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LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE FURNACE AND MICROWAVE EXCITED ELECTRODELESS DISCHARGE LAMPS FOR FLAME ATOMIC FLUORESCENCE SPECTROMETRY

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S e ltz e r , M ic h a e l D avid
LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE FURNACE
AND MICROWAVE EXCITED ELECTRODELESS DISCHARGE LAMPS FOR
FLAME ATOMIC FLUORESCENCE SPECTROMETRY
Ph.D.
The University o f C o n n e c tic u t
University
Microfilms
International
300 N. Zeeb Road, Ann Arbor, Ml 48106
Copyright 1986
by
Seltzer, M ichael David
All Rights Reserved
1986
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LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE FURNACE
AND MICROWAVE EXCITED ELECTRODELESS DISCHARGE LAMPS FOR
FLAME ATOMIC FLUORESCENCE SPECTROMETRY
Michael David S e l t z e r , Ph.D.
The U n iv e r s ity o f C o n n e c tic u t, 1986
The r e s e a r c h was p r i m a r i l y concerned w ith an i n v e s t i g a t i o n o f
methods aimed a t th e improvement o f a c c u ra c y , p r e c i s i o n , and
s e n s i t i v i t y o f atomic f lu o r e s c e n c e s p e c tro m e try , u sin g b o th
c o n v e n tio n a l l i g h t so u rc e and l a s e r e x c i t a t i o n , in b o th flame and
carbon fu rn a c e atom c e l l s .
B efore th e advent o f p u ls e d , h igh i n t e n s i t y hollow cathode lamps
and t u n a b le dye l a s e r s , e l e c t r o d e l e s s d is c h a rg e lamps were th e b e s t
l i g h t s o u rc e s f o r e x c i t a t i o n o f atomic f lu o r e s c e n c e .
Since th e
perform ance and r e p r o d u c i b i l i t y of e l e c t r o d e l e s s d is c h a rg e lamps
depends so c r i t i c a l l y on th e p r e p a r a t i o n o f th e lamps, an o p tim iz a tio n
o f t h i s p r e p a r a t i o n was u n d e rta k en in o rd e r t o c o n s t r u c t e l e c t r o d e l e s s
d is c h a rg e lamps f o r th e flame atomic f lu o r e s c e n c e d e te rm in a tio n of
manganese.
T h ir te e n c a r e f u l l y c o n t r o l l e d v a r i a b l e s in th e p r e p a r a t i o n
o f manganese e l e c t r o d e l e s s d is c h a rg e lamps were r ig o r o u s ly o p tim iz e d ,
r e s u l t i n g i n lamps t h a t p ro v id e d d e t e c t i o n l i m i t s t h a t were s u p e r io r
to th o s e o b ta in e d w ith lamps produced a t th e beginning o f th e
o p tim iz a tio n p ro c e s s and to p r e v io u s ly r e p o r te d l i t e r a t u r e v a lu e s f o r
manganese e l e c t r o d e l e s s d is c h a rg e lamps.
Michael David S e l t z e r —The U n iv e rs ity o f C o n n ec tic u t, 1986
The main body o f th e r e s e a r c h involved th e c o n s tr u c ti o n o f
in s tr u m e n ta tio n f o r l a s e r e x c it e d atomic flu o r e s c e n c e in b o th flames
and f u r n a c e s .
The crux of t h i s problem was t o i n t e g r a t e s e v e r a l
components, sy n chro nize t h e i r o p e r a tio n , and develop an in stru m e n t t o
a llo w s e n s i t i v e d e t e c t i o n o f u l t r a - t r a c e amounts o f m etals in samples.
Major emphasis was p la c e d on o p tim iz in g th e d e t e c t i o n system.
This
in c lu d e d th e development o f e l e c t r o n i c t r i g g e r i n g schemes to
sy n ch ro n ize th e d e t e c t i o n o f v e ry f a s t , pulsed f lu o r e s c e n c e s i g n a l s .
A s i g n i f i c a n t c o n t r i b u t i o n inv o lv e d a novel a p p l i c a t i o n o f gated
p h o t o m u l t i p l i e r tu b e o p e r a tio n t h a t r e s u l t e d in improvements in flame
l a s e r e x c ite d atomic f lu o r e s c e n c e d e t e c t i o n l i m i t s , by a llo w in g th e
use o f la r g e monochromator s l i t w id th s w ith o u t th e a t t e n d a n t r i s k o f
p h o to m u lti p lie r s a t u r a t i o n .
A carbon tu b e fu rn a c e a to m iz a tio n system, which employed the
method o f g r a p h ite probe sample i n t r o d u c t i o n , was developed f o r l a s e r
e x c i t e d atomic f lu o r e s c e n c e sp e c tro m e try , r e s u l t i n g in s e n s i t i v i t i e s
t h a t were comparable to g r a p h ite fu rn a c e atom ic a b s o r p tio n
s p e c tro m e try .
F i n a l l y , a F a b ry -P e ro t ty p e i n te r f e r o m e te r was c o n s tr u c te d , fo r
r a p i d and f a c i l e measurement o f l a s e r s p e c t r a l bandwidth.
An opto­
e l e c t r o n i c system f o r d e t e c t i o n o f th e c h a r a c t e r i s t i c i n t e r f e r e n c e
f r i n g e s , allow ed a sim ple b u t a c c u r a te measurement o f th e l a s e r
s p e c t r a l bandw idth in a m a tte r o f m inutes.
LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE FURNACE
AND MICROWAVE EXCITED ELECTRODELESS DISCHARGE LAMPS
FOR FLAME ATOMIC FLUORESCENCE SPECTROMETRY
Michael David S e l t z e r
B .A ., Rhode Is la n d C o llege, 1978
M .S., The U n iv e rs ity of C o n n ec tic u t, 1984
A D isse rta tio n
Subm itted in P a r t i a l F u lf illm e n t o f th e
Requirements f o r th e Degree of
Doctor of Philosophy
at
The U n iv e rs ity o f C onnecticut
1986
APPROVAL PAGE
Doctor of Philosophy D is s e r t a t i o n
LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE FURNACE
AND MICROWAVE EXCITED ELECTRODELESS DISCHARGE LAMPS
FOR FLAME ATOMIC FLUORESCENCE SPECTROMETRY
P resen ted by
Michael David S e l t z e r , B.A., M.S.
Major Adviser
o b e r t G. Michel
A sso ciate Adviser
James D. S t u a r t
A ssociate Adviser
Steven L. Suib
The U n iv e rs ity of C onnecticut
1986
- ii -
C opyright by
Michael David Seltzer
1 986
DEDICATION
To My Family and F rie n d s
- iii -
ACKNOWLEDGEMENTS
I t i s not o fte n easy to ex p ress g r a t i t u d e and a t th e same time e x p re ss
th e s i n c e r i t y w ith which t h a t g r a t i t u d e i s f e l t .
N e v e rth e le s s , I
would l i k e t o e x p re ss my d e ep e st a p p r e c ia tio n to th e fo llo w in g people
who have helped make t h i s a l l p o s s ib le :
Dr. R obert G. Michel, my r e s e a r c h a d v is o r , whose sense of
i n t e l l e c t u a l c h a lle n g e and n e v e r -s a y -d ie a t t i t u d e , in s p ir e d me to
achieve g o a ls t h a t I once c on sidered to be ou t of my re a c h .
My a s s o c i a t e a d v is o r s , Dr. James D. S t u a r t and Dr. Steven L.
S uib, whose ad v ice and i n s t r u c t i o n added a s o l i d c o n t r i b u t i o n to my
knowledge.
Dr. B ert Chamberland, whose wisdom and encouragement were
in v a lu a b le .
The o th e r members of the Chemistry f a c u l t y , by whom I had the
p r i v i l e g e of being educated.
And f i n a l l y , the o th e r g rad u ate s tu d e n t s both in my re s e a rc h
group and i n o th e r groups, w ith whom I had th e good f o r tu n e to work,
and sh a re id e a s and moral su p p o rt.
I would a ls o l i k e to thank the S ta te of C o n n ecticu t Department of
Higher E ducation f o r t h e i r generous award o f a High Technology
Graduate F e llo w sh ip .
- iv -
TABLE OF CONTENTS
APPROVAL P A G E ..................................................... ii
D E D I C A T I O N ...................................................... ill
ACKNOWLEDGEMENTS................................................. iv
Chapter
I.
page
GENERAL INTRODUCTION
......................................
1
Laser E x cited Atomic F luorescence Spectrom etry ...................... 1
Furnace Atomization f o r Atomic F lu o resc en c e
Spectrom etry ................................................................................... 4
Furnace L E A F S ...............................................
7
V a p o riz a tio n I n t e r f e r e n c e s i n Furnace Atomizers .................. 9
S p e c t r a l in t e r f e r e n c e s ............................................................. . . . 10
Background C o rre c tio n f o r LEAFS
.......................................... 11
Zeeman E f f e c t Background C o rre c tio n ...... ................................ 12
S p e c i f i c Aims o f th e P rese n t Research ; ...................................... 13
II.
PREPARATION OF ELECTRODELESS DISCHARGE LAMPS FOR ATOMIC
FLUORESCENCE SPECTROMETRY ............................ 14
I n tr o d u c ti o n ................................................................................................ 14
C l a s s i c a l Method o f P r e p a r a tio n .
..............................
18
M o d ific a tio n s to Method o f P r e p a r a tio n ................ . . . 19
E xp erim en tal ................................................................................................ 20
New Simplex V a ria b le s . . . . . . . . . . . . . . . . .
23
R e a g e n t s ............................................................
24
R e s u lts and D iscussion
.............................................. 24
R e p r o d u c ib ility o f P r e p a r a tio n . . . . .
.......................... 26
EDL Performance
. . . . . 27
C o n c l u s i o n .........................
29
III.
PHOTOMULTIPLIER DETECTION, LASER AND BOXCAR TRIGGERING
FOR L E A F S ............................................. 30
I n tr o d u c ti o n ............................................................................................... 30
Pulse-Mode P h o to m u ltip lie r S a t u r a t i o n
................................ 31
Dynode Chain D e s i g n .......................................
33
P h o to m u ltip lie r G a t i n g .................................................................... 34
Methods of P h o to m u ltip lie r Gating
. .............................. 36
E x perim ental ................................................................................................ 37
- v -
Laser T r i g g e r i n g .................................................................................37
Measurement of S ig rla l-to -N o ise R a tio s . . . . . . . . . 43
Boxcar Reference T r i g g e r i n g ..........................
43
A lte r n a te Methods o f Boxcar T rig g e rin g . . . . . . . 44
Advantages o f Adopted T rig g erin g System ...................... 46
R212UH PMT f o r Fluorescence D e tectio n . . . . .................. 47
9893QB PMT f o r F lu orescen ce D e te c tio n . . . . . . . . . 47
P h o to m u ltip lie r G a t i n g .......................... ; .....................................50
PMT G ating Board
...................................... . 5 0
Gating Pulse Generator
. . . . 52
L aser F ir in g Delay . . . . . .
. . . . 56
P h o to m u ltip lie r Gain C ontrol
.........................
61
A bbreviated Dynode Chain . . . . . . . . .
................. 61
R e s u lts and D iscussion . . . . .
. . . . . . 63
Gated v s. Non-Gated PMT O p eratio n
.............................63
Resonance D e tec tio n
. . . . 63
Non-Resonance D e tec tio n ................................... . . . . . 65
Advantages of Gated PMT O peration......................... . . . . . 67
E f f e c t o f PMT G ating on Dynode Chain C urrent .................. 69
L in ea r Dynamic R a n g e s ................................................... ; . . . . 72
R212UH PMT ....................................... . . . . . . . . . . . 72
9893QB PMT . . . . . . . . . . . . . . . . . . . . . 74
A bbreviated Dynode Chain f o r 9893QB PMT . . . . . . 74
R212UH v s . 9893QB PMTs....................................................... .. . 76
76
P r a c t i c a l C o n sid e ra tio n s .................. . . . . . . .
IV.
LASER EXCITED ATOMIC FLUORESCENCE IN A CARBON TUBE
F U R N A C E ...............................................78
I n tr o d u c ti o n ............................................................................................... 78
I d e a l i z e d Furnace fo r LEAFS .
.................................................. 79
I n s t r u m e n t a t i o n ..................................
81
L aser System . . . . . . . . .
................................................ 85
Laser T rig g e rin g . . . . . . . . . . . . . . . . . . .
86
Laser Beam Expansion and S te e r i n g O p tic s
.........................88
Furnace A t o m i z a t i o n ...................................................
95
Furnace Tube Design . . . . . ....................................... . 9 5
F a b r ic a tio n o f Furnace Tubes ................................................ 95
Furnace E lec tro d e Assembly . . . . .
.............................. 97
Furnace Enclosure fo r I n e r t Atmosphere . . . . . . . 99
O p tic a l B a f f lin g o f Furnace Emission .......................... 101
Furnace Sample I n tr o d u c tio n ...................... . ...................... 103
Furnace Wall S a m p l i n g ..............................................................103
Probe Atomization . . . . . . . . . . . . . . . .
104
C o n stru c tio n o f th e Probe Accessory . . . . . . .
105
P ro b e -a s -P la tfo rm Atom ization . . . . . . . . . .
107
D e te c tio n and S ig n a l P ro c e s sin g ............................................ 110
Data C o l l e c t i o n ..................................
111
R e s u lts and D iscussion . . . . . . .
. . 112
E v a lu a tio n o f Furnace H eating Performance . . . . . .
112
Furnace Tube L i f e t i m e ..............................................................117
E v a lu a tio n of Sample I n tr o d u c ti o n Methods . . . . . .
118
- vi -
Atomization C onditions fo r S e le c te d Elements . . . .
119
T h a l l i u m ............................................................................
120
123
L e a d ..............................
Iro n and Cobalt
.....................
123
Measurement o f D etection Lim its . . . . . . . . . . .
125
L inear Dynamic Ranges
. . . . . . . . . .
127
S c a t t e r and Resonance D e tectio n o f LEAFS in a
Carbon TubeF u r n a c e ..............................................................130
Conclusion
. . . . . . . . . . . . . .
133
V.
FABRY-PEROT INTERFEROMETRY FOR MEASUREMENT OF PULSED
LASER SPECTRAL BANDWIDTH .........................
137
In tr o d u c tio n ..........................................................................................
137
Laser S p e c tr a l Bandwidth and S p e c tr a l R esolu tio n in
LEAFS...................................................................................... 137
S p e c tr a l Scans o f E x c ita tio n and Fluo rescen ce
P r o f i l e s ...................................................................................138
Measurement of S p e c tr a l Bandwidth .
.....................
141
P r in c i p le s o f O peration o f th e F a b ry -P e ro t
In te r f e r o m e te r ................................................................. 143
Theory o f th e I n te r f e r e n c e F ring es Produced by the
F a b ry -P e ro t In te r f e r o m e te r ....................................... 145
Geometry o f Fabry-Perot I n t e r f e r e n c e F rin g e s . . .
147
C h a r a c t e r i s t i c s o f E talon s .................................................... 148
Free S p e c tr a l Range
.................................................148
F i n e s s e ......................................
149
Minimum R esolvable Bandwidth . . . . . . . . . . .
150
C a lc u la tio n o f S p e c tr a l Bandwidth ....................................... 150
In s tru m e n ta l Requirements
. . . . . . .
153
Experim ental ....................................... . . . . . .......................... 154
O ptics . . . . . . . . . . . . . . . . . . . . . . .
157
S e le c tio n of E talo n s . . . . . . . . . . . . . . . .
160
Reticon Photodiode Array . . . . . . . . . . . . . .
163
T r ig g e r in g E l e c tr o n ic s . . . . . . . . . .
.................. 164
Procedure fo r O btaining F abry-P erot In te rfe ro g ra m s . 168
R e s u lts and D iscussion ..................................................................... 170
S p e c tr a l Bandwidth o f Dye Laser a t 377.6 nm . . . . . 170
Fixed-A ir-G ap E talo n ....................................... . . . . .
170
S p e c tr a l Bandwidth o f Dye Laser a t 566.6 n m .......................173
Fixed-A ir-G ap E t a l o n ..............................
173
Solid Etalon
. . . . . . . . . . .
175
S p e c tr a l Bandwidth of Frequency Doubled R ad iatio n . . 179
S olid E t a l o n
. 179
S p e c tr a l Bandwidth of Helium-Neon CW Laser .................. 183
Fixed-Air-Gap Etalon ....................................... . . . . .
184
S o lid E t a l o n ..............................
186
D e-convolution of Laser S p e c tr a l Bandwidth
Measurements ..................................................................... 188
Summary of R esu lts
.....................
189
P r a c t i c a l C o n sid e ra tio n s . . . . . . . . .
.......................... 192
High Performance Fabry-Perot I n t e r f e r c m e t r y .......................193
- vii -
Mechanical I n s t a b i l i t y ........................................................
Thermal I n s t a b i l i t y
..............................
Appendix
A.
194
195
page
GATED I N T E G R A T O R .......................................... 196
P u r p o s e ................................................................................................... 196
Design and C o n stru ctio n . . . . . . . . . . . . . . . .
196
O peration o f the Gated I n t e g r a t o r . . . . . ....................... 201
BIBLIOGRAPHY
.................................................
- viii -
202
LIST OF TABLES
Table
page
1.
In s tr u m e n ta tio n and A p p a r a t u s ...................................................................... 22
2.
Optimized V a ria b les
3.
I n s t r u m e n t a t i o n .................................................................................................... 39
4.
D e te c tio n L im its ................................................................................................. 68
5.
R e la tio n s h ip of Average Background C u rren t to Peak P u lse
C u r r e n t..................................................................................................................70
6.
Background C urrent as a Function o f S l i t W i d t h ...................................71
7.
I n s tr u m e n ta tio n and A p p a r a t u s ..................................................................... 84
8.
H eating R ates and F in a l Temperatures o f 8 mm and 10 mm
T u b e s .................................................................................................................. 112
9.
R e p r o d u c ib i lity o f Maximum F in a l Temperature o f 8 mm and
10 mm Furnace T u b e s .................................................................................... 113
10.
H eating R ates fo r the Laboratory C o n stru cted Furnace
S y s t e m ..............................................................................................................115
........................................................................................ 25
11. H eating Rate and Temperature Measurements fo r a Commercial
Furnace Using th e HGA-500 or HGA-2000 Power Supply . . .
116
12. R e p r o d u c ib i lity o f Sample I n tr o d u c ti o n f o r Wall Sampling,
Probe, and P ro b e -a s -P la tfo rm A tom ization ...............................
125
13.
Carbon Tube Furnace LEAFS D e tec tio n L im its ....................................
126
14.
S c a t t e r and the Resonance D e te c tio n o f Cesium
............................
131
15.
I n te r f e r o m e te r System Components ..........................................................
156
16.
Performance Param eters f o r S o lid E ta lo n s .........................................
162
17.
Performance Param eters fo r Fixed-A ir-G ap E talo n s .......................
163
- ix -
18.
Summary o f Laser S p e c t r a l Bandwidth Measurements ......................
- x -
191
LIST OF FIGURES
F ig u re
page
1.
Block Diagram of EDL E x c ite d AFS I n s tr u m e n ta tio n ................................. 21
2.
I n f lu e n c e o f th e Weight o f S i l i c a Chips in EDL on the
Fluorescence S ig n a l ........................................................................................28
3.
C urren t Workload fo r C onventional PMT O p e ra tio n ................................... 32
4.
C urrent Workload f o r Gated PMT O p e ra tio n .................................................. 35
5.
Block Diagram o f LEAFS In s tr u m e n ta tio n .................................................. 41
6.
Line Frequency Synchronized O s c i l l a t o r .......................................................42
7.
Tapered Dynode Chain fo r R212UH P M T .........................................................48
8.
Tapered Dynode Chain f o r 9893QB P M T ......................................................... 49
9.
G a tin g Board C i r c u i t r y w ith Dynode Chain (Thorn-EMI)........................ 51
10.
Gate Pulse G enerator C i r c u i t ....................................................................... 53
11.
Timing Diagram f o r Laser T r ig g e r in g and F ir in g , PMT
Gating, and Boxcar S ign al P r o c e s s in g ...................................................55
12.
