iement
Pb
Zn
c'd
cr
Ni
107 = 4.6
(94)
1240 = 68
(107)
330 = 17
(94)
na'
22 = 0.35
<471
*.5 = 0.2
73)
II
u
atmospheric pressure microwave-assisted methods' (jtg if'1 = SD)
temp controi 2
temp control 1 temp control 3
power control
(86)
1130 = 20
(97)
312 = 2
(89)
39.6 = 3.9
(95)
21 = 1
•45)
17 = 2
83)
98 = 3.8
(86)
1120 = 29
(96)
307 = 12
(88)
40.9 = 1.9
•98)
15= 1.1
•32)
15 = 1.6
•73)
113 = 8.1
(99)
1119 = 60
•96)
326 = 3.7
(93)
39.4 = 1.2
•94)
17.3 = 1.3
•37)
15.5 = >.75
175-
' Numbers in parentheses are percent recoveries. ' Reference 27
or imormauon onlv.
hot plate*
u g g ' 1 = SD)
111 =6.4
(97)
1240 = 38
(107)
340 = 13
(97)
na'
23 = 0.9
(49)
16 = 0.4
•78)
.>1 0 1 leacnaoie ci
using method 31
iftg g - :V
100
(91-110)
1I0O
(930-1500)
310
•290-140)
40
•32 - 46)
20
•15-25)
16
14-20)
s
NIST certified values
for total digestion
O igg'1 = 95% CD
114=2
;i»i2 = :i
350.4 = 4.S
41.7 = 9.25
47'
20.0 = 1.1
Numbers :n parentheses are ranges. • na. not available. ■Noncertified values,
without boiling. At a temperature of 95 ;C. concentrated nitric
acid should not boil. Since temperature controi :s difficult to
maintain on i hot plate, this is easier to reproduce and maintain
by using microwave temperature feedback controi instruments.
However, the temperature profile for the microwave assisted
modified method J050B using power control, obtained by simul­
taneous temperature measurement with a fiber-optic temperature
sensor, was more like that u a hot plate. The resultant temper­
ature was -yptcaily found to be 1 0 - i 5 C above the required 95
C Programming the power to maintain i certain temperatur*
is difficult, since microwave digesters with power control are
rr-<tncted m the control to the power increments and time settings
available similar to hot piates. In addition, heat loss and evapora­
tion of reagents must be considered. The alternative to this
procedure is to controi the power by temperature feedback during
microwave leaching. L'ntil 1994. power control was the only form
of control in dedicated atmospheric pressure microwave systems.
This was the single most limiting ieature of these earlier
microwave instruments.
Temperature feedback controi makes atmospheric pressure
microwave systems much more capable. However, -c-mperature
measurements m a microwave field ar* limited to lemces that
ore transparent to the field. The use ot shielded thermocouples
s ^appropriate in comparison with its use :n a •:io<e<i-v»‘<«e!
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
system. w-nere it? shielding i? grounded tc a cavin wall. as the
micro wav- neid «.tl; be conducted trom the vessel or. the surface
of the shielding Fiber-optic thermometry is commoniy used in
microwave cavity systems- and is compatible with open vessel
systems Gas bulb thermometers and IR sensors are two
alternatives currently developed tor the use in atmospheric
pressure microwave digesters. The gas bulb thermometer is
based on gas law principles, measuring the pressure difference
o: heated air in a glass bulb inside the sample flask. The gas
bulb has to be fully covered by solution to assure an accurate
temperature measurement
IR sensors were used for noninvasive temperature control. The
intensity of IR radiation emitted from the vessel base is measured
through a hole in the bottom of the cavity under the flask. In the
Prolabo instrumentation, the IR radiation emitted is reflected by
a mirror in an angle of 90c toward the IR detector. IR sensor
calibration was accomplished by simultaneous temperature ac­
quisition with a fiber-optic sensor or the gas bulb thermometer.