L aser F i r i n g Delay C ir c u it/D a ta Link T r a n s m itte r ................................ 57
13.
Timing Diagram fo r Laser F ir in g Delay C i r c u i t r y ...................................58
14.
Block Diagram o f T r ig g e r in g System Components....................................... 60
15.
A bbreviated Dynode Chain f o r 9893QB
16.
SNR v s . S l i t Width f o r Calcium (422.7 nm/422.7 nm)............................... 64
17.
SNR v s . S l i t Width fo r Ir o n (296.7 rro/373.5 nm)................................... 66
18.
L in e a r Dynamic Range fo r R212UH (1P28) PMTs
19.
L in e a r Dynamic Range o f 9893QB PMT............................................................ 75
- xi -
62
.................................... 73
20.
I d e a liz e d LEAFS Furnace.....................................................................................80
21.
Block Diagram of Furnace LEAFS In s tru m e n t..............................................82
22.
Carbon Tube Furnace P o s itio n e d Between Pole Pieces of
E lectrom ag net....................................................................................................83
23.
V a ria b le Frequency O s c i l l a t o r fo r Laser T rig g e rin g ..................... 87
24.
L aser Beam Expansion and S te e r in g O p tic s ................................................92
25.
Gaussian Laser Beam C h a r a c t e r i s t i c s ...........................................................93
26.
F u n c tio n o f Beam Expanding O p t i c s ..........................................................94
27.
Design f o r Furnace Tube F a b r i c a t i o n ...........................................................96
28.
Furnace E le c tro d e Assembly............................................................................. 98
29.
Furnace Enclosure f o r I n e r t Atmosphere.................................................. 100
30.
O p tic a l B a ffle s fo r Furnace Em ission...................................................... 102
31.
Probe Accessory fo r LEAFS i n a Carbon Tube Furnace........................ 108
32.
D e t a i l s o f Probe Tip and P o s itio n in Furnace Tube.......................... 109
33.
Probe Atom ization of R e p lic a te 500 pg Thallium Samples.
34.
P ro b e -a s -P la tfo rm A tomization o f Lead Samples...................................124
35.
L in e a r Dynamic Ranges fo r Iro n and L e a d ........................................... 129
36.
F a b ry -P e ro t I n t e r f e r e n c e F rin g es and I n t e n s i t y P r o f i l e .
37.
M u ltip le R e f le c tio n s o f Light Within an E talon .........................
38.
Design o f Laboratory C onstructed F a b ry -P e ro t
I n t e r f e r o m e t e r ................................................................................................155
39.
Block Diagram o f I n te r f e r o m e te r D etec tio n System............................ 158
40.
Gimbal Adapter fo r Fixed-Air-Gap E ta lo n s ............................................. 159
41.
T r ig g e r in g C ir c u itr y fo r Synchronizing D e tec tio n of Laser
I n t e r f e r e n c e F rin g es by th e Photodiode A rray............................... 166
42.
Timing Diagram fo r T rig g e rin g C i r c u i t r y ..........................................167
43.
In te rfe ro g ra m fo r Laser R ad iatio n a t 377.6 nm.................................. 172
- xii -
. .
122
. . 144
146
44.
In te rfe ro g ra m of Laser Output a t 566.6 nm Using a FixedAir-Gap E t a lo n ................................................................................................ 174
45.
In te rfe ro g ra m of Laser Output a t 566.6 nm Using a S o lid
E ta lo n ................................................................................................................. 176
46.
In te rfe ro g ra m o f Frequency Doubled L a ser O utput........................ 181
47.
I n te rf e ro g r a m o f Helium-Neon Laser Output Obtained With a
Fixed-Air-Gap E ta lo n ...........................................................................185
48.
In te rf e ro g r a m f o r He-Ne Laser Output Obtained With Solid
E ta lo n .........................................................................................................187
49.
Block Diagram and P a r t i a l Schematic o f Gated I n t e g r a t o r .
. .
199
50.
S im p lifie d I l l u s t r a t i o n of M o d ific atio n s to I n t e g r a t o r .
. .
200
- xiii -
Chapter I
GENERAL INTRODUCTION
1.1
LASER EXCITED ATOMIC FLUORESCENCE SPECTROMETRY
Laser E xcited Atomic Fluorescence Spectrom etry (LEAFS) has been shown
to be one of th e most s e n s i t i v e s p e c tro s c o p ic te ch n iq u es f o r the
d e te rm in a tio n o f t r a c e m e ta ls in samples by use of flames (1 -1 0 ),
plasmas (1 1 -1 3 ), and carbon fu rn ac es (1 4 -2 0 ).
E arly a n a l y t i c a l
a p p lic a tio n s o f flame LEAFS (1) r e s u l t e d in d e t e c t i o n l i m i t s t h a t were
only s l i g h t l y b e t t e r than f o r flu o re s c e n c e e x c i t a t i o n by con ven tion al
sources and were poorer than th o se of g ra p h ite fu rn ace atomic
a b s o rp tio n sp ectro m etry (GFAAS).
Kuhl and S p itsc h a n (3) introduced
the use of frequency-doubled l a s e r r a d i a t i o n which allowed U.V.
e x c i t a t i o n of t r a n s i t i o n s f o r elem ents (Mg 285.2 nm, Ni 305.1 nm, and
Pb 283.3 nm) t h a t had not been p re v io u s ly r e p o r te d .
Weeks e t a l . (5)
demonstrated t h a t flame LEAFS s e n s i t i v i t y could be improved by
expanding the l a s e r beam to i r r a d i a t e a la rg e volume of a n a ly te atoms
in the flame as long as th e atoms were s a t u r a t e d by th e l a s e r
ra d ia tio n .
Under s a t u r a t i o n c o n d itio n s , th e r a t e o f flu o re s c e n c e
approached th e r a t e of e x c i t a t i o n .
The d e t e c t i o n l i m i t s re p o rte d in
r e f . 5, re p re s e n te d an improvement over p re v io u s ly r e p o rte d r e s u l t s
(1-3) f o r LEAFS.
Bolshov e t a l . (15) in r e p o r t i n g fu rn a c e LEAFS
work, suggested t h a t flame a to m iz atio n may not perm it th e r e a l i z a t i o n
of th e advantages of l a s e r e x c i t a t i o n .
This was due p r im a rily to the
high s p e c t r a l background o f a n a l y t i c a l l y u s e f u l flam es and th e
p o t e n t i a l f o r quenching o f flu o re s c e n c e by flame s p e c ie s .
This may
have been a v a lid assum ption in view o f th e s t a t e of development of
tu n a b le dye l a s e r s a t t h a t tim e.
P u ls e d -tu n a b le dye l a s e r s a re
c u r r e n t l y a v a i l a b l e t h a t p rov id e s u p e r io r performance i n terms of both
i n t e n s i t y and narrow s p e c t r a l bandwidth, r e l a t i v e t o tho se a v a il a b le
a s r e c e n t l y as f iv e y e a rs ago.
Flame LEAFS in s tru m e n ta tio n i s s i g n i f i c a n t l y more expensive than
commercial GFAAS in s tr u m e n ta tio n , but th e s e n s i t i v i t i e s of th e two
techn iqu es a re only comparable.
Flame LEAFS has not been developed
commercially a s an a n a l y t i c a l technique p r im a r i ly because o f th e
c o m p le x itie s and in th e p a s t , o fte n poor r e l i a b i l i t y of l a s e r
o p e ra tio n .
Omenetto and Human (21) reviewed th e g e n e ra l
c o n s id e r a tio n s f o r LEAFS in flam es, plasm as, and f u r n a c e s , and
recommend th a t the flame no t be e n t i r e l y abandoned as an atom c e l l fo r
LEAFS, because of i t s r e l a t i v e s im p l i c i t y and r a p i d i t y o f a n a l y s i s .
These a u th o rs su g g est t h a t f o r optimum flame LEAFS work, th e l a s e r
peak power should approach s a t u r a t i o n a t a l l w avelengths t o o b ta in the
maximum flu o re s c e n c e i n t e n s i t y .
Piepmeier (22) reviewed the
t h e o r e t i c a l a s p e c ts o f s a t u r a t i o n in LEAFS.
O liv a re s and H i e f t j e (23)
ex p erim en tally measured th e s a t u r a t i o n s p e c t r a l power d e n s i t i e s of
elem ents and
compared them to p re d ic te d t h e o r e t i c a l valu es (24).
Omenetto e t a l . (7) dem onstrated flame LEAFS t o be a v ia b le tech n iq u e
f o r the d e te rm in a tio n o f le a d i n whole blood.
The a u th o rs were a b le
t o expand the l a s e r beam d ia m e te r, th ereby i r r a d i a t i n g a la r g e volume
of a n a ly te atoms, while m a in ta in in g s a t u r a t i o n c o n d itio n s .
S c a tte r
was avoided by u sin g a d i r e c t l i n e non-resonance t r a n s i t i o n (283*3
nm/405.8 nm).
Hovis and Gelbwachs (10) used CW l a s e r e x c i t a t i o n to
o b ta in flu o r e s c e n c e o f the barium ion in a n i t r o u s - o x i d e / a c e t y l e n e
flame.
A t h e r m a l l y - a s s i s t e d , non-resonance t r a n s i t i o n was used and
was found to be s u p e r io r t o re s o n a n t f lu o r e s c e n c e o f n e u t r a l barium.
The 0 .7 ng/ml d e t e c t i o n l i m i t o b ta in e d f o r barium io n flu o r e s c e n c e was
lower than Ba d e t e c t i o n l i m i t s o b ta in e d by both n e u t r a l atom flame
LEAFS ( 5 ,8 ,9 ) and l a s e r enhanced i o n i z a t i o n (2 5 ).
Omenetto e t a l . (1 1 ), Human e t a l . (1 2 ), and Huang e t a l . (13)
have r e p o rte d d e t e c t i o n l i m i t s f o r l a s e r e x c ite d atomic and io n ic
flu o r e s c e n c e f o r s e v e r a l elem ents in the i n d u c tiv e ly coupled plasma
(ICP).
The ICP i s l e s s s u b je c t to p h ysico-ch em ical i n t e r f e r e n c e s than
a flame and t h e r e f o r e may be a more i d e a l atom c e l l f o r LEAFS.
S ev eral atomic and io n ic non-resonance t r a n s i t i o n s , not e a s i l y
a c c e s s i b l e in flam es, a r e a v a i l a b l e f o r use in th e ICP (12 ).
1.2
FURNACE ATOMIZATION FOR ATOMIC FLUORESCENCE SPECTROMETRY
E arly atomic flu o r e s c e n c e i n v e s t i g a t i o n s were c a r r i e d out i n flames
u s in g c o n v e n tio n a l l i g h t so u rc es f o r e x c i t a t i o n (2 6 ,2 7 ).
R ese arch e rs,
re c o g n iz in g the advantages o f non-flame a to m iz a tio n f o r atomic
a b s o r p tio n work, soon experim ented w ith t h i s approach f o r atomic
flu o r e s c e n c e s p e c tro m e try .
K irk b rig h t (28) reviewed th e use o f
fu rn ace a to m iz ers in atomic s p e c tro m e try , in c lu d in g atomic
flu o r e s c e n c e a p p l i c a t i o n s .
Massmann (29,30) r e p o r te d a h eated
g r a p h ite c e l l f o r atomic flu o r e s c e n c e measurements w ith hollow cathode
lamp (HCL) e x c i t a t i o n .
His samples (30 pi or l e s s ) were in tro d u c e d by
a sy rin g e i n t o a g r a p h ite cup t h a t had s l i t s in o p p o s ite s i d e s .
In
g e n e r a l, th e s e n s i t i v i t y o f fu rn a c e atomic f lu o r e s c e n c e was observed
to be comparable to t h a t o f a b s o r p tio n measurements made in th e same
c e ll.
Massmann however, p r e d i c t e d t h a t fu rn a c e atomic flu o r e s c e n c e
s e n s i t i v i t i e s could be improved w ith more in t e n s e l i g h t s o u rc e s .
West
e t a l . (31 —31*) > in a fo u r p a r t a r t i c l e , r e p o rte d th e d e sig n ,
c o n s tr u c ti o n , and use of a h ig h ly e f f i c i e n t carbon f ila m e n t atom
r e s e r v o i r f o r atomic flu o r e s c e n c e and atomic a b s o r p tio n sp e c tro m e try .
Small volumes (5 pi or l e s s ) o f a n a ly te s o l u t i o n were p laced on the
fila m e n t and atomized by p a ss in g a 100 A c u r r e n t through th e fila m e n t
(in an argon atm osphere).
A v a r i e t y o f m e ta ls were i n v e s t i g a t e d u sing
both hollow cathode lamp and e l e c t r o d e l e s s d is c h a rg e lamp e x c i t a t i o n .
The d e te c tio n l i m i t s o b ta in e d were among the b e s t r e p o r te d in the
l i t e r a t u r e f o r t h i s te c h n iq u e .
O utstanding were th e d e t e c t i o n l i m i t s
r e p o r te d f o r zin c (32) and cadmium (33) which were 2 x 10
-14
g
5
and 1.5 x 10
-13
J g, r e s p e c t i v e l y .
In r e f . 3H, gold a n a ly te was
vaporized from one fila m e n t while a second f ila m e n t, in c lo s e
p ro x im ity , was used t o v a p o riz e p o t e n t i a l i n t e r f e r i n g s p e c ie s in order
to i n v e s t i g a t e vapor phase i n t e r f e r e n c e s .
th e r e were no such i n t e r f e r e n c e s .
The a u th o rs concluded t h a t
Amos e t a l . (35) re p o rte d a
c l i n i c a l a p p lic a tio n f o r carbon rod a to m iz a tio n AFS t h a t involved th e
d i r e c t d e te rm in a tio n o f le ad in blood.
These a u th o rs in v e s t ig a te d a
number o f vapor phase i n t e r f e r e n c e s t h a t were normally a s s o c ia te d w ith
th e a n a ly s is of le ad and found t h a t th e s e i n t e r f e r e n c e s could be
reduced c o n sid e ra b ly by surrounding th e carbon rod atom izer w ith a
hydrogen flam e.
B r a tz e l e t a l . (36) r e p o r te d a platinu m loop
ato m izer which was shown to be an e f f i c i e n t atom izer f o r v o l a t i l e
elem ents such a s cadmium, g a lliu m , and mercury in a v a r i e t y of
m a tr ic e s .
N o n -v o la tile elem ents were not s tu d ie d .
Bartschmid (37)
used a flow -through g r a p h ite fu rn ac e atom izer f o r n o n -d is p e rs iv e
atomic flu o re s c e n c e sp ec tro m e try .
E x c ita tio n was c a r r i e d out u sing
e l e c t r o d e l e s s d isch a rg e lamps (EDLs).
Atomic flu o re s c e n c e was
d e te c te d above the end of the furnace tu b e .
N o n -d isp ersive d e te c tio n
was accomplished u sin g a s o l a r - b l i n d p h o to m u ltip lie r tube (PMT).
h ig h - a p e r tu r e system allowed high l i g h t throughput as w ell as the
sim u ltaneou s d e te c tio n o f a l l resonance l i n e s o c c u rrin g w ith in the
s p e c t r a l response range of th e PMT.
Black e t a l . (38) r e p o rte d a
platinum tube furnace fo r continuous n e b u liz a tio n AFS w ith EDL
e x c ita tio n .
I n t e r f e r e n c e s t h a t suppressed the flu o re s c e n c e s ig n a l
were removed when th e fu rnace tem perature was r a i s e d from 1350° to
1600°C.
This
P a te l e t a l . ( 3 9 ,MO) used a g ra p h ite rod atom izer f o r atomic
f lu o re s c e n c e e x c ite d w ith m u ltip le elem ent th e rm o s ta tte d EDLs.
D e te c tio n l i m i t s in th e range of 0.1 ng to 0.01 pg were o b ta in e d f o r
the elem ents Ag, Cd, Cu, Hg, Pb, Sn, T l, and Zn.
chemical i n t e r f e r e n c e s were observed.
No s p e c t r a l or
A sim ultan eou s d e te rm in a tio n of
s i l v e r and copper in j e t engine o i l s was performed (MO).
The
d e te c tio n l i m i t s ( r e f s . 3 9 , MO) were comparable, i f not s u p e r io r , to
those r o u t i n e l y o b ta in a b le w ith g ra p h ite fu rn ace atomic a b s o r p tio n
sp ec tro m e try .
Murphy e t a l . (M1) compared th e use of a carbon rod
atom izer fo r d i s c r e t e sampling, along w ith a s l o t t e d carbon rod and
carbon tube fu rn ace f o r con tin uo us n e b u liz a tio n , f o r AFS, where photon
counting and l o c k - i n d e t e c t i o n were compared.
Molnar and Winefordner
(M2) used a v i t r e o u s carbon furn ace f o r co ntinu ou s sample in tr o d u c tio n
f o r AFS.
A pneumatic n e b u liz e r w ith a d e s o lv a tio n chamber was used
with th e v i t r e o u s carbon fu rn ac e to give an a s p i r a t i o n e f f i c i e n c y of
93$.
The a u th o rs in v e s t i g a t e d the decay of atom p o p u la tio n s above th e
furnace o u t l e t u sin g e i t h e r an Ar-H2 d i f f u s i o n flame o r an Ar sh e a th .
Chuang and Winefordner (M3) employed a Molnar type (MM) g ra p h ite
fila m e n t atom izer f o r continuum source e x c ite d AFS.
D e te c tio n l i m i t s
were r e p o rte d f o r s e v e ra l m e ta ls.
The e a r ly use o f fu rn ac e a to m iz atio n f o r atomic flu o re s c e n c e
spectrom etry gave r e s u l t s which i n d ic a te d t h a t t h i s might be an id e a l
combination f o r t r a c e m etal a n a l y s i s .
However, as p r e d ic te d by
Massmann (30 ), th e f u l l p o t e n t i a l o f furnace atomic flu o re s c e n c e
spectrom etry may only be r e a l i z e d w ith in te n s e l i g h t s o u rc e s .
1.3
FURNACE LEAFS
LEAFS d e t e c t i o n l i m i t s , in th e low femtogram range f o r some m e ta ls,
have been o b tain ed w ith furnace a to m iz a tio n (1M ,15,17).
These
d e te c tio n l i m i t s a re th e b e s t of any a n a l y t i c a l tech niq ue to d ate and
a re from te n to one thousand tim es lower than th o se re p o rte d f o r
GFAAS.
Bolshov e t a l . (14—16) used a g r a p h ite cup fu rn a c e , b u t
p o in te d out t h a t tu b e fu rn ac es such a s th o se commonly used in AA may
provide h ig h e r a to m iz a tio n e f f i c i e n c y , and su gg ested t h a t an in c re a s e
i n furnace LEAFS s e n s i t i v i t y may be o b ta in e d by improving the design
of the a to m iz e r.
Bolshov e t a l . (15,16) employed cup fu rn ace LEAFS
f o r tr a c e m etal d e te rm in a tio n s in a g r i c u l t u r a l samples, and r e p o rte d a
b o ric a cid i n t e r f e r e n c e f o r the d e te rm in a tio n o f ir o n .
Hohimer and
H argis (17) employed a v itr e o u s carbon rod atom izer to o b ta in a 20
femtogram d e t e c t i o n l i m i t fo r th a lliu m .
The atom izer was designed
e x p re ss ly f o r atomic flu o re s c e n c e work and was equipped w ith o p t i c a l
b a f f l e s to reduce th e amount o f fu rn ace continuum r a d i a t i o n re a c h in g
the d e t e c t o r .
The a u th o rs used an i n t e r f e r e n c e f i l t e r fo r
n o n -d is p e rs iv e d e te c tio n of th e th a lliu m flu o r e s c e n c e .
Neumann and
K riese (18) a ls o used a carbon rod ato m izer s i m i l a r to t h a t re p o rte d
by Amos e t a l . (35) f o r LEAFS d e te r m in a tio n s o f le a d .
For the purpose
o f comparison, e x c i t a t i o n was a l s o c a r r i e d out u sin g both hollow
cathode lamps and EDLs.
The LEAFS d e t e c t i o n l i m i t o b ta in e d here was
about one o rd e r o f magnitude lower th an t h a t o b ta in e d w ith th e
c o n v en tio n al s o u rc e s .
The l a s e r ir r a d i a n c e was high enough to
approach s a t u r a t i o n c o n d itio n s , r e s u l t i n g in an improvement in l i n e a r
dynamic range r e l a t i v e t o EDL o r HCL e x c i t a t i o n .
More r e c e n t l y ,
Wittman and Winefordner (19 ), and G oforth and Winefordner (2 0 ), have
r e p o r te d th e use of carbon rod a to m iz a tio n f o r LEAFS.
Wittman and
W inefordner (19) used an atom izer s i m i l a r to t h a t r e p o r te d by Wynn e t
a l . (4 5 ).
The a u th o rs r e p o rte d d e t e c t i o n l i m i t s f o r manganese,
sodium, and t i n .
A study o f quenching o f th e f lu o r e s c e n c e by v a rio u s
s h e a th gases was c a r r i e d o u t.
G oforth and W inefordner (20) p aid
p a r t i c u l a r a t t e n t i o n to p r e - tr e a tm e n t o f th e g r a p h ite fu rn a c e s in an
a tte m p t t o avoid problems a s s o c i a t e d w ith the fo rm atio n o f r e f r a c t o r y
c a r b id e s allo w in g some improvement in d e te c tio n l i m i t s f o r elem en ts
t h a t form such m a t e r i a l s .
A lso, sub-picogram d e te c tio n l i m i t s f o r
le a d and indium were r e p o r te d .
The advantages of fu rn ac e a to m iz a tio n over flame and plasma
a to m iz a tio n f o r LEAFS are a s f o llo w s .
Furnace a to m iz ers produce a
denser atomic vapor w ith in c re a s e d sample re s id e n c e time r e l a t i v e to
fla m e s.
The a to m iz atio n e f f i c i e n c y o f fu rn a c e s approaches 100$ as
compared t o about 10$ f o r fla m e s.
Furnace a to m iz ers pro v id e a
r e l a t i v e l y i n e r t environment which i s f r e e of many o f th e m olecu lar
s p e c ie s t h a t quench atomic flu o r e s c e n c e in fla m e s.
Furnace a to m iz ers
only r e q u ir e small samples and th e a n a l y s i s o f s o l i d samples i s
p o s s ib le (4 6 ,4 7 ).
1.4
VAPORIZATION INTERFERENCES IN FURNACE ATOMIZERS
D esp ite the h igh s e n s i t i v i t y dem onstrated f o r fu rn a c e LEAFS, th e r e
have been few r e p o r t s of r e a l sample a n a ly s e s by t h i s te c h n iq u e .
This
may be due i n p a r t , to th e poor r e p u t a t i o n o f fu rn ac e a to m iz ers
re g a rd in g p h y sico -ch em ical v a p o r iz a ti o n i n t e r f e r e n c e s .
Although i t
has not been explored r i g o r o u s l y , i t i s very l i k e l y t h a t many o f th e
in t e r f e r e n c e e f f e c t s a s s o c i a t e d w ith r e a l sample a n a ly s e s by GFAAS may
be j u s t a s complex in fu rn a c e LEAFS.
Research a c t i v i t y i n GFAAS has
g e n e ra te d s e v e r a l approaches to re d u c e, e l i m i n a t e , or compensate f o r
chemical or v o l a t i l i z a t i o n i n t e r f e r e n c e s .
The main approaches have
been th e use of m atrix m o d ifie rs (4 8 ,4 9 ) , r a p id ly h e a te d fu rn aces
(4 9 -5 1 ), th e L'vov p la tfo rm (4 9 ,5 2 ,5 3 ) and probe a to m iz a tio n (5 4-5 6 ).
These approaches a l l in v o lv e a tte m p ts to v a p o riz e the sample i n t o an
id e a l is o th e r m a lly h e ate d environm ent.
This has been shown to promote
a to m iz a tio n e f f i c i e n c y by m inim izing i n t e r f e r e n c e s (5 7 ,5 8 ) .
Physico-chem ical i n t e r f e r e n c e s a s s o c ia te d w ith r e a l samples m a tric e s
can cause accuracy problems f o r GFAAS.
The r e s u l t i s u s u a lly a s ig n a l
su p p re ssio n r e l a t i v e to pure aqueous s o l u t i o n s c o n ta in in g the same
mass o f a n a l y t e .
This may a ls o be t r u e f o r fu rn a c e LEAFS.
However,
the combined use o f most o f th e a to m iz a tio n s t r a t e g i e s noted above has
been s u c c e s s f u l f o r GFAAS and in a wide v a r i e t y o f c ase s have allowed
aqueous s ta n d a rd s to be used f o r th e d e te r m in a tio n o f m etals in r e a l
samples.
1.5
SPECTRAL INTERFERENCES
S p e c tr a l i n t e r f e r e n c e s t h a t occur in GFAAS a r e e q u a lly p ro b le m a tic .
These in c lu d e a b s o r p tio n o f source r a d i a t i o n by m olecular s p e c ie s ,
s c a t t e r i n g o f sou rce l i g h t by unvaporized p a r t i c l e s , and furnace
blackbody em issio n .
The s p e c t r a l background in c r e a s e s th e a n a l y t i c a l
s i g n a l and u n le s s i t i s compensated f o r , can s e v e re ly l i m i t the
accuracy of a g iv en d e te rm in a tio n .
A number of p o t e n t i a l s p e c t r a l
i n t e r f e r e n c e s e x i s t f o r flame LEAFS, in c lu d in g s c a t t e r and m olecular
f lu o r e s c e n c e .
Several examples o f flame m olecular flu o re s c e n c e have
been documented.
Weeks and W inefordner (5) were unable to observe
atomic f lu o r e s c e n c e o f vanadium (385.54 nm) due to th e d i f f i c u l t y o f
f in d i n g t h i s resonance t r a n s i t i o n amongst th e CN m olecular
flu o r e s c e n c e in t h a t s p e c t r a l r e g io n .
Barnes e t a l . (59) observed
flu o r e s c e n c e o f th e CH r a d i c a l a t 431.5 nm w ith dye l a s e r e x c i t a t i o n .
M olecular f lu o r e s c e n c e in flames (60-63) i s more i n te n s e w ith l a s e r
e x c i t a t i o n th an w ith c o n v en tio n al l i g h t so u rc e s and t h e r e f o r e may be
a l s o more e v id e n t and cause s e r io u s background problems in fu rn ace
LEAFS.
However, many o f the m olecular s p e c ie s found in flames a re
probably not p r e s e n t in fu rn ac e a to m iz ers and as y e t ,
such
i n t e r f e r e n c e s have not been r e p o rte d pro b ab ly due to th e e a r ly s ta g e
o f development o f fu rn ace LEAFS.
In a d d i t i o n t o m olecular
f lu o r e s c e n c e , s c a t t e r of in c id e n t l a s e r r a d i a t i o n and blackbody
continuum fu rn a c e em ission a re p o t e n t i a l s p e c t r a l i n t e r f e r e n c e s in
fu rn ac e LEAFS.
1.6
BACKGROUND CORRECTION FOR LEAFS
In p r i n c i p l e , s c a t t e r , m olecular flu o r e s c e n c e , and fu rn ace em ission
can be c o r r e c te d f o r by scanning th e l a s e r wavelength to one or b oth
s i d e s o f th e a n a ly te e x c i t a t i o n l i n e t o make an e s tim a te of th e
background ( 6 ) .
This assumes t h a t th e s c a t t e r , m olecular
flu o r e s c e n c e , and fu rn ace em ission a r e th e same o f f - l i n e as they a r e
a t the a n a ly te l i n e .
This i s p ro b ab ly tr u e f o r s c a t t e r and fu rn ace
e m issio n , bu t may not be t r u e f o r m olecular flu o r e s c e n c e i f any
m olecular bands have s u f f i c i e n t s t r u c t u r e c lo s e to the a n a ly te l i n e .
C a re fu l s p e c t r a l a n a l y s i s o f th e background may then be ne ce ssa ry to
o b ta in an a c c u ra te c o r r e c t i o n .
The techn iq ue of scanning th e l a s e r
w avelength to make an e s tim a te o f th e background i s not p r a c t i c a l f o r
furnace LEAFS because o f th e t r a n s i e n t n a tu re o f the s ig n a l and
s e v e r a l a to m iz a tio n s would be r e q u ir e d to complete a scan.
In
a d d i t i o n , the slow scanning tech n iq u e does not d is c r im in a te a g a in s t
low frequency n o is e (64,65) which can degrade d e t e c t i o n l i m i t s .
A
scanning technique such as wavelength modulation o f th e dye l a s e r
o u tp u t could provide a ra p id background measurement during a s i n g le
a to m iz a tio n and would d is c r im in a te a g a in s t most low frequency n o i s e .
Wavelength modulation o f CW l a s e r s has been r e p o rte d (66-68) and i s
probably f e a s i b l e f o r pulsed dye l a s e r s as w e ll.
Wavelength
m odulation, l i k e th e slow -scanning te c h n iq u e , depends on i n t e r p o l a t i n g
the background from o f f - l i n e measurements and may not pro v id e an
a c c u ra te e s tim a te o f the background a t th e a n a ly te l i n e i f the
background has s u f f i c i e n t s t r u c t u r e .
Frueholz and Gelbwachs (69)
r e p o r te d a method fo r p o l a r i z a t i o n r e j e c t i o n of s c a t t e r e d l i g h t in
resonance atomic flu o r e s c e n c e d e t e c t i o n .
The a u th o rs were a b le to
d is c r im in a te a g a in s t s c a t t e r o f source l i g h t (CW l a s e r ) by ta k in g
advantage of th e high p o l a r i z a t i o n of th e s c a t t e r e d l i g h t and
i n s e r t i n g a p o l a r i z e r in th e d e te c tio n o p t i c s .
The p o l a r i z e r provided
e f f i c i e n t r e j e c t i o n of s c a t t e r e d l i g h t and allowed s i g n i f i c a n t
improvements in s i g n a l - t o - n o i s e r a t i o .
1.6.1
Zeeman Effect Background Correction
Zeeman e f f e c t background c o r r e c t i o n i s c u r r e n t l y one o f th e most
e f f e c t i v e methods of background c o r r e c t io n used in fu rn a c e atomic
a b s o r p tio n s p e c tro m e try .
The th eo ry and a p p l i c a t i o n o f Zeeman e f f e c t
background c o r r e c t i o n have been reviewed in d e t a i l by Stephens (7 0),
Yasuda e t a l . (7 1 ), and de L oo s-V ollebregt and De Galan (7 2 ).
Zeeman
background c o r r e c t i o n in modern commercial AA in stru m e n ts in v o lv e s
p la c in g the carbon fu rn ac e between th e pole p ie c e s o f an
e le c tro m a g n e t.
A magnetic f ie ld - i n d u c e d modulation o f th e atomic
a b s o r p tio n allow s a measurement o f the background at th e a n a ly te l i n e ,
in th e absence o f th e a n a l y t i c a l s i g n a l .
Zeeman e f f e c t background
c o r r e c t i o n , a ra p id modulation te c h n iq u e , d is c r im in a te s a g a in s t most
low frequency n o is e .
The atomic a b so r p tio n p ro c e ss i s a p re c u rs o r to
f lu o r e s c e n c e , hence Zeeman background c o r r e c t i o n should be, in
p r i n c i p l e , d i r e c t l y a p p lic a b le to LEAFS.
1.7
SPECIFIC AIMS OF THE PRESENT RESEARCH
There are a number o f s i m i l a r i t i e s between atomic a b s o r p tio n
spectrom etry and atomic flu o re s c e n c e spectrom etry which involve
p r im a r i ly , the req u ire m e n ts f o r the performance o f th e a to m iz e r.
This
i s because, f o r maximum s e n s i t i v i t y , both AAS and AFS r e q u i r e the
e f f i c i e n t p ro d u c tio n of a ground s t a t e atomic v ap or.
Recent
developments in a to m iz a tio n technology have have in s u re d t h a t t h i s can
be achieved f o r both aqueous s ta n d a rd s and a n a ly te in r e a l samples.
The carbon tu be fu rn a c e has been shown to be a more optimum
atom izer f o r a n a l y t i c a l work than o th e r fla m e le ss a to m iz e r s .
As has
been dem onstrated in fu rn a c e atomic a b s o rp tio n sp ec tro m e try , tube
fu rn ac es can more e f f e c t i v e l y m itig a te v a p o riz a tio n in t e r f e r e n c e s and
are th u s a more p r a c t i c a l choice f o r LEAFS a to m iz a tio n .
The primary
o b je c tiv e of t h i s r e s e a r c h i s to e x p lore th e f e a s i b i l i t y o f t h i s
approach and i f s u c c e s s f u l , e x p l o i t th e advantages o f carbon tube
furnace a to m iz atio n f o r l a s e r e x c ite d atomic flu o re s c e n c e spectrom etry
(LEAFS).
More s p e c i f i c a l l y , a tte m p ts w i l l be made to o b ta in d e te c tio n
l i m i t s th a t are comparable, i f not s u p e r io r to p re v io u s ly published
furnace LEAFS work.
Chapter II
PREPARATION OF ELECTRODELESS DISCHARGE LAMPS FOR ATOMIC
FLUORESCENCE SPECTROMETRY
2.1
INTRODUCTION
E le c tr o d e le s s d is c h a rg e lamps (EDLs) have been s tu d ie d f o r many y e a rs
as p o t e n t i a l so u rc es f o r atomic flu o r e s c e n c e sp ec tro sc o p y (AFS)
(73 -8 7 ), and d e s p it e th e la ck of optimum sou rce c o n d itio n s , good
r e s u l t s were o f t e n o b ta in e d .
S tu d ie s have been r e p o r te d (81) where
d e te c tio n l i m i t s f o r some m e ta ls, e . g . copper 8 x 10“ ^ pg/ml, ob tained
w ith EDL e x c ite d atomic flu o r e s c e n c e , were around 1000 tim es lower
than th o se o b ta in e d w ith low i n t e n s i t y hollow cathode lamp (HCL)
e x c ita tio n .
I t i s im po rtan t to note h e re , t h a t th e hollow cathode
lamps used in r e f . 81 were probably lim ite d in o u tp u t r e l a t i v e to th e
h i g h - i n t e n s i t y HCls r e p o r te d by S u ll iv a n and Walsh (8 8 ,8 9 ), and to th e
demountable, boosted o u tp u t HCLs r e p o rte d by S u ll iv a n (9 0 ), and
S u lliv a n and van Loon (9 1 ) , which were p o s s ib ly , more id e a l e x c i t a t i o n
sou rces f o r AFS.
The high-perform ance hollow cathode lamps a v a i l a b l e
today are based i n p a r t , on th e se d e s ig n s .
In some c a s e s , th e s e HCLs
may be comparable or s u p e r io r to EDLs a s so u rces f o r AFS.
The
req uirem ents n e ce ssa ry to a s s u re good accu racy , p r e c i s i o n ,
s e n s i t i v i t y , and low l i m i t s of d e t e c t i o n f o r EDL e x c it e d atomic
- 14 -
f lu o r e s c e n c e sp ectrom etry a r e : h igh i n t e n s i t y , s t a b i l i t y , and long
s h e l f and o p e r a tin g tim es (8 3 ).
In g e n e r a l , th e performance o f EDLs
i s a d i r e c t r e s u l t o f the method of p r e p a r a t i o n .
However, in s p i t e o f
d e t a i l e d d e s c r i p t i o n s o f EDL p r e p a r a t io n t h a t have appeared in th e
l i t e r a t u r e (9 2 -1 0 3 ), i t has o f te n been d i f f i c u l t to reproduce th o se
p ro c e d u res to o b ta in EDLs w ith p r o p e r t i e s s i m i l a r to th o se d e sc rib e d
(8 2 ).
S e v e ra l a u th o rs (76,84,101-103) have i d e n t i f i e d c e r t a i n
experim ental p aram eters f o r th e p r e p a r a t io n and o p e ra tio n of EDLs as
being c r i t i c a l to good perform ance.
Most o f th e e a r ly work (82) was concerned w ith o p tim iz in g th e lamp
m a t e r i a l s , s e l e c t i o n of m etals or m etal compounds, and s e l e c t i o n and
p r e s s u r e o f th e i n e r t f i l l g a s e s .
Much o f th e more r e c e n t EDL work
has inv o lv e d a tte m p ts to s t a b i l i z e th e lamps during o p e ra tio n in o rd e r
to o b ta in more s u i t a b l e l i g h t so urces f o r a n a l y t i c a l work.
The work
o f Browner e t a l . (97,104) in d ic a te d t h a t lamp tem p eratu re was th e
s i n g l e most im p ortant v a r i a b l e in c o n t r o l l i n g th e r a d i a n t o u tpu t from
an EDL.
Browner £ t a l . (97,104) found t h a t th e key to EDL s t a b i l i t y
was t h e r m o s ta ttin g th e lamps during o p e r a tio n to m ain ta in a c o n s ta n t
EDL o p e r a tin g tem p e ratu re independent o f microwave power.
P a te l,
Browner, and W inefordner (105) employed p r e c i s e tem p eratu re c o n t r o l
f o r m u lti-e le m e n t EDLs and optim ized th e o p e r a tin g te m p e ra tu re s f o r
s i n g le e l s n e n t and m u lti-e le m e n t a n a ly s e s .
B a ll (106) a l s o r e p o r te d a
d ev ice f o r te m p e ratu re s t a b i l i z i n g EDLs which employed r e s i s t i v e
h e a tin g of a nichrome w ire .
w ith in 2%.
The a u th o r r e p o r te d s t a b l e lamp o u tp u t
Baranov e t a l . (107) f a b r i c a t e d h igh frequency (RF) EDLs
t h a t were th e rm a lly s t a b i l i z e d by e n c lo s in g th e lamps in a vacuum
slee v e .
W alters and Smit (108) r e p o r te d th e use o f th e rm o s ta tte d RF
EDLs as so u rc e s f o r b o th AAS and AFS.
Here, s p e c t r a l bandwidths of
EDLs, o p e ra te d a t v a rio u s te m p e ra tu re s , were measured u s in g
in te rfe ro m e t r y .
M ansfield £ t a l . (76) dem onstrated, by s t a t i s t i c a l e v a lu a tio n of
t h e i r d a t a , t h a t EDL i n t e n s i t i e s u s e f u l f o r th e p ro d u c tio n o f atomic
f lu o r e s c e n c e , were d i r e c t l y r e l a t e d to lamp d ia m e te r, gas f i l l
p r e s s u r e , and th e form of element (m etal or m etal h a l i d e ) , b u t were
independent o f the weight o f m a te r ia l in tro d u c e d in t o th e lamp.
S y lv e s te r and McCarthy (80) c o r r e l a t e d lamp i n t e n s i t y to s e v e ra l
p aram eters o f p r e p a r a t io n , and e v a lu a te d EDL i n t e n s i t y b o th by d i r e c t
measurement and by using th e lamps t o e x c i t e atomic flu o re s c e n c e in
o rd e r to compare the i n t e g r a t e d l i n e i n t e n s i t y and th e i n t e n s i t y a t
th e l i n e c e n t e r .
Jansen e t a l . (109) p re p a re d s m a l l - s i z e EDLs f o r
t h e i r Zeeman scanning s t u d i e s of a b s o r p tio n l i n e p r o f i l e s in fla m e s.
The i n t e n s i t i e s o f EDLs and hollow cathode lamps (HCLs) were compared
as were the s p e c t r a l bandw idths.
I t was observed t h a t when the HCL
was o p e ra ted a t maximum c u r r e n t and th e EDL a t maximum microwave
power, th e HCL was a c t u a l l y more i n t e n s e , b u t th e EDL e x h ib ite d a
narrow er lin e w id th .
C h ild s and Schrenk (110) r e p o r te d th e c h a r a c t e r i s t i c s of low p r e s s u r e
s u l f u r EDLs which were prep ared u s in g s ix d i f f e r e n t s u l f u r c o n ta in in g
compounds and f i v e d i f f e r e n t i n e r t f i l l g a s e s .