This allows the emissivity of the quartz glass vessel used during
the experiments to be corrected by an emissivity factor (range
0.10-1.00 =0.01) within the controlling software. In the CEM
microwave unit, the IR sensors are placed several centimeters
below each vessel. The IR sensors were individually calibrated
by a low (concentrated nitric acid 121 °C) and high solvent boiling
point (concentrated sulfuric arid 330 -Q and verified by an
independent fiber-optic measurement system.
A major advantage of the IR sensor is that no cross contamina­
tion can occur between samples from this source since the probe
has no contact with the sample. The IR sensors measure energy
reflected or emitted by any surface directly behind the vessel base
and therefore have a response that changes slightly with solution
volume. The IR sensors require frequent checking of the
calibration and have been found to be volume-dependent
The temperature accuracy of all systems was checked with
simultaneous temperature acquisition using an independent fiber­
optic thermometry system. Temperature spikes, seen as a result
of overshooting the goal temperature, were mainly observed with
temperature controls 2 and 3. primarily due to deficiencies in the
control algorithms. Adjustments of die feedback control algorithm
could improve their temperature control. Less major thermal and
power spikes were observed with temperature control 1. The
maximum temperature is maintained to a much higher accuracy,
as shown in Figure 1.
The regulation of a certain temperature on a hot plate has been
found to be much more difficult than with the use of microwave
digestors. Calibration of a hot plate to produce 95 °C in a single
flask resulted in the other flasks' temperatures ranging from 85
and 118 :C It was commonly observed that, depending on the
flask measured, the distribution pattern, number of flasks, surface
temperature, and air movement the temperature may vary by 35
:C. This is in agreement with results found by other researchers.3
The advantage of preparing several samples at the same time (hot
plate) is counteracted by the problem of regulating and controlling
a certain temperature for all sample flasks. In comparison, the
microwave IR sensor is able to regulate and control 95 =C with a
mean temperature of 95.4 =1.1 °C (four replicates. 300 data points
in 5 nun).
Furthermore, reagents were programmed to be added auto­
matically on all microwave systems. Temperatures and speed of
_*m
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K
’
M ir m u c i^
C ne*i
1 9 9 5
1 9 8 7
2 C
Ana
HNO,
H
100
M C I
O
El
Fi
IR sensor
Coal
Temperature
Not
Power
20
Am
30
40
50
Time (Min.)
Figure 1. Temperature vs time profile of microwave-assisted
method 3050B using temperature feedback control 1 (IR sensor)
reagent addition were coordinated through sensor and software
control This is in contrast to the manual hot plate implementa­
tion.
Both designs of atmospheric pressure, temperature-controlled,
microwave equipment produced similar results, demonstrating the
appropriateness of microwave-induced heating and its ability to
be implemented in very different manners while producing
consistent results.
CONCLUSIONS
This study demonstrates that microwave control is an efficient
and effective alternative to conventional heating sources for EPA
method 3050B. Control of temperature rather than microwave
power is better at maintaining specific sample preparation tem­
peratures. Temperature feedback control microwave systems are
capable of controlling the temperature at the required 95 °C with
an accuracy of ±2 °C that is not achievable by either hot plate or
microwave power controL Moreover, precision is improved, leach
time is reduced by 60%. and reagent addition is automated.
This application demonstrates that atmospheric pressure
microwave sample preparation, applying temperature feedback
controL is an appropriate alternative to traditionally implemented
convection and conduction heating on hot plates. Additional
environmental methods, such as total decomposition of oils, and
polymers in EPA method 3031. or species extractions, such as
chromium (VI) in method 3060. are also being implemented using
this newer technology with similar improvements. Other tradi­
tional sample preparation methods requiring control of reaction
temperature will find advantages in microwave temperature
feedback control implementation.
ACKNOWLEDGMENT
The authors thank Dr. Peter Walter for providing the ICPMS
data. In addition, we acknowledge Dr. Stuart Chalk for his
valuable suggestions. The research was supported by a grant
from Prolabo Corp. (Prance' and CEM (Matthews. NO.
Received for review June 5. 1996. Accepted September
25. 1996.
AC960553L
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Afiatyocal Chemistry. Vol. 68. No. 2*. December 15. 1996
Reproduced with permission of the copyright owner. Further reproduction prohibited without permission.
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