Goode and Otto (111)
p re s e n te d a c r i t i c a l e v a lu a tio n o f EDL f a b r i c a t i o n d e t a i l s and
o p e r a tin g c o n d itio n s and emphasized t h a t the shape o f th e lamp bulb i s
a c r itic a l fa c to r.
Novak and Browner (112,113) in v e s t i g a t e d RF
e x c it e d EDLs and observed t h a t pulsed-mode o p e ra tio n was s u p e r io r to
CW o p e r a tio n .
In r e f . 1 1 3 a r e l a t i o n s h i p between atomic flu o re s c e n c e
s i g n a l s i z e and lamp duty c y cle was i l l u s t r a t e d .
Alexandrov e t a l .
(114) r e p o r te d the use o f RF e x c ite d EDLs f o r atomic a b s o r p tio n o f
l i t h i u m and a l k a l i n e e a r t h m e ta ls.
The e a r ly EDL l i t e r a t u r e ( p r i o r t o 1974) was reviewed by Haarsma
e t a l . (8 2 ).
In fo rm a tio n was compiled on i n e r t gas and p re s s u re s
u sed , m e ta ls or compounds used, and dimensions o f th e lamps, e t c .
Michel e t a l . (101-103) have more r e c e n t l y c o n c e n tra te d on improving
th e r e p r o d u c i b i l i t y of p r e p a r a t io n o f th e s e lamps.
th e l a t t e r work i s r e p o r te d h e re .
A c o n tin u a tio n of
Michel e t a l . (101-103) showed t h a t
th e r e p r o d u c i b i l i t y o f the manufacture o f EDLs depends p r im a r i ly on
c a r e f u l l y id e n t i f y i n g and c o n t r o l l i n g th e v a r i a b l e s which a re in h e re n t
i n the p r e p a r a t io n o f th e lamps.
This approach was s u c c e s s fu l f o r
cadmium (101,102) and selenium (103) when te n v a r i a b l e s were
i d e n t i f i e d and r ig o r o u s ly optim ized u s in g th e Simplex a lg o rith m
(1 1 5 ,1 1 6 ).
S th a p it e t a l . (86,87) used th e method of Michel e t a l .
(101-103) to p re p a re lead EDLs fo r AFS d e te r m in a tio n s o f le a d in blood
(86) and in ta p water (87).
2.1.1
Classical Method of Preparation
The c l a s s i c a l method of p r e p a r a t io n of EDLs (82) was very sim ple.
U sually the lamp blank was clean ed by flame h e a tin g th e quartz to
white h e a t under vacuum.
Then th e a p p r o p r ia te m etal o r m etal h a lid e
was put in th e lamp and sublimed w hile in th e lamp by flame h e a tin g to
d riv e o f f th e v o l a t i l e i m p u r itie s .
Then a few t o r r of an i n e r t gas
(u s u a lly argon) was added and th e lamp s e a le d .
obvious and
Some v a r i a b l e s were
always c a r e f u l l y o ptim ized, f o r example, the weight of
m a te r ia l and p re s s u re o f the
i n e r t gas in th e lamp.
The method of
Michel e t a l . (101-103) dem onstrated, however, t h a t th e su b lim a tio n
was c r i t i c a l .
In t h a t work, th e r e q u ir e d amount of m a te ria l was put
in th e lamp and the s u b lim a tio n was then brought about in a c o n tr o lle d
manner by i n i t i a t i n g a microwave d is c h a rg e in th e lamp blank while i t
was on th e vacuum system.
The v a r i a b l e s t h a t were c o n tr o lle d were th e
d u r a tio n and a p p lie d power o f the d is c h a rg e and th e f i l l p re s s u re of
argon d u rin g th e d is c h a rg e .
T his p rocedure worked f i n e f o r the
r e l a t i v e l y v o l a t i l e cadmium and selenium EDLs, but t h i s re s e a rc h
determ ined t h a t i t did not work f o r th e manganese EDLs.
This was
because the
h e a t provided by th e microwave f i e l d was not s u f f i c i e n t to
sublime th e
manganese io d id e t h a t was in tro d u c e d in t o th e lamp.
2.1.2
Modifications to Method of Preparation
Browner e t a l . (97,104) showed t h a t tem p erature c o n tr o l was c r i t i c a l
to lamp o p e r a tio n .
For the p re s e n t work, i t was p o s tu la te d t h a t
tem p eratu re c o n tr o l during lamp p r e p a r a t io n , i . e . du rin g the
su b lim a tio n s te p , may a l s o be c r i t i c a l t o lamp performance and
re p ro d u c ib ility .
In prev io u s EDL work r e p o r te d in th e l i t e r a t u r e , the
tem perature of the lamps during p r e p a r a t io n was a r e s u l t o f h e a tin g by
the microwave d is c h a rg e , or by flame h e a tin g in the e a r l i e s t work.
This i s probably not optimum f o r th e l e s s v o l a t i l e m e ta ls or metal
compounds.
Two approaches are d e sc rib e d h e re which were designed to
f a c i l i t a t e the su blim ation of l e s s v o l a t i l e m a t e r i a l s .
F i r s t , th e EDL
was th e rm o s ta tte d (97,104) while i t was on the vacuum system p r i o r to
and du rin g the su b lim atio n stag e and, second, ground s i l i c a c h ip s o f
approxim ately 0.5 mm average diam eter were put i n to the lamp blank.
Therm ostating the EDL w ith hot a i r r a i s e d th e tem perature to encourage
su b lim a tio n .
The s i l i c a c h ip s probably in c re a s e d th e r a t e of
s u b lim a tio n by in c r e a s in g th e s u rfa c e a re a from which th e m a te ria l
could sublim e.
2.2
EXPERIMENTAL
Except f o r th e m o d if ic a tio n s d e sc rib e d h e r e , manganese EDLs were
p re p a red in the manner d e sc rib e d in r e f s . 101-103 and the output o f
each EDL was measured by u sin g i t t o e x c it e atomic flu o re s c e n c e of
manganese i n a s to ic h io m e tr ic n itr o g e n - s e p a r a te d a i r - a c e t y l e n e flam e.
Figu re 1 i s a block diagram of th e AFS in s tr u m e n ta tio n used to
e v a lu a te th e Mn EDLs.
The in s tru m e n ta tio n used (Table 1) was s im ila r
to t h a t used in r e f s . 101-103.
Atomic flu o r e s c e n c e s i g n a l s were
d e te c te d by u sin g a s l i t width which encompassed a l l l i n e s o f th e
manganese t r i p l e t a t 280 nm.
21
E
F ig u re 1:
Block Diagram of EDL E x cited AFS In s tru m e n ta tio n .
TABLE 1
In s tru m e n ta tio n and Apparatus
component
m anufacturer
model n o .
monochromator
HR 320
f o c a l le n g th , 0.32 m
g r a t i n g , 1800 g/mm
ap ertu re , f/4 .2
d i s p e r s i o n , 1.67 nm/mm
s l i t w id th, 1.1 mm
In stru m e n ts SA, Metuchen, NJ
p h o to m u lti p lie r tube
9789QB
Thorn-EMI, F a i r f i e l d , NJ
PMT housing
PR-1M00RF
Products fo r Research
Danvers, MA
photon c o u n te r
1112
P rin c e to n Applied R esearch,
P r in c e to n , NJ
la b o r a to r y c o n s tr u c te d
m echanical chopper
microwave g e n e r a to r
M icrotron 200
Broida c a v ity
210 L (3MX)
tem p eratu re c o n t r o l l e r
TC-1000
h e a tin g elem ent
CHE 29767
a i r h e a ti n g assembly
EHA 129763
E lec tro -M e d ica l S u p p lie s ,
Wantage, UK
T h e a ll Engineering Co.,
Oxford, PA
GTE-Sylvania, E x e te r, NH
flame gas premix chamber
P erkin -E lm er, Norwalk, CT
c a p i l l a r y bu rn er and flame s e p a r a to r
la b o r a to r y c o n s tr u c te d
vacuum system
U n iv e rs ity o f C on n ecticut
T ec h n ica l s e r v ic e s
see r e f s .
101-103
EDL b lank s
ii
it
q u a rtz f o r EDL blank s
V itre o sil
Thermal American Fused Quartz
M o n tv ille , NJ
s i l i c a c h ip s
CFX 3600-0H
G&M A s s o c ia te s , Oakland, CA
2.2.1
New Simplex Variables
The v a r i a b l e s used t o c o n tr o l the p r e p a r a t io n o f th e EDLs were tho se
d e s c rib e d i n r e f s . 102 and 103 p lu s th r e e more v a r i a b l e s which were
r e l a t e d t o th e m o d if ic a tio n s making a t o t a l o f 13.
The new v a r i a b l e s
were d e fin e d as fo llo w s:
Q weight o f ground s i l i c a c h ip s (mg)
This i s th e weight o f ground s i l i c a c h ip s in tro du ced i n to
th e lamp bulb p r i o r to p r e p a r a t io n o f th e EDL.
The s i l i c a
c h ip s provided a la r g e s u rfa c e a re a f o r su b lim a tio n o f th e
m etal h a l i d e .
The s i l i c a c h ip s were cleaned by flame
h e a tin g th e lamp blank to white h e a t , under vacuum.
Tj th e rm o s ta tte d a i r te m p e ratu re (°C) used p r i o r t o and
d u rin g the su b lim a tio n s ta g e of th e p r e p a r a t io n .
T his h e a tin g s te p r a i s e d th e tem p e ratu re of the lamp to
encourage s u b lim a tio n o f th e m etal h a l i d e .
Heat was a p p lie d
to th e lamp f o r a p re h e a tin g p e rio d ( t c ) p r i o r to the
b
su b lim a tio n s te p , and a l s o f o r th e d u ra tio n o f the
s u b lim a tio n s te p ( t g ) .
p r e h e a tin g time (s) p r i o r to s u b lim a tio n .
This was the p e rio d during which th e lamp was heated b e fo re
th e microwave power was sw itched on to i n i t i a t e the
su b lim a tio n o f th e m etal h a lid e du rin g lamp p r e p a r a t io n .
This v a r i a b l e was designed to encourage ra p id s u b lim a tio n as
soon as th e microwave power was switched on.
24
2.2.2
Reagents
Manganese ( I I ) io d id e , was used f o r EDL p r e p a r a t io n , and a stock
1000 ppm manganese c h lo r id e s o lu tio n which was used f o r p re p a rin g
d i l u t e s ta n d a rd s , was o b tain ed from Alfa P ro d u cts, Danvers, MA.
2.3
RESULTS AND DISCUSSION
A computer program was w r i t t e n in BASIC language (117) to allow th e
Simplex a lg o rith m d e sc rib e d by Deming and Morgan (115) to be run on a
PDP 11-03 minicomputer.
A Simplex o p tim iz a tio n (115,116) o f the 13
v a r i a b l e s was c a r r i e d ou t.
The r e c i p r o c a l of th e d e te c tio n l i m i t
o b ta in e d w ith an EDL f a b r i c a t e d acc o rd in g to the c o n d itio n s suggested
by each Simplex v e r te x , was used as th e in p u t response to e v a lu a te
t h a t v e r te x .
The Simplex o p tim iz a tio n was h a lte d a f t e r 62 i t e r a t i o n s
because th e re was no longer a s i g n i f i c a n t change in th e performance o f
th e EDLs from v e r t i c e s 57-62.
The o p tim iz a tio n r e s u l t e d in EDLs t h a t
gave s i g n i f i c a n t l y b e t t e r d e te c tio n l i m i t s than th ose p re v io u s ly
prep ared a t th e beginning of th e o p tim iz a tio n p ro c e ss .
The optimum
v a lu e s fo r th e se v a r i a b l e s , a s determ ined by the Simplex proced ure,
a re given in Table 2.
TABLE 2
Optimized V a ria b les
V a ria b le
D e sc rip tio n o f
each v a r i a b l e
Optimum
le v e l
Boundaries3
Q
weight o f s i l i c a c h ip s
30 mg
5-50 mg
w
weight o f Mn
(in tro d u ce d as
Mnl2 s o lu tio n )
69 yg
10-300 yg
t^
time under vacuum
( a f t e r water removal)
385 s
60-800 s
argon p re s s u re
fo r d is c h a rg e
8 .0 mbar
1-25 mbar
T1
th e rm o s ta tte d h o t a i r
tem perature du rin g
sub lim a tio n
464 °C
400-520°C
t5
p re h e a tin g p e rio d
p r i o r to s u b lim a tio n
394 s
100-900 s
P.
a p p lie d microwave power
125 W
a f t e r on set of su b lim a tio n
50-220 W
t2
d u ra tio n o f
su b lim atio n d is c h a rg e
112 s
30-210 s
t_
time f o r c o o lin g
a f t e r d is c h a rg e
292 s
100-540 s
tjj
time under vacuum
a f t e r c o o lin g
586 s
100-1080 s
A2
argon f i l l p r e s s u r e
before s e a lin g
13.0 mbar
1-32 mbar
P2
microwave power f o r
o p e ra tio n
125 W
50-220 W
T2
th e rm o s ta tte d h o t a i r
tem perature f o r o p e ra tio n
480°C
400-540°C
a
Range over which each v a r i a b l e was
t e s t e d during the Simplex o p tim iz a tio n .
26
2.3.1
Reproduclb111ty of Preparation
In o rd e r t o e v a lu a te th e r e p r o d u c i b i l i t y o f p r e p a r a t io n , te n EDLs were
made by u sin g the optimized l e v e l s o f v a r i a b l e s in Table 2.
The
average d e te c tio n l i m i t was 0 .2 pg/1 (10 s i n t e g r a t i o n tim e,
s i g n a l - t o - n o i s e r a t i o equal t o 2 , where th e n o ise was measured by
ta k in g th e square ro o t of th e background a s measured on a photon
c o u n te r ) .
The d e te c tio n l i m i t was found to be determined by sho t
n o is e .
A 2 pg/1 manganese s o l u t i o n was used to measure th e d e t e c t i o n
lim it.
The average d e t e c t i o n l i m i t (0 .2 yg/1) was b e t t e r than
p re v io u s ly re p o rte d d e t e c t i o n l i m i t s f o r manganese EDLs ( t y p i c a l l y
6.0 pg/1 ( 8 2 ) ) .
The flu o re s c e n c e s i g n a l s o b ta in e d from each o f th e
te n EDLs, when u sin g a 100 pg/1 manganese s o l u t i o n , gave an average
1.1 x 104 c o u n ts /s w ith a r e l a t i v e stan d a rd d e v ia t io n of 1558.
T his i s
comparable to th e p re v io u s ly r e p o r te d r e p r o d u c i b i l i t i e s (101-103).
The 100 pg/1 flu o re s c e n c e s i g n a l was w ith in th e l i n e a r p a r t o f the
c a l i b r a t i o n curve.
of
The co n fid en ce i n t e r v a l based on th e t
d i s t r i b u t i o n and a 95$ co n fid en ce l e v e l was between 1.0 x 101* and
H
1.2 x 10 c o u n ts / s .
27
2.3«2
EDL Performance
The s t a b i l i t y o f a l l EDLs was such t h a t over about M h o f o p e r a tin g
time t h e i r output would d r i f t by ab o u t 3% per hour.
The d r i f t may
have been due to f l u c t u a t i o n s in th e microwave power or in the
tem p eratu re of the EDLs.
L ife tim e s , to h a l f o u tp u t, were a t l e a s t
50 h b u t were not measured beyond t h a t .
The m o d ific a tio n s to the method were aimed a t p ro v id in g f o r th e
same ra p id su b lim a tio n o f th e r a t h e r i n v o l a t i l e manganese io d id e a s
was achieved f o r the more v o l a t i l e cadmium and selenium compounds
(101-103).
The a tte m p t to make manganese EDLs with the unmodified
method r e s u l t e d i n a slow s u b lim a tio n (up t o 20 min) and demanded h ig h
microwave power.
The a d d i t i o n of s i l i c a c h ip s to th e lamp blank and
h e a tin g the lamp blank w ith th e rm o s ta tte d ho t a i r made ra p id
s u b lim a tio n p o s s ib le ( l e s s than 2 min).
A u n i v a r i a t e search (F ig u re
2) re v e a le d t h a t a t l e a s t 20 mg o f s i l i c a c h ip s in the EDL were
ne ce ssa ry to ach iev e flu o r e s c e n c e s i g n a l s comparable to th o se o b tain ed
w ith th e Simplex optim ized l e v e l o f 30 mg of s i l i c a c h ip s .
In c re a s in g
th e w eight of s i l i c a c h ip s beyond 30 mg did not r e s u l t in a p p re c ia b le
in c r e a s e s i n flu o re s c e n c e s i g n a l (Figure 2).
FLUORESCENCE SIGNAL
counts per second 1x10 )
28
00-
CM“
10
20
30
40
50
60
70
80
WEIGHT OF SILICA CHIPS IN EDL (mg)
Fig ure 2:
In flu e n c e o f th e Weight o f S i l i c a Chips in EDL on th e
Fluorescence S ig n a l. The e r r o r b a rs r e p r e s e n t th e 15$
r e l a t i v e sta n d a rd d e v i a t i o n o f th e f lu o r e s c e n c e s i g n a l s of
ten EDLs (see t e x t ) .
29
2.4
CONCLUSION
I t may be t h a t th e improved d e t e c t i o n l i m i t over p u b lish e d work was a
r e s u l t o f op tim al in s tru m e n ta tio n (a h igh throughput monochromator and
photon coun tin g) and c a r e f u l c o n tr o l o f a l l v a r i a b l e s which le d to
f a c i l e and hence more a c c u r a te o p tim iz a tio n .
However, th e r e i s a
p o s s i b i l i t y t h a t th e p resence o f th e s i l i c a c h ip s during th e o p e ra tio n
o f the lamp a f f e c t s i t s r a d i a n t o u tp u t, f o r example by promoting more
ra p id v a p o riz a tio n -c o n d e n s a tio n p ro c e sse s in th e lamp.
This could
le a d to more r a p id r e g e n e r a tio n of e x c ite d s t a t e s p e c ie s in th e lamp
and more ra p id p u rg ing o f quenching s p e c ie s by c o n d en sa tio n .
An
a d d i t i o n a l b e n e f i t o f r e s u l t i n g from th e m o d if ic a tio n s appeared to be
a re d u c tio n in warmup time f o r th e lamp.
f o r a l l lamps were reached w ith in 5 min.
F u ll i n t e n s i t y and s t a b i l i t y
Chapter III
PHOTOMULTIPLIER DETECTION, LASER AND BOXCAR TRIGGERING
FOR LEAFS
3.1
INTRODUCTION
Laser Excited Atomic Fluorescence Spectrometry (LEAFS) i s p o t e n t i a l l y
one o f the b e st tech niqu es f o r th e d e term in a tio n o f tr a c e m etals in
samples from th e p o in ts of view of both s e n s i t i v i t y and accuracy.
For
many m etals, d e te c tio n l i m i t s have been re p o rte d f o r flame (5) and
fu rn ace (15) LEAFS which a re b e t t e r than most o th e r s p e c tro s c o p ic
approaches in th e se atom c e l l s .
One o f th e primary a n a l y t i c a l
advantages of LEAFS i s th e very long l i n e a r dynamic ranges o f the
c a l i b r a t i o n curves which can extend over g r e a t e r th an s ix o rd e rs of
magnitude.
To cope w ith t h i s wide range of s ig n a l s i z e s , i t i s
normally necessary to vary th e p h o to m u ltip lie r tube (PMT) o p e ra tin g
v o lta g e in o rd er to reduce the c u r r e n t flow ing in th e dynode chain and
thus keep th e PMT w ith in i t s own l i n e a r o p e ra tin g ra n g e .
However,
when a high power nanosecond or microsecond pu lsed dye l a s e r i s used
f o r e x c i t a t i o n , problems may a r i s e because th e s ig n a l s durin g each
p u ls e are very la rg e (up to 100 mA a t the o u tp u t of th e WIT). T his i s
not q u ite as s e r io u s a problem as i t ap pears because th e average
c u r r e n t caused by the flu o re s c e n c e i s u s u a lly l e s s than th e t y p i c a l
- 30 -
average c u r r e n t t h a t th e PMT and i t s a s s o c ia te d dynode chain i s
cap ab le of s u s ta in in g (abo ut 100 to 200 pA f o r a dynode ch ain p ro p e rly
designed fo r pulsed s i g n a l s ) .
3.1.1
Pulse-Mode Photomultiplier Saturation
The s i t u a t i o n i s agg ravated i n any atom c e l l by broadband background
which alone could cause th e PMT t o o p e ra te c lo s e to i t s maximum
average c u r r e n t o u tp u t.
A PMT o p e ra tin g under such c o n d itio n s i s
o f te n unable to d e l i v e r th e high p u ls e c u r r e n ts in response t o h ig h
am plitude l i g h t p u ls e s .
This b ehav ior i s known as pulse-mode
s a t u r a t i o n (118), and o ccurs when p ulsed l i g h t s ig n a l s a re d e te c te d in
the presence of an i n t e n s e , co n tin u o u s background.
Figu re 3
i l l u s t r a t e s th e c u r r e n t workload f o r c o n v en tio n al PMT d e t e c t i o n of
pu lsed s ig n a l s in th e pre sen c e of a con tin uo us background s i g n a l .
Excessive dynode c u r r e n t s brought about by th e p resence o f an i n te n s e
background cause u n d e sired changes in dynode p o t e n t i a l s which in tu rn
le a d to changes in PMT g a in (119,120).
N o n -lin e a r d e t e c t i o n o f p u lsed
s i g n a l s i s a d i r e c t r e s u l t o f th e s e unwanted g a in changes t h a t a l t e r
th e s ig n a l s i z e .
Pulse-mode s a t u r a t i o n in LEAFS does not a r i s e j u s t
a t high c o n c e n tr a tio n s , where one would expect la r g e s i g n a l s , b u t a l s o
c lo s e to the d e t e c t i o n l i m i t .
Near th e d e te c tio n l i m i t , i t i s u s u a l
to use very wide monochromator s l i t w idths t h a t improve
s i g n a l - t o - n o i s e r a t i o s (SNRs). But a t th e same tim e, more background
i s allowed onto th e PMT, r e s u l t i n g in an in c re a s e in average c u r r e n t .
32
CONVENTIONAL
PMT OPERATION
O
LlJ
l/>
_J
3 '
Q_
I
\ S 'o '= s :
(X
LlJ
I—
5
o
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o3
UJ
o
z
I
llJ
to
UJ
UJ
G£
O
28
O
U_ —
s
o
5
iN a a y n o
F igu re 3:
sqonv
iw d
C urrent Workload fo r C onventional PMT O p eratio n.
3.1.2
Dynode Chain Design
The p r i n c i p a l l i m i t a t i o n on c u r r e n t o u tp u t of a p h o to m u lti p lie r i s the
form ation of a space charge a t th e l a s t few s t a g e s .
This space charge
can be overcane i f the p o t e n t i a l d if f e r e n c e a c r o s s th e l a s t few s ta g e s
i s in c re a s e d by use of a ta p ered dynode chain r a t h e r than an
e q u a l - v o l t s - p e r - s t a g e v o lta g e d i v i d e r .
A ta p e re d d i v id e r c i r c u i t
c r e a t e s a p ro g r e s s iv e v o lta g e in c re a s e le a d in g to a th r e e to fou r
times th e normal i n t e r s t a g e p o t e n t i a l d i f f e r e n c e , a c r o s s th e l a s t
s ta g e (121).
Tapered dynode c h a in s were c o n s tr u c te d f o r th e PMTs used
in t h i s work (see l a t e r s e c t i o n ) .
For th e d e te c tio n o f p u lsed l i g h t s i g n a l s i t i s n ecessary to
employ charge s to ra g e c a p a c ito r s on those l a s t few s ta g e s of a tap ered
dynode chain where peak p u ls e c u r r e n t demand i s th e g r e a t e s t (2 ,1 2 1 ).
The high peak c u r r e n ts re q u ire d d u rin g la rg e am plitude l i g h t p u ls e s
can be su p p lied by th e s e c a p a c i t o r s while th e dynode chain c u r r e n t
need only be s u f f i c i e n t to provide th e average anode c u r r e n t f o r the
PMT.
This approach works only when average anode c u r r e n t s a re "o rd e rs
of magnitude" (121) l e s s th an th e peak p u lse c u r r e n t s . (The f i g u r e of
" o rd e rs of magnitude" i s quoted from m anufacturers l i t e r a t u r e and th e
p re s e n t a u th o rs have not been a b le to f in d a more a c c u ra te f i g u r e .
However, the p re s e n t work does d e fin e t h i s b e t t e r ) .
Under th e above
c o n d itio n s , p ro p e rly s e le c te d c a p a c i t o r s can m ain tain dynode
p o t e n t i a l s to w ith in 1$ o f the d e s ir e d v alu e (121).
High average
c u r r e n t s due to an i n t e n s e , c o n tinu ou s background, p re v e n t th e
c a p a c i t o r s from m a in ta in in g t h e i r f u l l charge between l i g h t p u ls e s and
th e c a p a c i t o r s become i n e f f e c t i v e a s sou rces o f p u lsed c u r r e n t s .
T his
problem was a dd ressed in t h i s r e s e a r c h .
3.1.3
Photomultiplier Gating
To make charge s to ra g e c a p a c i t o r s e f f e c t i v e , i t i s d e s i r a b l e t o reduce
th e average c u r r e n t w ith o u t s a c r i f i c i n g peak p u lse c u r r e n t s .
PMT
g a tin g does t h i s by sw itch in g th e PMT o f f f o r over 99.9$ of th e tim e,
which p re v e n ts i t from seeing th e background when i t i s u nn ecessary to
do so.
The PMT i s sw itched on only long enough t o d e t e c t th e p u lsed
atomic flu o r e s c e n c e s ig n a l and whatever background t h a t o ccu rs d urin g
th a t short in te rv a l.
The r e s u l t i s a dram atic r e d u c tio n in average
anode c u r r e n t , but no t a t th e expense of pu lsed c u r r e n t s .
i l l u s t r a t e d in f i g u r e 4 .
This i s
The charge s to ra g e c a p a c i t o r s remain
e f f e c t i v e because t h e i r charge i s no t being c o n s ta n tly d ra in e d by th e
demand fo r high average c u r r e n t .
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C u rren t Workload f o r Gated PMT O peration.
2
In g e n e ra l i t has been shown t h a t th e l a r g e s t pulsed o u tp u t c u r r e n t s ,
l i n e a r w ith r e s p e c t to in c i d e n t l i g h t i n t e n s i t y , can be obtained when
a PMT i s op e ra ted in a g ated mode with a small duty f a c t o r (122).
In
r e c e n t y e a r s , gated p h o to m u lti p lie r s have been used i n a v a r i e t y of
s p e c tro s c o p ic a p p l i c a t i o n s (123-126).
3.1.3-1
Methods of Photoaultlplier Gating
Several methods fo r g a tin g of p h o to m u ltip lie r s have been re p o rte d in
th e l i t e r a t u r e .
In g e n e r a l, no in d iv id u a l method i s s u p e r io r , b u t
each has i t s advantages as w ell as d is a d v a n ta g e s , and th e s e l e c t i o n of
a g a tin g method most o f te n depends on the p a r t i c u l a r PMT a p p l i c a t i o n .
Among the re p o rte d g a tin g methods a r e :
p u ls in g th e o v e r a l l high
v o lta g e supply (1 23 ,12 7 ,12 8), p u ls in g groups o f dynodes (128-130),
p u lsin g the photocathode (123), p u ls in g th e f i r s t dynode (125), and
p u ls in g a focussin g e le c tr o d e (1 2 2 ,12M).
In th e p re s e n t work, a PMT
g a tin g scheme which in vo lv es p u ls in g th e voltag e of th e first dynode,
i s employed.
This d i s s e r t a t i o n r e p o r t s ex p erim en tal work t h a t shows t h a t PMT
g a tin g , when a p p lie d to LEAFS, a llo w s s ig n a l s to be d e te c te d w ith in
th e l i n e a r o p e ra tin g range of th e p h o to m u ltip lie r while using much
l a r g e r s l i t w idths than a re o therw ise p o s s i b l e .
Larger s l i t w idths
improved the SNRs of LEAFS measurements a t a l l c o n c e n tr a tio n s .
D etection l i m i t s have been improved i n l i n e w ith th e o v e r a l l
improvements in SNRs.
Long and W inefordner (131) have emphasized the
importance of m easuring d e te c tio n l i m i t s by e x t r a p o l a t i o n from
c a l i b r a t i o n curves t h a t a re l i n e a r .
The use of a g ated
p h o to m u ltip lie r tu be allo w s d e te c tio n l i m i t s t o be measured a t wide
s l i t w idths w ithout th e a tte n d a n t r i s k of n o n - l i n e a r i t y of th e
c a l i b r a t i o n curves caused merely by th e d e te c tio n system.
D etection
l i m i t s measured under th e Long and Winefordner c o n d itio n s then more
n e arly r e p r e s e n t th e p r a c t i c a l SNR ( i . e . p r e c i s i o n and s e n s i t i v i t y )
c a p a b i l i t i e s of LEAFS.
3.2
3.2.1
EXPERIMENTAL
Laser Triggering
The LEAFS in s tr u m e n ta tio n used fo r t h i s work i s l i s t e d in Table 3 and
i s shown in f i g u r e 5.
The excimer pump l a s e r was e x t e r n a l l y tr i g g e r e d
a t 60 Hz by a la b o r a to r y c o n s tru c te d l i n e frequency synchronized
o s c i l l a t o r (132).
shown in f i g u r e 6.
The schematic drawing o f th e o s c i l l a t o r c i r c u i t i s
The o s c i l l a t o r used a step-down tra n s fo rm e r w ith a
6.3 v o l t secondary to produce a re fe re n c e s ig n a l correspo nd in g to th e
60 Hz l i n e frequ en cy.
T his r e f e r e n c e s ig n a l was used t o d e riv e the
t r i g g e r p u ls e s f o r th e excimer l a s e r .
This was accomplished by u sin g
an o p e r a tio n a l a m p l i f i e r a s a comparator and c o n fig u re d a s shown in
f i g u r e 6 to c l i p th e 60 Hz s in e wave and produce s q u a re d -o ff p u lses
which were synchronized to th e l i n e frequency.
A v a r i a b l e phase s h i f t
(Rt and Ct ) allowed th e r e f e r e n c e s ig n a l and subsequent t r i g g e r p u lses
38
to be phase s h i f t e d w ith r e s p e c t to th e l i n e v o lta g e waveform.
T his
phase s h i f t e r pro vided c o n tro l over which p a r t o f th e sine-wave was
used t o charge th e l a s e r c a p a c ito r s .
Optimum charging o ccu rred i f the
beginning o f th e charging p ro c e ss co in cid ed w ith a z e r o -c r o s s in g of
th e l i n e freq u en cy .
S ince th e p o s i tiv e - g o in g edge of th e l a s e r
t r i g g e r p u lse i n i t i a t e d th e charging p ro c e s s , and i t could be e a s i l y
p o s itio n e d u s in g th e phase s h i f t e r , ch arg in g can be optim ized.
R2
a d ju s te d th e am plitude a t which the r e f e r e n c e sine-wave was c lip p e d
and th u s c o n t r o l l e d th e duty f a c t o r o f th e o u tp u t p u ls e and allowed
the d u r a tio n o f l a s e r c a p a c ito r ch arging to be c o n t r o l l e d .
The t r i g g e r s i g n a l was tr a n s m itte d between th e o s c i l l a t o r and the
l a s e r power supply by use of an o p t i c a l f i b e r d a ta l i n k .
This
e l e c t r i c a l l y i s o l a t e d the l a s e r from a l l o th e r p a r t s o f th e in stru m en t
and helped d is c r im in a te a g a in s t ra d io frequency in te r f e r e n c e (RFI)
(133).
TABLE 3
In s tru m e n ta tio n
D e s c rip tio n
Model Number
M anufacturer
Excimer l a s e r
XeF, 351 nm, 20 ns p u ls e
800-1XR
T ach isto Laser Systems
Needham, MA
Dye l a s e r
10 ns p u ls e
DL-19P
M olectron
Cooper Laser Sonics
Santa C la ra , CA
Frequency doubler
5-12
Inrad
N orth vale, NJ
The freq u en cy doubler u s e s a b i r e f r i n g e n t c r y s t a l such as
p otassium dihydrogen phosphate (KDP) to produce the second
harmonic (U.V.) of fundamental ( v i s i b l e ) l a s e r r a d i a t i o n .
V a ria b le l a s e r a tt e n u a t o r
935-3
Newport C o rpo ration
Fountain V alley, CA
Boxcar i n t e g r a t o r
162/165/165
15 ns a p e r tu re d u ra tio n
1 s time c o n s ta n t
P rin ce to n Applied Research
P rin c e to n , NJ
Monochromator
H-10
F / 3 .5 , 0.1 m fo c a l le n g th
8 nm/mm l i n e a r d is p e r s io n
In stru m e n ts S.A.
Metuchen, NJ
P h o to m u ltip lie r tube
R212UH
Hamamatsu C o rpo ration
M iddlesex, NJ
P h o to m u ltip lie r tube
9893QB
Thorn-EM I
F a i r f i e l d , NJ
PMT housing w ith magnetic
and RFI s h ie ld in g
B293
Table 3 c o n tin u e d .
PMT g a tin g board
GB1001B
Laboratory c o n s tr u c te d
The PMT g a tin g b o a rd , when used w ith th e 9893QB PMT,
sw itch es th e p o t e n t i a l o f th e f i r s t dynode, to
e f f e c t i v e l y t u r n th e PMT on or o f f , on command.
PMT g a te p u ls e g e n e r a to r
Laboratory c o n s tr u c te d
The PMT g a te p u lse g e n e r a to r produces a p u ls e to t r i g g e r
th e g a t i n g board and sy n ch ro n ize th e g a tin g o f the PMT t o
th e f i r i n g o f th e l a s e r .
Laser t r i g g e r c i r c u i t r y
Laboratory c o n s tr u c te d
The l a s e r t r i g g e r c i r c u i t r y p ro v id e s a v a r i a b l e r a t e p u lse
t r a i n to f i r e th e l a s e r a t a u s e r s e l e c t e d r e p e t i t i o n r a t e .
Boxcar t r i g g e r system
Laboratory c o n s tr u c te d
The boxcar r e f e r e n c e t r i g g e r i n g system in s u r e s t h a t th e
boxcar d e t e c t i o n g a te opens in a c c u r a te synchrony w ith
f i r i n g o f th e l a s e r t o op tim iz e d e t e c t i o n o f th e pulsed
atomic f lu o r e s c e n c e s i g n a l s .
C hart r e c o r d e r
Omni-Scribe
Houston In stru m e n ts
Houston, TX
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Block Diagram of LEAFS In s tru m e n ta tio n
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Line Frequency Synchronized O s c i l l a t o r .
3.2.2
Measurement of Slgnal-to-Nolse Ratios
For a l l s i g n a l - t o - n o i s e c a l c u l a t i o n s and d e t e c t i o n l i m i t measurements,
one f i f t h o f the peak-to-peak n o ise on th e blan k , measured on a c h a r t
re c o rd e r w ith a 0 .5 second response tim e , was taken as the n o is e , and
th e n e t flu o r e s c e n c e s ig n a l s were o b ta in e d by s u b t r a c t i n g th e blank
from th e t o t a l s i g n a l .
The n o ise measurements o b ta in e d w ith t h i s
method a re n u m erically e q u iv a le n t to th o se o b ta in e d by using an
e l e c t r o n i c i n t e g r a t o r (5) and ta k in g th e sta n d a rd d e v ia tio n of s e v e ra l
blank measurements.
D etec tio n l i m i t s were measured a t about 5 ng/ml
of each m etal and b la n k s were t y p i c a l l y e q u iv a le n t t o 0 .5 ng/ml of
both m e ta ls.
Such b la n k s included c o n t r i b u t i o n s from calcium and iro n
con tam in atio n i n the water and a c id s u s e d , which was v e r i f i e d by
scanning th e wavelength o f the l a s e r t o confirm th e presen ce o f atomic
l i n e f lu o r e s c e n c e .
Other noise so urces were probably s c a t t e r o f l a s e r
r a d i a t i o n (c a lciu m ), ra d io frequency i n t e r f e r e n c e (RFI), flame
background and boxcar n o is e ( i r o n ) .
S ig n a ls could no t be measured
from c o n c e n tr a tio n s below 1 ng/ml because o f c o n tam in atio n
u n c e r t a i n t i e s in th e c o n c e n tra tio n s of th e s ta n d a rd s and between
d i f f e r e n t b la n k s.
T his contam ination problem i s c u r r e n tly being
addressed by c a r e f u l a t t e n t i o n to the use o f c le a n room te c h n iq u e s.
3.2.3
Boxcar Reference Triggering
The boxcar r e f e r e n c e t r i g g e r i n g system used f o r t h i s work involved the
use of an a d d i t i o n a l PMT (R212UH) to d e te c t t h e U.V. outp ut pu lse of
th e excimer pump l a s e r (see f i g . 5 ).
The c o rre sp o n d in g e l e c t r i c a l
o u tp ut p u ls e from th e PMT was used as a r e f e r e n c e to t r i g g e r th e
b oxcar.
Excimer l a s e r em ission was tr a n s m itte d t o th e t r i g g e r PMT by
means of a bundle o f fused s i l i c a o p t i c a l f i b e r s (E nsign-B ickford Co.,
Simsbury, CT.).
A w e ll s h ie ld e d , 18 m, c o a x ia l c able was used to
delay the a r r i v a l o f the flu o re s c e n c e p u lse from th e d e te c tio n PMT to
th e boxcar s i g n a l in p u t in o rd e r to compensate f o r t r i g g e r i n g d e la y s
in h e r e n t to the boxcar (131*). This r e s u l t e d in an approximately 75 ns
s i g n a l d e la y .
Boxcar a p e r tu r e d e la y s o f 65-70 ns and boxcar a p e r tu r e
d u r a tio n s of 15 ns were r o u ti n e ly used.
3-2.3.1
Alternate Methods of Boxcar Triggering
Various methods of boxcar t r i g g e r i n g were t r i e d b e fo re th e p re s e n t
method was adopted. I n i t i a l l y , a system s i m i l a r t o t h a t used by
Weeks
e t a_l. (5) which used e x te r n a l c i r c u i t r y to t r i g g e r b o th the n itr o g e n
l a s e r and the b o x c ar, was employed.
I t was a p p a r e n tly r e l i a b l e fo r
th e se a u th o r s , a s in d ic a te d by the high q u a l i t y o f t h e i r r e s u l t s .
However, t h i s system d id not t r i g g e r the boxcar re p ro d u c ib ly when used
in c o n ju n c tio n w ith our excimer l a s e r .
J i t t e r (tem poral u n c e r ta in t y )
i n the opening o f th e boxcar a p e r tu r e was ab o u t 50 ns and r e s u l t e d in
noisy d e t e c t i o n .
be u s e d .
Boxcar a p e r tu r e d u r a tio n s l e s s th an 50 ns could not
The poor performance of t h i s system was p a r t i a l l y due to
pickup o f ra d io frequency i n t e r f e r e n c e (RFI) by th e tr i g g e r i n g
c i r c u i t r y and t r i g g e r p u lse tra n s m is sio n l i n e s .
The in te r f e r e n c e was
e m itte d p r im a r ily by th e excimer l a s e r d u rin g i t s d is c h a rg e .
A second system which was s i m i l a r t o t h a t used by E p s te in e t a l .
(6) f o r t h e i r fla sh lam p pumped dye l a s e r , was t r i e d w ith e q u a lly
d is a p p o in tin g r e s u l t s .
This system used an e l e c t r i c a l s y n c h ro n iz a tio n
p u lse from th e l a s e r power supply, to t r i g g e r th e b o x c a r.
This sync
p u ls e could n o t be a t r u e r e f e r e n c e due to tem poral u n c e r t a i n t i e s in
th e l a s e r em ission p ro c e s s and i t s use could le a d t o n oisy boxcar
d e tec tio n .
However, in r e f . (6) a l a s e r p u ls e o f 1 ys and a boxcar
a p e r tu r e o f s i m i l a r d u r a tio n were u s e d .
I f any j i t t e r in boxcar
t r i g g e r i n g d id o c cu r, i t was probably i n s i g n i f i c a n t in comparison t o
th e la r g e a p e r tu r e d u r a tio n t h a t was used .
When th e method o f E p s te in
e t a l . (6) was u sed, j i t t e r was observed t o be ab o u t 50 ns and boxcar
a p e r tu r e d u r a tio n s s m a lle r than t h i s could not be u sed .
The system
a l s o p ick ed up RFI.
The t r i g g e r i n g arrangem ent of O liv a re s and H i e f t j e (23) used a
f a s t photodiode to d e t e c t dye l a s e r e m is s io n .
The d e t e c t i o n of l a s e r
em ission i s prob ably th e most r e l i a b l e sou rce o f a r e f e r e n c e s ig n a l
fo r boxcar t r i g g e r i n g .
The e l e c t r o - o p t i c a l t r i g g e r i n g system used in
th e p r e s e n t work was based on t h a t used by O liv a re s and H i e f t j e (2 3 ).
A PMT was used f o r d e t e c t i o n o f th e l a s e r l i g h t p u ls e in s te a d of a
f a s t photodiode and an o p t i c a l f i b e r bundle was used t o tra n s m it th e
excimer r a d i a t i o n t o th e t r i g g e r PMT.
3.2.3.2
Advantages of Adopted Triggering System
The system was found to be immune to RFI, r e l i a b l e , and allow ed th e
use o f narrow 10-15 ns boxcar a p e r t u r e d u r a t i o n s .
The immunity t o RFI
was ensured by use o f th e o p t i c a l f i b e r to tr a n s m it th e excimer U.V.
r a d i a t i o n over th e f a i r l y long d i s t a n c e ( 3 m) to th e t r i g g e r PMT.
A
s h o r t c o a x ia l c a b le (30 cm) was then used to t r a n s m i t th e e l e c t r i c a l
s i g n a l from th e t r i g g e r PMT t o th e b oxcar.
Use o f lo n g c o a x ia l
c a b le s , r a t h e r th an th e o p t i c a l f i b e r , would have in c re a s e d th e
s u s c e p t i b i l i t y o f th e system t o RFI.
T h is i s p a r t i c u l a r l y a problem
w ith excimer l a s e r s due t o th e in te n s e e l e c t r i c a l d is c h a r g e a s s o c ia te d
w ith th e f i r i n g o f th e l a s e r .
A f a c t o r o f 2 to 3 improvement in d e t e c t i o n l i m i t s was observed
upon ado p tio n o f th e PMT/optical f i b e r method of b oxcar t r i g g e r i n g .
This i s i l l u s t r a t e d in Table 4 where f i g u r e s in p a r e n th e s e s show an
improvanent over a d ja c e n t f i g u r e s o b ta in e d w ith th e o th e r boxcar
t r i g g e r i n g systems t h a t we t r i e d .
A ll rem aining f i g u r e s shown in
Table 4 were o b ta in e d w ith th e PMT/optical f i b e r system .
rem aining p o r t i o n s o f Table 4 a r e d is c u s s e d l a t e r .
The
3.2.4
R212PH PMT for Fluorescence Detection
The f i r s t flame LEAFS work in t h i s la b o r a to r y was done u sin g a
Hamamatsu R212UH PMT which was a ls o used in r e f . (5) and was s i m i l a r
to the 1P28 used i n r e f s . 2 and 6 .
F ig u re 7 i l l u s t r a t e s th e d esig n of
a la b o r a to r y c o n s tr u c te d ta p e re d dynode chain t h a t was used in
c o n ju n c tio n w ith th e R212UH (n o te th e u se o f c a p a c i t o r s on th e l a s t
few dynodes).
3.2.5
T h is design was s i m i l a r t o t h a t used in r e f . 2,
9893QB PMT for Fluorescence Detection
The EMI 9893QB which was used fo r t h i s work, was a 2" l i n e a r fo cu ssed
p h o to m u lti p lie r tube comparable to th e R212UH in terms of s p e c t r a l
re s p o n se , and re sp o n se tim e.
However, th e 9893QB has alm ost a f a c t o r
o f th r e e h ig h e r g a in than th e R212UH and a f a c t o r o f f i v e g r e a te r
anode s e n s i t i v i t y (Amperes/lumen).
The 9893QB prov id ed l a r g e r s i g n a l s
and hence g r e a t e r f l e x i b i l i t y f o r th e d e t e c t i o n of pulsed s ig n a l s over
a wide range o f i n t e n s i t i e s .
A ta p e r e d dynode ch ain ( f i g . 8 ) , based
on th e d esig n used f o r the R212UH b u t m odified f o r th e 9893QB was
c o n s tr u c te d . T h is dynode chain was used f o r th e comparisons between
g a ted and n on-gated o p e ra tio n o f t h e 9893QB PMT.
48
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Tapered Dynode Chain f o r R212UH PMT
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Tapered Dynode Chain f o r 9893QB PMT
3.2.6
Photomultiplier Gating
3.2.6.1
PMT Gating Board
A p h o to m u ltip lie r g a tin g board, model number GB1001B (EMI), was used
w ith th e 9893QB PMT.
f i r s t dynode.
The g a tin g board c o n t r o l s th e p o t e n t i a l o f th e
The g a tin g board was c o n fig u re d w ith th e dynode c h ain
shown i n f i g u r e 8.
The g a tin g board c i r c u i t r y i s shown i n f i g u r e 9.
For th e m a jo rity o f th e tim e, the f i r s t dynode i s held a t a p o t e n t i a l
of about 30 v o l t s n e g ativ e w ith r e s p e c t to the photocathode which
p la c e s th e PMT in an e s s e n t i a l y " o f f ” s t a t e .
When e x te r n a ll y
t r i g g e r e d , by a u s e r su p p lied p u ls e , th e g a tin g board sw itch es th e
p o t e n t i a l o f th e f i r s t dynode to ab ou t 200 v o l t s p o s i t i v e w ith r e s p e c t
to the photocathode, which momentarily p la c e s th e PMT i n an "on"
sta te .
The g a tin g board has a maximum duty f a c t o r o f o p e ra tio n o f 1$
and a minimum gate w idth of 2 ys.
More d e t a i l s about th e g a tin g board
and i t s o p e ra tio n can be obtained from th e m anufacturers l i t e r a t u r e
(135,136).
C e r ta in components on the g a tin g board have f a i l e d from time to
tim e .
These in c lu d e the zener d io d e s , 1N5252 (24 V), 1N5271 (200 V),
and 1N4100 (7 V), and th e 6N135 o p t o - i s o l a t o r .
The f a i l u r e of th e s e
d e v ice s i s u n p r e d ic ta b le , th e r e f o r e i t i s prud en t to have rep lacem en ts
on hand.
51
(_>
^ u
; t uj
■vw
1o
-l-l
tfl
<n
ID
Figu re 9:
G ating Board C i r c u i t r y w ith Dynode Chain (Thorn-EMI).
3.2.6.2
Gating Pulse Generator
Figure 10 i s a schem atic drawing o f the la b o r a to r y designed and
c o n s tr u c te d , p u ls e g e n e r a to r and p u ls e d e lay c i r c u i t t h a t was used fo r
th e purpose o f e x t e r n a l l y t r i g g e r i n g th e g a tin g bo ard .
o f t h i s c i r c u i t were tw o -f o ld .
The f u n c tio n s
F i r s t i t pro vid ed a t r i g g e r p u ls e o f
a d j u s t a b l e d u r a tio n to th e g a tin g bo ard, and second, i t allow ed th e
g a tin g o f th e PMT t o be sy nch ron ized w ith th e f i r i n g o f th e l a s e r
system.
The ga te p u ls e g e n e r a to r c i r c u i t employed two h a lv e s o f a 4528
tim e r to p ro v id e p o s i t i o n i n g of th e PMT g a te p u lse and ad ju stm en t of
g a te p u ls e w idth .
P o te n tio m e te rs Rx and R2 p ro v id e d a c o a rs e and f in e
d e la y a d ju stm e n t, r e s p e c t i v e l y , f o r th e PMT g a te p u ls e .
ad ju stm e n t of PMT g a te p u ls e width from 1 to 50 p s.
R3 allow ed an
The PMT g a te
p u ls e o f s e le c te d w idth co u ld e a s i l y be p o s itio n e d to c o in c id e w ith
th e a c t u a l f i r i n g o f th e l a s e r and subsequent f lu o r e s c e n c e s i g n a l .
53
in
in
cm
CM
CM
in
CM
oo
00
CN
in
in
in
in
? 5
m
O
oo
in
m
m
F igu re 10:
Gate Pulse G enerato r C i r c u i t
A tim ing diagram ( f i g . 11) shows th e tem poral r e l a t i o n s h i p s between
l a s e r t r i g g e r i n g and f i r i n g , PMT g a tin g , and boxcar sig n a l p ro c e s s in g .
The PMT g a tin g sequence was t r i g g e r e d by th e f a l l i n g edge of the l a s e r
t r i g g e r p u lse (a) which a l s o i n i t i a t e d th e f i r i n g sequence o f th e
excimer l a s e r .
S ince a c t u a l l a s e r em ission o ccu rred about 150 ys
a f t e r the h ig h to low t r a n s i t i o n of the l a s e r t r i g g e r p u lse ( e ) , th e
PMT g a te p u ls e had t o be delayed ( b ) .
A PMT g a te p u ls e o f v a ria b le
d u r a tio n was then se n t to th e g a tin g board ( c ) .
The a c tu a l PMT ga te
width was a f u n c tio n o f th e ga te p u ls e w idth b u t was s l i g h t l y longer
due to tu r n - o n and t u r n - o f f tim es in h e re n t in th e g a tin g board (d ).
65-70 ns boxcar a p e r tu r e d e la y (g) and a 15 ns a p e r tu r e d u r a tio n
A
(h)
were used to c a p tu re the a n a l y t i c a l s ig n a l which had been delayed fo r
75 ns ( f ) using th e 18 m c o a x ia l d elay l i n e .
An a p p lie d g a tin g pulse
o f 4 ys d u r a tio n , was found t o be a co nv enient compromise between
minimizing th e duty f a c t o r o f th e PMT and ease o f o p e r a tio n .
A pulse
o f 4 ys r e s u l t e d in th e PMT being on f o r about 6 ys because of the
in h e r e n t d e la y s in the g a t i n g board.
Thus th e 4 ys p u ls e used to
t r i g g e r the g a tin g board a t 60 Hz r e s u l t e d in t h e PMT being tu rn e d o f f
f o r more th an 99.9% of th e tim e.
The GB1001B g a tin g board produced
sw itch in g t r a n s i e n t s ( f ) t h a t appeared a t th e PMT anode o u tp u t.
However, th e se t r a n s i e n t s occurred well b e fo re and a f t e r th e boxcar
a p e r tu r e opening and a s a consequence, were not d e te c te d .
a.
LASER TRIGGER PULSE (60Hz)
VARIABLE
PMT GATE PULSE DELAY
PMT GATE PULSE
off
J
I
i
PMT
LASER OUTPUT
'
►J75nsf*—
lu rr
SWITCHING TRANSIENT
I BOXCAR APERTURE
DELAY
15ns
BOXCAR APERTURE
DURATION
Figure 11:
Timing Diagram f o r Laser T rig g e rin g and F ir in g , PMT
G ating, and Boxcar Signal P ro ce ssin g .
3.2.6.3
Laser Firing Delay
For sy n c h ro n iz a tio n purposes, the PMT ga te p u lse c i r c u i t was tr i g g e r e d
by th e f a l l i n g edge of the 10 ms l a s e r t r i g g e r p u ls e .
O r i g i n a l l y , th e
excimer l a s e r f i r e d about 150 ps a f t e r t h i s f a l l i n g edge.
The excimer
l a s e r power supply has s in c e been upgraded to work a t h ig h e r
r e p e t i t i o n r a t e s and one r e s u l t o f t h i s has been t h a t th e excimer now
f i r e s about 20 ps a f t e r th e f a l l i n g edge of th e t r i g g e r p u ls e .
caused problems with t r i g g e r i n g th e PMT g a tin g b o a rd .
This
In h e re n t d elays
in th e g a tin g board c i r c u i t r y and in th e g a te p u ls e g e n e r a to r
n e c e s s i t a t e d t h a t a t l e a s t a 100 ps delay occur between th e f a l l i n g
edge o f the t r i g g e r p u lse and a c t u a l f i r i n g o f th e l a s e r , i n o rd e r fo r
PMT g a tin g to be synchronized w ith th e f i r i n g o f th e l a s e r .
A la se r
f i r i n g delay was in c o rp o ra te d in t o th e d a ta l i n k t r a n s m i t t e r p o r ti o n
o f the tr i g g e r i n g c i r c u i t r y to postpone th e f i r i n g o f th e l a s e r f o r a
s u f f i c i e n t i n t e r v a l to allow th e PMT g a tin g sequence to commence.
The
d e lay was accomplished by leng th e n in g the t r i g g e r p u ls e s e n t t o th e
l a s e r by about 100 ps ( v a r i a b l e ) .
A schem atic drawing o f th e l a s e r
f i r i n g delay c i r c u i t r y i s shown i n f i g u r e 12.
Figure 13 i s a tim ing
diagram i l l u s t r a t i n g th e f u n c tio n o f the l a s e r f i r i n g d e la y c i r c u i t .
57
LULU
Offi
z<
U iZ
OC LlI
CM
m
m
>
m
*
m
04
in
eo
^
(VI
00
CM
Lf> lO
>
m
CO
m
INPUT
m
u
F igu re 12:
Laser F ir in g Delay C ir c u it/D a ta Link T r a n s m itte r .
PMT GATING SEQUENCE
COMMENCES HERE — — i
LASER FIRING DELAY10 m s PULSE
500/j s PULSE (4528A)
i
i
I
!
30-150jJS PULSE U528 BI­
LASER TRIGGER PULSE
(7473)
LASER FIRES HERE
Figure 13:
*j
Timing Diagram f o r Laser F i r i n g Delay C i r c u i t r y .
The c i r c u i t in f i g u r e 12 o p e ra te d a s fo llo w s :
One h a l f o f a 4528
tim e r (A) was t r i g g e r e d by th e p o s itiv e - g o in g edge o f the 10 ms l a s e r
t r i g g e r p u ls e and produced a 500 ys p u ls e o f i t s own.
A second 4528
tim e r (B) was t r i g g e r e d by th e f a l l i n g edge o f the 10 ms l a s e r t r i g g e r
p u ls e (th e t r a n s i t i o n t h a t a l s o t r i g g e r d th e PMT g a tin g sequence) and
produced a v a r i a b l e (100-150 ys) p u ls e .
The o u tp u t p u ls e s o f th e se
tim e rs were fed through an e x c lu s iv e OR g a te (7486) to a 7473
f l i p - f l o p which was c o n fig u re d to to g g le on th e s e p u ls e s .
Since the
to g g lin g a c t i o n occurred on th e f a l l i n g edge o f th e s e p u ls e s , th e
d u r a tio n o f th e o u tp u t p u l s e o f th e f l i p - f l o p could be v a r ie d by
a d j u s t i n g the d u ra tio n of th e p u ls e from th e second 4528 tim e r.
The
o u tp u t p u ls e from th e f l i p - f l o p was approxim ately 10 ms lon g, b u t the
f a l l i n g edge o c cu rred s l i g h t l y l a t e r by ab ou t 100-150 ys than t h a t of
th e 10 ms in p u t p u ls e .
T his caused the excimer l a s e r to f i r e l a t e r
(by 100-150 y s ) , th u s allo w in g s u f f i c i e n t time f o r th e PMT g a t i n g
sequence to commence.
The PMT g a tin g sequence was s t i l l i n i t i a t e d by
th e f a l l i n g edge o f th e o r i g i n a l 10 ms t r i g g e r p u ls e .
O ther tim ing
f u n c tio n s such as boxcar t r i g g e r i n g were u n a f f e c te d by th e l a s e r
f i r i n g d e la y .
The o u tp u t o f th e d elay c i r c u i t was fe d t o th e d a ta
l i n k t r a n s m i t t e r module f o r tra n s m is s io n along an o p t i c a l f i b e r to th e
d a ta lin k r e c e i v e r lo c a t e d in th e excimer l a s e r power sup ply.
The
p u ls e t r a i n to the d a ta l i n k t r a n s m i t t e r could be g a te d on or o f f by
sw itch c o n t r o l o r by a remote e n able such as a computer.
F ig u re 14 i s
a block diagram showing th e r e l a t i o n s h i p between th e v a rio u s
components o f th e l a s e r and PMT t r i g g e r i n g system.
LASER
POWER
SUPPLY
DATA
LINK
RECEIVER
DATA
■+LINK
TRANSMITTER
LASER
FIRING
DELAY
Ui
“u.©
U)
Figure 1M:
Block Diagram of T r ig g e r in g System Components.
3.2.7
Photomultiplier Gain Control
The gain o f a p h o to m u ltip lie r i s a f u n c tio n o f the v o lta g e a p p lie d to
th e dynode ch ain v o lta g e d i v i d e r .
The p ro c e ss of measuring a l i n e a r
dynamic range f o r LEAFS may in v o lv e a range o f s ig n a l s i z e s o f s e v e r a l
o rd e rs o f magnitude.
I t i s o f te n n e ce ssa ry to reduce th e
p h o to m u lti p lie r gain 'in o rd e r t o keep s ig n a l r e l a t e d anode c u r r e n ts
w ith in the l i n e a r range of th e PMT and i t s dynode c h a in .
This i s most
e a s i l y accomplished by reducing th e a p p lie d v o ltag e to th e dynode
c h a in .
However, re d u c tio n s in PMT dynode chain v o lta g e may r e s u l t in
d e g ra d a tio n of s i g n a l - t o - n o i s e r a t i o s f o r LEAFS measurements.
3.2.7.1
Abbreviated Dynode Chain
M a n u fa c tu re r's l i t e r a t u r e (121) su g g ests t h a t i t i s b e t t e r t o o p e ra te
a PMT with l e s s s ta g e s o f g a in w ith h igh interdynode v o lta g e s , than i t
i s to simply lower th e a p p lie d v o lta g e .
An a b b re v ia te d dynode chain
was c o n s tr u c te d fo r th e 9893QB PMT to be used with th e g a tin g b oard.
Figu re 15 shows the d e sig n o f t h i s a b b re v ia te d dynode c h a in .
four dynodes were t i e d to th e anode.
The l a s t
FOR 9
DYNODE CHAIN
ABBREVIATED
LU
o"l
o
Figure 15:
A bbreviated Dynode Chain f o r 9893QB
3.3
RESULTS AND DISCUSSION
3.3.1
Gated vs. Non-Gated PUT Operation
A d i r e c t comparison was made between
g ated and non-gated o p e ra tio n of
th e 9893QB p h o to m u lti p lie r , f o r resonance as w ell as non-resonance
d e te c tio n o f LEAFS s i g n a l s .
In each c a s e , s l i t width was v a rie d and
both SNRs and d e t e c t i o n l i n e a r i t y were e v a lu a te d .
3«3.1.1
Resonance Detection
Figure 16 i l l u s t r a t e s th e e f f e c t o f in c re a s in g th e s l i t w idth fo r th e
resonance d e te c tio n o f calcium flu o re s c e n c e a t 422.7 nm.
Two calcium
s o l u t i o n s , 5 ng/ml and 10 ng/ml were a s p i r a t e d in t o a n itr o g e n
s e p a ra te d , a i r - a c e t y l e n e flam e, and th e atomic flu o r e s c e n c e s ig n a l s
were measured.
The s lo p e s of the flu o re s c e n c e v s . c o n c e n tr a tio n were
c a lc u la te d fo r each s l i t w idth a t a c o n s ta n t PMT v o lta g e of -2200
v o lts.
As i s shown in f i g u r e 16, when th e s l i t w idth was in c re a s e d , a
slo p e of one was m aintained only with th e g a te d PMT.
The non-gated
PMT gave an improvement in SNR w ith in c re a s in g s l i t w idth, b u t
d e te c tio n l i n e a r i t y d e t e r i o r a t e d .
The e r r o r in s lo p e measurement was
e stim a te d to be ab ou t 5%, in canmon w ith th e r e s u l t s re p o rte d by o th e r
r e s e a r c h e r s (5 ).
RATIO Sng-mrcn
SIGNAL-TO-NOfSE
o ■
o.
Ca 422.7nm/4227nm
(.93)
CO
W «
(O
2
1
SLIT WIDTH (mm)
Figure 16:
SNR v s . S l i t Width f o r Calcium (422.7 nm/422.7 nm).
Time c o n s ta n t = 1 s . All f i g u r e s in p a re n th e s e s are
th e slop es o f the c a l i b r a t i o n c urv es o b tain ed between
th e two calcium c o n c e n tr a tio n s 5 and 10 ng/ml .
□ Gated 9893QB;
© Non-gated 9893QB.
65
3-3.1.2
Non-Resonance Detection
In a s i m i l a r experim ent, both g ated and non-gated o p e ra tio n of a
9893QB PMT were used fo r th e non-resonance d e t e c t i o n o f iro n
f lu o r e s c e n c e .
E x c i t a t i o n was a t 296.7 nm by means of frequency-
doubled l a s e r r a d i a t i o n and flu o r e s c e n c e d e t e c t i o n a t 373.5 nm was
c a r r i e d out w hile th e s l i t was v a rie d between 2.0 mm and M.O mm.
At
one p o in t th e monochromator s l i t s were com pletely removed. Ir o n
s o l u t i o n s o f 5 ng/ml and 20 ng/ml were a s p i r a t e d and flu o re s c e n c e
s i g n a l s were measured.
Again, SNRs and s lo p e s were c a l c u l a t e d .
Figure 17 shows t h a t i n c r e a s e s in s l i t w idth caused an improvement in
SNR f o r b o th g a te d and non-gated PMT o p e r a t i o n .
However, a s lo p e of
one was not o b ta in e d w ith th e non -g ated PMT a t a H mm s l i t w idth.
L in ea r d e t e c t i o n , w ith a s lo p e o f one, was o b ta in e d w ith th e g a ted PMT
up to th e M mm s l i t width and became f a i r l y n o n - li n e a r only when th e
s l i t s were removed e n t i r e l y .
This s u g g e s ts t h a t , when th e PMT i s
g a te d , l i n e a r d e t e c t i o n might be o b tain ed even w ith s l i t s as la r g e as
6 or 8 mm.
However, s l i t s i z e s between H mm and no s l i t were not
a v a i l a b l e a t th e time of t h i s experiment so l i n e a r i t y was not t e s t e d
a t 6 or 8 mm.
66
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C9)£]
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.
SLIT WIDTH (mm)
F ig u re 17:
SLITS
REMOVED
SNR v s . S l i t Width f o r Iro n (296.7 nm/373.5 nm).
Time c o n s ta n t = 1 s . All f i g u r e s in p a re n th e se s
a re th e slo pes o f c a l i b r a t i o n cu rv es o b tain ed
between th e two i r o n c o n c e n tr a tio n s , 5 and 20 ng/ml.
□ Gated 9893QB;
© Non-gated 9893QB.
Both f i g . 16 and 17 appear t o in d ic a te t h a t th e non-gated 9893QB
gave s l i g h t l y b e t t e r SNRs a t some s l i t w idths.
This phenomenon can be
a t t r i b u t e d to th e r o u ti n e o b s e rv a tio n t h a t a PMT a t th e th re s h o ld of
s a t u r a t i o n showed an a t t e n u a t i o n o f the n o ise on a g iven sig n a l p r i o r
to an a t t e n u a t i o n of th e s ig n a l i t s e l f .
improvement in SNR.
This r e s u l t e d in a d e ce p tiv e
I t was probably because the peaks o f th e n o ise
re p re s e n te d a h ig h e r c u r r e n t than th e average s i g n a l and hence
s a t u r a t i o n occurred f o r th e n o ise p r i o r to i t o ccu rring fo r th e
average s i g n a l .
3.3.2
Advantages of Gated PMT Operation
F ig u re s 16 and 17 in d ic a te t h a t g a ted p h o to m u ltip lie r o p e ra tio n has
allow ed the use of l a r g e r s l i t w idths than could be used w ith
non-gated PMT o p e r a tio n , and has in c re a s e d the SNRs of LEAFS s i g n a l s
while m ain tain in g a slo p e o f one f o r th e c a l i b r a t i o n cu rv es.
improvement was a ls o e v id e n t a t th e d e t e c t i o n l i m i t (Table 4 ) .
This SNR
The
s l i t w idths l i s t e d a re th o s e w ith which th e measurements were made and
th e l a r g e s t t h a t would a llo w l i n e a r d e te c tio n (s lo p e = 1) from th e
d e t e c t i o n l i m i t to about 100 times th e d e t e c t i o n l i m i t .
TABLE 4
D e tec tio n L im its
(ng/ml) SNR=3
Element
g
Gated PMT
(slit)
10 s time c o n s ta n t3
Non-gated PMT
Ca
0.05 ( 0 .0 3 ) b
2 mm
0.2 ( 0 . 1 ) b
Fe
0.07
4 mm
0.1
(slit)
lite ra tu re
0.1
mm
0.0 8 c
2
mm
0 .6 d
The d e t e c t i o n l i m i t s were
sh o t n o is e l im it e d a t both 1 and 10 second time c o n s ta n ts .
This was dem onstrated by takin g measurements o f th e blank
a t both time c o n s ta n ts . The d if f e r e n c e in th e n o ise on th e
blank was a f a c t o r o f /T o .
b Calcium d e te c tio n l i m i t s in p a re n th e se s re p re s e n te d
improvements due to a re d u c tio n i n boxcar a p e r tu r e d u r a tio n
allowed by improvements in th e boxcar t r i g g e r i n g system (se e t e x t ) .
c Weeks e t a l . (5)
d E p ste in e t a_l. (6)
The d e t e c t i o n l i m i t o b ta in e d fo r calciu m (Table 4) w i t h a gated PMT
and a 2 mm s l i t (0 .0 3 ng/ml) was a d e f i n i t e improvement i n comparison
to t h a t o b ta in e d w ith th e non-gated PMT and 0.1 mm s l i t (0.1 ng/m l).
The d e te c tio n l i m i t s o b tain ed f o r iro n w ith both g ated (4 mm s l i t ,
0.07 ng/ml) and non-gated PMTs (2 mm, 0.1 n g/m l), were s u p e r io r to
th o se re p o rte d by E p s te in e t a l . , (6) f o r s i n g l e - p a s s and m u lti- p a s s
e x c i t a t i o n , a t 0.6 and 0 .2 ng/ml r e s p e c t i v e l y .
This s i g n i f i c a n t
improvement was a t l e a s t p a r t i a l l y due to th e a b i l i t y t o open the
s l i t s to 4 mm.
I t i s a n t i c i p a t e d t h a t f u r t h e r in c re a s e s in s l i t
w idth, ( to 6 or 8 mm), a llo w a b le w ith g a ted p h o to m u lti p lie r o p e ra tio n ,
w i l l r e s u l t in f u r t h e r improvement in th e iro n d e t e c t i o n l i m i t .
3-3.3
Effect of PMT Gating on Dynode Chain Current
F ig u re s 16 and 17 in d ic a te d t h a t g a tin g t h e PMT allowed th e u se of
l a r g e r s l i t s to improve the SNR o f LEAFS measurements in the p resence
o f high background s i g n a l s .
The average background c u r r e n t had been
reduced by a t l e a s t a thousand fo ld because th e PMT was e s s e n t i a l l y
o f f f o r more th a n 99.955 of th e tim e .
The use o f charge s to ra g e
c a p a c i t o r s on a dynode chain can
only
be e f f e c t i v e when th e average
c u r r e n t i s sm all compared to th e
peak
p u lse c u r r e n t s . Table 5 shows
th e t y p i c a l range o f average background c u r r e n t s encountered in t h i s
work over th e working range of s l i t w idths f o r th e non-gated PMT.
The
background c u r r e n t did not vary (Table 6) a s th e square of th e s l i t
width as i t sh ou ld (137) because th e n on-gated PMT was probably i n
some degree o f s a t u r a t i o n a t th e wider s l i t
w id th s. In a d d itio n ,
e s tim a te s a re g iv en in t a b l e 5 o f the range of average
c u r r e n ts when th e g a ted PMT was used.
background
These e s tim a te s were based on a
knowledge o f th e o f f time of th e g a ted PMT and by assuming t h a t th e
background c u r r e n t in c re a s e d as th e square o f th e s l i t w idth.
For th e
purpose of comparison, r a t i o s o f t y p i c a l peak p u lse c u r r e n ts to
average background c u r r e n t s were c a l c u l a t e d and a r e a ls o shown.
It
can be seen t h a t f o r a non-gated PMT, measured background c u r r e n ts
were only 10 2 to 10J3 times s m a lle r than th e expected peak p u ls e
cu rren ts.
t h a t a re 10
cu rren ts.
The g a te d PMT gave e stim a te d average background c u r r e n ts
tim e s, or more, sm a lle r th an th e expected peak p u ls e
TABLE 5
R e la tio n s h ip o f Average Background C u rre n t t o Peak P u ls e C u rre n t.
C urrent Source
Range of C u rre n ts
( s l i t s 0.1 - 4 mm)
Range o f r a t i o s :
p u ls e c u r r e n t /
average c u r r e n t
Average Background
Non-gated PMT
measured3
4 pA - 200 pAb
102 t o 103
Average Background
Gated PMT
estimated*3
4 nA - 4 pA
g r e a t e r th an 10
Peak p u ls e
Gated and NonGated PMT
measured**
100 pA - 100 mA
Average background c u r r e n t f o r n itr o g e n s e p a r a te d , a i r / a c e t y l e n e
flame a t 422 nm as measured w ith an o s c i l l o s c o p e .
b 200 pA i s th e m a n u fa ctu rers maximum
recommended average c u r r e n t f o r th e 9893QB.
C (see t e x t )
d
Peak p u ls e c u r r e n t s o b ta in e d from boxcar v o lta g e measurements.
TABLE 6
Background C u rre n t as a Fun ctio n o f S l i t Width
Average Background C urren t
S lit
Non-gated PMT3
Gated PMTb
Slope o f C a l i b r a t i o n Curve
Non-gated PMT
Gated PMT
0.1 mm
4 pA
4 nA
1.00
1.00
1.0 mm
40 pA
400 nA
0.93
1.00
2 .0 mm
180 pA
1600 nA
0.8 5
1.00
a measured (see t a b l e 5)
e s tim a te d (se e t a b l e 5)
Table 6 shows average background c u r r e n t s a s a f u n c tio n o f s l i t
w idth along w ith th e s lo p e s o f c a l i b r a t i o n c u rv e s ( f o r calcium from
f i g . 16) o b ta in e d w ith each s l i t w id th .
The g a te d PMT w ith i t s
s i g n i f i c a n t l y lower average background c u r r e n t gave l i n e a r d e t e c t i o n
a t a l l s l i t w id th s, while th e n o n-gated PMT was e a s i l y s a t u r a t e d under
th e same c o n d itio n s .
This in d ic a te d t h a t th e a c c e p ta b le r a t i o o f peak
p u ls e c u r r e n t to average background c u r r e n t f o r LEAFS d e t e c t i o n in
flam es was in a range g r e a t e r than 10 4 and t h a t a range o f 10 2 t o 10-'3
was no t a c c e p ta b le . I f the l a t t e r ran ge had been a c c e p ta b le , th e
n o n -gated PMT would probably have g iv e n l i n e a r c a l i b r a t i o n c u rv e s a t
a l l s l i t w id th s.
3-3-4
3.3.4.1
Linear Dynamic Ranges
R212UH PMT
The R212UH PMT, when used w ith th e ta p e re d dynode c h a in , appeared to
a llo w f o r a dram atic e x te n s io n (by alm ost th r e e decades) o f th e l i n e a r
range o f resonance d e t e c t i o n o f calcium atomic flu o r e s c e n c e when
compared to th e range r e p o r te d in r e f . 5.
The dynamic range ( f i g . 18)
extended from th e d e t e c t i o n l i m i t to 100 yg/ml
seven decades.
which was a range of
However, th e e x te n s io n of t h i s l i n e a r range may have
been a r e s u l t o f th e e lim in a tio n of p o s t - f i l t e r e f f e c t s (138) which
can cause n o n - l i n e a r i t y o f th e atomic flu o r e s c e n c e a t h igh
c o n c e n tr a tio n s .
The high power (up to 10 m J/pulse w ith th e S ti lb e n e
420 dye) and th e narrow s p e c t r a l bandwidth (0.001 nm a t 422 nm) of th e
l a s e r system used h e re allow ed expansion o f th e l a s e r beam and
i r r a d i a t e a la r g e volume o f th e flame, th u s p re v e n tin g p o s t - f i l t e r
e f f e c t s while s t i l l m a in ta in in g s a t u r a t i o n o f th e calciu m energy
le v e ls.
73
co
c 8
A R212UH
O R212UH (LITERATURE)
o
*>6
CALCIUM CONCENTRATION (ng-ml*1)
Fig ure 18:
L in ear Dynamic Range f o r R212UH (1P28) PMTs
3.3.4.2
9893QB PMT
The a p p l i c a t i o n of a ta p e re d dynode ch ain t o th e
9893QB high gain PMT
r e s u l t e d in s ig n a l s t h a t were c o n s id e ra b ly l a r g e r (by a f a c t o r o f 25)
than calcium atomic flu o re s c e n c e s i g n a l s o b ta in e d w ith th e R212UH
under s i m i l a r c o n d itio n s .
The l i n e a r range o f the c a l i b r a t i o n curves
( f i g . 19) were comparable a s were th e Ca d e t e c t i o n l i m i t s . Without
g a t i n g , f o r bo th the R212UH and 9893QB p h o t o m u l t i p l i e r s , resonance
d e t e c t i o n o f calcium was lim it e d t o th e u se o f a 0.1 mm monochromator
s l i t i n o rd e r to m aintain l i n e a r i t y .
l i n e a r i t y a t all c o n c e n tra tio n s .
Wider s l i t w idths p re v e n ted PMT
Successive re d u c tio n s in PMT
o p e ra tin g v o lta g e s were made in o rd e r to m ain tain l i n e a r i t y o f th e PMT
a t a l l c o n c e n tra tio n s when using th e 0.1 mm s l i t .
3-3-4.3
Abbreviated Dynode Chain for 9893QB PMT
The use of an a b b re v ia te d dynode ch ain w ith th e 9893QB PMT allowed
h ig h e r PMT v o lta g e s to be used w ith l e s s s ta g e s o f gain.
This
approach a ffo rd e d no advantages in terms o f prev e n tin g PMT s a t u r a t i o n
and allow ed no a p p aren t improvement in s i g n a l - t o - n o i s e r a t i o .
75
9693
R212UH
CO
«VLEAFS
CALCIUM 422.7nm
0.1 mm slit
o ss-
CALCIUM CONCENTRATION (ng-mf1)
Figure 19:
Linear Dynamic Range o f 9893QB PMT
3-3.1*.1*
R212UH
ts .
9893QB PMTs
There was no improvement in l i n e a r dynamic range of c a l i b r a t i o n curves
when g a te d and non-gated PMT o p e ra tio n were compared.
The e f f e c t of
g a tin g th e PMT was only to allo w th e s l i t width t o be in c re a s e d
w ith o u t l o s s in l i n e a r i t y (s lo p e = 1) o r l i n e a r dynamic range o f th e
c a l i b r a t i o n c u rv e s.
The in c re a s e d s l i t w idth was th e cau se o f th e
improvement i n SNR d e t a i l e d above.
3
.
PRACTICAL CONSIDERATIONS
The u s e o f a ta p e re d dynode c h ain w ith a h ig h g ain p h o to m u lti p lie r was
no t in i t s e l f s u f f i c i e n t to overcome th e problem o f PMT s a t u r a t i o n
LEAFS d e t e c t i o n .
in
Reducing th e o p e r a tin g v o lta g e o f th e PMT allow ed
r e d u c tio n of dynode c u r r e n ts to w ith in th e in h e re n t l i n e a r range o f
th e PMT/dynode chain system.
However, reducing th e PMT o p e ra tin g
v o lta g e r e s u l t e d i n a d e g ra d a tio n i n SNRs.
Previous workers (5) have
employed c a l i b r a t e d n e u t r a l d e n s it y f i l t e r s to a t t e n u a t e flu o r e s c e n c e
s i g n a l s i n an a tte m p t to m a in ta in l i n e a r d e te c tio n over a la r g e range
of sig n al in t e n s i t i e s .
This method i s very e f f e c t i v e and u s e f u l a t
h ig h a n a ly te c o n c e n tr a tio n s .
I t i s not a p r a c t i c a l s o l u t i o n fo r PMT
s a t u r a t i o n t h a t can occur when l a r g e s l i t s a r e used to measure th e
d e tec tio n lim it.
The improvement in SNR obtained by opening th e s l i t s
may be c a n c e lle d o u t by the e f f e c t o f a tt e n u a t i n g th e flu o r e s c e n c e
sig n al.
This a t t e n u a t i o n means t h a t l e s s l i g h t w i l l f a l l on th e PMT
and r e s u l t s In a d e c re a s e in SNR w ith th e square r o o t of th e
a tte n u a tio n fa c to r.
Gated PMT o p e r a tio n appeared to be a more
p r a c t i c a l a l t e r n a t i v e f o r a l l e v i a t i n g PMT s a t u r a t i o n in LEAFS
d e t e c t i o n because i t was g e n e r a lly a p p l i c a b l e a t t h e d e te c tio n l i m i t
a s w ell a s a t high a n a ly te c o n c e n tr a tio n s , and i t promoted l i n e a r
d e t e c t i o n w ith o u t s a c r i f i c i n g s i g n a l - t o - n o i s e r a t i o .
Perhaps a
com bination o f g ated PMT o p e r a tio n , a t t e n u a t i o n o f flu o r e s c e n c e
s i g n a l s by n e u t r a l d e n s ity f i l t e r s a t h ig h c o n c e n tr a tio n s , and
j u d i c i o u s o p tim iz a tio n o f PMT o p e r a t i n g v o lta g e , would be th e most
b e n e f i c i a l approach to the d e t e c t i o n o f LEAFS s i g n a l s .
Chapter IV
LASER EXCITED ATONIC FLUORESCENCE IN A CARBON TUBE
FURNACE
4.1
INTRODUCTION
The b e s t r e p o rte d LEAFS d e t e c t i o n l i m i t s t o d a te , in th e low femtogram
ra n g e , have been obtained w ith fu rn ac e a to m iz a tio n (1 4 ,1 5 ,1 7 ) .
Carbon
cup (14-16) and carbon rod (17-20) a to m iz ers have been used
s u c c e s s f u lly a s atom c e l l s fo r LEAFS b u t th e use of a carbon tube
fu rn a c e f o r LEAFS h as n o t y e t been r e p o r te d in th e l i t e r a t u r e .
Carbon
tub e fu rn a c e s more c lo s e ly approximate th e i d e a l is o th e rm a lly h e a te d
environment than does e i t h e r th e rod or cup fu rn a c e .
The advantages
o f an is o th e rm a lly h e ated environment f o r a to m iz a tio n in AAS have been
d is c u s s e d in d e t a i l (5 7 ,5 8 ), and i t i s th e o b je c tiv e o f th e p r e s e n t
work to employ th e se advantages f o r LEAFS a to m iz a tio n .
The carbon
tu b e fu rn ace i s a sem i-enclosed atom c e l l , u n lik e rod and cup
a to m iz e rs , which a re c o n sid e re d t o be open atcm c e l l s (4 9 ).
Analyte
r e s id e n c e tim e i s probably lo n g e r i n th e tube fu rn ace due to
r e s t r i c t e d d i f f u s i o n o f the atom cloud r e l a t i v e to rod or cup
a to m iz e r s .
The use o f carbon tube fu rn a c e s fo r AAS has r e s u l t e d in
th e h i g h e s t s e n s i t i v i t i e s f o r t h a t te c h n iq u e .
The primary aim o f t h i s
p r o j e c t was to i n v e s t i g a t e th e a n a l y t i c a l f e a s i b i l i t y of LEAFS in a
carbon tub e fu rn ac e.
- 78 -
4.1.1
Idealized Furnace for LEAFS
F ig u re 20 i l l u s t r a t e s an i d e a l i z e d co n ce p tio n o f a LEAFS tu b e fu rn a c e .
The req u irem en ts of a carbon tube fu rn ac e fo r LEAFS are s i m i l a r to
tho se f o r AA w ith two b a s ic e x c e p tio n s .
F i r s t , long path le n g t h
fu rn a c e s such as those used in AA a re not r e q u ir e d .
I t i s more
d i f f i c u l t to c o l l e c t flu o r e s c e n c e from w ith in a lo ng tube w itho u t
c o l l e c t i n g an excess o f fu rnace blackbody em ission which degrades
d e tec tio n lim its .
Secondly, the LEAFS tu b e fu rn ac e must s a t i s f y th e
illu m i n a ti o n and o b s e r v a tio n geometry req u irem en ts o f AFS.
That i s ,
p o r t s f o r the e n tra n c e and e x i t o f th e l a s e r beam, as w ell as a p o r t
to allow f o r c o l l e c t i o n o f flu o r e s c e n c e a t r i g h t a n g le s to th e
i n c i d e n t l a s e r beam.
The g e o m e tric a l design of th e LEAFS furnace tube
w ith r e s p e c t to i llu m i n a ti o n and o b s e r v a tio n , i s c r i t i c a l because th e
atomic p o p u la tio n both i n and o u ts id e th e illu m in a te d volume can have
a s tro n g in f lu e n c e on th e flu o r e s c e n c e curves o f growth (1 39 ).
This
i s due p r im a r i ly to th e occurrence of b o th p r e - and p o s t - f i l t e r
effects.
I t was t h e r e f o r e im p o rtan t t o c o n sid e r th e se f a c t o r s in th e
desig n and f a b r i c a t i o n o f LEAFS tube f u r n a c e s .
In g e n e r a l, th e id e a l
LEAFS tu b e fu rn ac e should prov ide ra p id h e a tin g , a llo w f o r b o th
e f f i c i e n t illu m in a tio n o f th e a n a ly te and e f f i c i e n t c o l l e c t i o n of
flu o r e s c e n c e .
Figure 20:
Id e a liz e d LEAFS Furnace.
4.2
INSTRUMENTATION
F ig u re 21 i s a block diagram showing th e major components o f th e
fu rn ac e LEAFS in s tru m e n t.
A l i s t o f th e se components and t h e i r
m an u factu rers i s shown in Table 7 .
The fu rn ac e LEAFS in s tr u m e n ta tio n
i s th e same a s t h a t used f o r flame LEAFS with th e e xceptio n t h a t a
la b o r a to r y c o n s tr u c te d fu rn ac e atom izer system (s e e l a t e r s e c tio n ) has
re p la c e d the flame a s th e a n a l y t i c a l atom c e l l .
However, flame atomic
flu o r e s c e n c e i s used to tune th e outp ut o f th e dye l a s e r to th e
e x c i t a t i o n wavelength of th e flu o r e s c e n c e t r a n s i t i o n .
This was
n e ce ssa ry because o f the t r a n s i e n t n a tu re of th e furn ace flu o r e s c e n c e
sig n a l.
Furnace a to m iz a tio n r e q u i r e s sm all, d i s c r e t e samples,
t y p i c a l l y 5 to 50 p i .
The a n a ly te atom cloud r e s u l t i n g from a sample
t h i s s m a ll, r e s i d e s i n th e fu rn a c e f o r a few seconds o r l e s s , hence
th e t r a n s i e n t atomic flu o r e s c e n c e s ig n a l .
The furnace atom izer was p o s itio n e d between th e p o le p ie c e s of an
A.C. electro m ag net ( f i g u r e 22) f o r f u t u r e i n v e s t i g a t i o n s o f th e
a p p l i c a t i o n of Zeeman e f f e c t , background c o r r e c t i o n to fu rn ace LEAFS.
A lso, a v a r ia b le frequency l a s e r t r i g g e r i n g c i r c u i t (se e s e c t i o n
4 . 2 .2 ) has re p la c e d th e 60 Hz l i n e frequency o s c i l l a t o r .
82
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F igu re 21:
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u
Block Diagram of Furnace LEAFS In s tru m e n t.
MAGNET POLE PIECES
FURNACE
LASER BEAM
FLUORESCENCE
Figure 22:
Carbon Tube Furnace P o s itio n e d Between Pole P ieces of
E lectrom ag net.
TABLE 7
In s tru m e n ta tio n and Apparatus
D e s c rlp tio n
Model number
M anufacturer
Excimer l a s e r
XeCl, 308 nm, 20 ns p u ls e
8 0 0 -1 XR
T a c h isto Laser Systems
Needham, MA
Dye l a s e r
10 ns p u ls e
DL-1 9P
M olectron,
Cooper Laser Sonics
S anta C la ra , CA
Frequency Doubler
5-12
I n r a d , N o rth v a le , NJ
Boxcar a v e ra g e r
162/165
P r in c e to n Applied Research
P r in c e to n , NJ
Monochromator f / 3 . 5 ,
0.1 m f o c a l le n g th ,
8 nm/mm l i n e a r
d isp ersio n
H-10
In stru m e n ts SA,
Metuchen, NJ
P h o to m u ltip lie r Tube
9893QB/350
Thorn-EMI, F a i r f i e l d , NJ
PMT g a tin g board
GB1001B
PMT housing w ith
magnetic and RFI s h ie l d in g
B293
Furnace Power Supply
HGA-2000
T rig g e rin g c i r c u i t r y
Furnace e l e c t r o d e assembly
Perk in-E lm er, Norwalk, CT
Laboratory c o n s tr u c te d
4.2.1
Laser System
The fu rn a c e LEAFS in stru m e n t was based on an excimer pumped, tun able
dye l a s e r system .
The excimer l a s e r was o p erated w ith xenon c h lo r id e
which em its l a s e r r a d i a t i o n a t 308 nm.
The dye l a s e r , w ith an
e x te r n a l a u to - tr a c k in g frequency d o u b le r, provided tu n a b le outp u t from
about 220 nm through th e v i s i b l e .
s te p p e r m otor.
The dye l a s e r was tuned w ith a
The dye l a s e r outpu t was focused through th e frequency
doubling c r y s t a l by use o f a 300 mm f o c a l le n g th l e n s to enhance th e
co n v ersio n e f f i c i e n c y .
A tw o-lens beam expander (see s e c t i o n 4 .2 .3 )
was used t o a d j u s t th e diam eter o f th e l a s e r beam to th e d e s ir e d
v a lu e .
The excim er l a s e r was t r i g g e r e d a t 80 Hz by e x t e r n a l c i r c u i t r y
(s e e fo llo w in g s e c t i o n ) .
An o p t i c a l d a ta l i n k was used to tra n s m it
t r i g g e r p u ls e s from t h i s c i r c u i t r y to th e excimer l a s e r .
This helped
to minimize RF i n t e r f e r e n c e by i s o l a t i n g th e excimer l a s e r from th e
r e s t o f th e in stru m e n t.
4.2.2
Laser Triggering
Fig ure 23 i s a schematic drawing o f th e v a r ia b le frequency o s c i l l a t o r
c i r c u i t r y used to e x te r n a ll y t r i g g e r th e excimer l a s e r a t a u s e r
selected r e p e titio n r a te .
The c i r c u i t was based on th e ICL8038
fu n c tio n g e n e r a to r chip which i s u sed , in a v o lta g e c o n tr o lle d
o s c i l l a t o r c o n f ig u r a tio n , to g e n erate a v a r ia b le frequency (10-100 Hz)
sine-wave.
A 356 o p e ra tio n a l a m p lif ie r was used as a comparator to
c l i p the sine-wave a t th e d e s ir e d am plitude to produce a s q u a re d -o ff
p u lse t r a i n o f s e le c te d duty c y c le (10 ms pulse w idths a t a l l
freq u en cies).
The p u l s e - t r a i n o u tp u t o f th e op-amp was then c u r r e n t
a m p lifie d and used to t r i g g e r the excimer l a s e r .
All o th e r instru m en t
t r i g g e r i n g f u n c t i o n s in c lu d in g boxcar r e f e r e n c e t r i g g e r i n g , PMT
g a tin g , and l a s e r f i r i n g delay o perated th e same as with th e flame
LEAFS in s tru m e n ta tio n .
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F ig ure 23:
m
V ariab le Frequency O s c i l l a t o r f o r Laser T rig g erin g
4.2.3
Laser Beam Expansion and Steering Optics
Since i t was d e s i r a b l e to i r r a d i a t e th e l a r g e s t p o s s ib le volume of
a n a ly te atcms r e g a r d le s s o f th e c h o ic e o f atom c e l l , i t was necessary
to expand the l a s e r beam from i t s nominal dimensions o f 1 mm by 2 mm,
i n t o a c i r c u l a r beam w ith a diam eter c lo s e t o 4 mm which was th e
d iam eter o f the beam e n tra n c e p o r t o f th e carbon fu rn ac e tu b e s (se e
l a te r sec tio n ).
A tw o -le n s beam expander ( f ig u r e 24) was c o n s tr u c te d
to accomplish t h i s t a s k .
Two fused s i l i c a bi-convex le n s e s , 12.5 mm
and 25 mm f o c a l le n g th r e s p e c t i v e l y , were mounted on l i n e a r
t r a n s l a t i o n s ta g e s on an o p t i c a l r a i l , to allow p r e c i s e ad ju stm en t of
l e n s s e p a r a tio n .
The 12.5 mm l e n s was p la c e d n e a r e s t th e l a s e r and
th e 25 mm le n s was lo c a t e d 37 mm from th e f i r s t le n s , towards th e
fu rn a c e ato m iz er.
C e rta in c o n s id e r a tio n s must be observed in fo c u ssin g o f Gaussian
l a s e r beams (140).
Gaussian l a s e r beams c h a r a c t e r i s t i c a l l y d iv e rg e
from t h e i r minimum dia m e ter o r "beam w a ist" ( f i g u r e 2 5 ).
This
d ivergence must be accounted f o r when d esig nin g o p t i c a l systems f o r
beam expansion.
In th e p r e s e n t c a s e , i t was im p o rtan t t h a t th e l a s e r
beam be m aintained a t a n e a r ly c o n s ta n t diam eter as i t tr a v e l e d
through th e fu rn a c e ato m iz er which was p o s itio n e d between th e pole
p ie c e s of an elec tro m ag n e t ( f i g u r e 22).
This was n e ce ssa ry to
minimize l o s s e s in beam i n t e n s i t y and to minimize s c a t t e r e d and s t r a y
l i g h t in th e fu rn ace.
The o p t i c a l r a t i o n a l e f o r s e l e c t i n g t h e
components (bi-convex le n s e s ) f o r th e beam expander were a s fo llo w s .
The beam waist w0 ( f i g u r e 25) i s r e l a t e d to th e nominal d iam eter
o f th e l a s e r beam acco rd ing t o th e formula:
d iam eter = 2 /2 w0
This nominal d iam eter i s m aintained over a d is ta n c e in th e d i r e c t i o n
o f p ro p a g a tio n c a l l e d th e confocal parameter which i s r e l a t e d t o the
beam w a ist according to th e formula:
2
2 irw 0
b i = ------X
where:
b j = co n fo cal param eter
w0 = beam w a ist
and
\ = wavelength o f l a s e r l i g h t
A l a s e r beam w ith a wavelength o f MOO nm and a nominal dia m e ter of
2 mm then has a beam w a ist of 0.707 mm and a c o n fo c a l param eter of
7.85 m.
In o rd e r to expand t h i s beam to a diam eter o f M mm, a
tw o -le n s system can be u sed.
One le n s w ill fo c u s th e beam such t h a t
i t w i l l a c q u ire new p r o p e r t i e s , i . e . beam w a ist and confocal param eter
( f i g u r e 2 6 ).
The new c o n fo c a l param eter can be c a l c u l a t e d acc o rd in g
to th e formula:
b 2 --------b:
where:
b2 =
new co n fo cal param eter
f i = f o c a l le n g th o f f i r s t le n s
and
bl =
c on fo cal le n g th o f o r i g i n a l l a s e r beam
A second l e n s w i l l r e - f o c u s th e new beam again g iv in g t o i t new
p r o p e r t i e s ( f i g u r e 2 6 ).
The c o n fo c al le n g th o f th e beam a f t e r two
le n s e s can be c a l c u l a t e d a s fo llo w s .
b 3 ---------
where:
b 3 = c o n fo c a l p aram eter a f t e r 2nd le n s
f 2 = f o c a l le n g th o f 2nd le n s
and
b 2 = c o n fo c al le n g th a f t e r 1 s t le n s
From the new c o n fo c a l p a ra m e te r, b 3, the a s s o c ia te d beam w a is t w3 can
be c a lc u la te d as fo llo w s .
The expanded beam d ia m e te r ( f i g u r e 26) i s giv en by:
d ia m e ter = 2/5w3
I f two bi-convex le n s e s o f f o c a l le n g th 12.5 mm and 25 mm r e s p e c tiv e ly
a re used to expand a 2 mm l a s e r beam a t 400 nm, and th e le n s e s are
s e p a ra te d by the sum o f t h e i r f o c a l le n g th s , 37 mm (se e f i g u r e 24),
th e expanded beam w i l l have a diam eter o f 4 mm and a c on focal
p aram eter o f 31.4 m.
T his means t h a t th e expanded l a s e r beam can
m aintain a nominal d ia m e ter of abo ut 4 mm over a d is ta n c e o f 31.4 m
which i s more than s u f f i c i e n t to t r a v e r s e th e d is ta n c e from th e o p tic s
to the fu rn ace ato m izer and beyond.
A f i n e adjustm ent of the beam
diam eter can be o b ta in e d by varying s l i g h t l y , the d is ta n c e between the
lenses.
Figure 26 i l l u s t r a t e s th e fu n c tio n of th e beam expansion
o p tic s.
The o u tp u t a p e r tu r e o f th e dye l a s e r i s about 240 mm above the
p la n e of th e ta b le to p , and th e o p t i c a l a x is of th e furnace atom izer
i s about 325 mm above th e t a b l e to p .
The l a s e r beam was t r a n s l a t e d to
the h e ig h t of the fu rnace by a p a i r of fused s i l i c a r i g h t angle prisms
on o p t i c a l mounts secured t o the same o p t i c a l r a i l a s th e beam
expander (f ig u r e 2 4 ).
th e l a s e r beam.
The prism s allowed f l e x i b i l i t y in p o s itio n in g
A box w ith e n tra n c e and e x i t a p e r tu r e s was
c o n s tr u c te d to house th e beam expansion and s t e e r i n g o p t i c s .
The box
reduced th e amount o f s t r a y l a s e r l i g h t g e n erate d by r e f l e c t i o n s o f f
th e fa c e s of the o p t i c s , from reach in g th e d e te c tio n system.
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F igu re 24:
Laser Beam Expansion and S te e rin g O p tic s.
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F igure 25:
. . . la s e r Beam C h a r a c t e r is t ic s .
G aussian Laser
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F igu re 26:
Function o f Beam Expanding O p tic s
4.2.4
4.2.4.1
Furnace Atoalzation
Furnace Tube Design
A ll fu rn a c e tubes used in th e p r e s e n t work were f a b r i c a t e d in our
la b o r a to r y a ccord ing t o th e d e sig n shown in f i g u r e 27.
The dim ensions
shown were chosen a s a compromise between d u r a b i l i t y , h e a tin g r a t e ,
and p h y s ic a l l i m i t a t i o n s imposed by th e l o c a t i o n o f th e fu rn a c e w ith in
the p o le gap o f th e e le c tro m a g n e t.
Furnace tu b e s o f s m a lle r o u te r
d iam eter (8 mm) were t r i e d , b u t i t was d i f f i c u l t t o e f f i c i e n t l y
c o l l e c t flu o r e s c e n c e w ith o u t a l s o c o l l e c t i n g a l a r g e amount o f furnace
em ission a s w e ll.
A lso , th e use o f probe sample i n t r o d u c t i o n (see
s e c tio n 1 .2 . 5 .2 ) r e q u ir e d t h a t the 10 mm fu rn a c e tu b e s be used to
accomodate th e s i z e o f th e probe t i p .
4.2.4.2
Fabrication of Furnace Tubes
Furnace tu b e s were machined from h i g h - p u r i t y , 10 mm d ia m e ter g r a p h ite
rod s which were o b ta in e d fra n S i g r i C o rp ., S o m e rv ille , NJ.
G raphite
rods were h e ld i n a c o l l e t and bored out on a l a t h e t o make tube stock
with a 7 .5 mm I . D . .
I n d i v i d u a l tubes were c u t t o le n g th and th e tube
ends were f i n i s h e d on t h e l a t h e .
T ransverse h o le s o f 4 mm d ia m e ter
were d r i l l e d u sin g a d r i l l p r e s s .
The fu rn a c e tu b e s were v i s u a l l y
in s p e c te d f o r d e f e c t s and c lea n e d i n an u l t r a s o n i c w ater b a th .
The
fu rn ac es were th en p y r o l y t i c a l l y c o ate d u s in g methane in a la b o ra to ry
c o n s tr u c te d chamber.
96
2
F ig u re 27:
Design f o r Furnace Tube F a b r ic a tio n .
4.2.4.3
Furnace Electrode Assembly
A furn ace e l e c t r o d e assembly ( f i g u r e 28) was c o n s tr u c te d to p o s i t i o n
th e carbon tu be fu rn a c e between the pole p ie c e s o f th e e le c tro m ag n e t,
and allow f o r f a c i l e removal and replacem ent o f fu rnace tu b e s .
The
d e sig n i s s i m i l a r t o t h a t o f o ld e r Varian fu r n a c e a to m iz e rs in which
th e furnace tu b e s were held between two carbon rod e le c tr o d e s .
The
p r e s e n t d e sig n i s d i f f e r e n t in t h a t a s p r in g p lu n g e r i s employed (se e
fig u re 28) to apply c o n s ta n t p re s s u re and promote p o s i t i v e e l e c t r i c a l
c o n ta c t between th e fu rn ac e tube and th e e l e c t r o d e s .
Furnace tubes
could e a s i l y be re p la c e d by a pp lyin g a s l i g h t p r e s s u r e to th e arms a t
th e r e a r o f th e e l e c t r o d e assembly to c o u n te r th e p re s s u re a p p lie d by
th e s p rin g p lu n g e r.
Furnace e l e c t r o d e s were f a b r i c a t e d from 4.6 mm d iam eter g ra p h ite
ro ds ( S ig r i C o rp ., S o m e rv ille, NJ).
The ends o f th e e l e c t r o d e s were
contoured w ith a 5 mm r a d iu s cu rve, to e x a c tly match th e o u te r
d iam eter (10 mm) o f th e furnace tubes ( f i g u r e
2 7 ).
The furnace
e le c tr o d e assembly was mounted on a th r e e dim en sion al t r a n s l a t i o n a l
s ta g e to a llo w p r e c i s e p o s itio n in g o f th e fu rn a c e tube w ith r e s p e c t to
th e magnet p o le p ie c e s .
HGA-2000 power supply.
The fu rn ac e was h e a te d by a Perk in-Elmer
98
%
1
F ig ure 28:
Furnace E le c tro d e Assembly.
4.2.4.4
Furnace Enclosure for Inert Atmosphere
A fu rn ace e n c lo s u re was n ecessary to exclude a i r from th e fu rnace
( f i g u r e 29) to p re v e n t o x id a tio n o f th e carbon fu rn a c e tube and
e lec tro d e s.
I t was designed to f i t between th e magnet c o i l w indings,
and was c o n s ta n tly flu s h e d w ith argon (e x c e p t during th e a to m iz a tio n
s te p ) to m ain tain a s l i g h t l y p o s i t i v e p r e s s u r e , i n e r t atmosphere.
The
s id e s and f r o n t o f th e e n c lo s u re were made o f p l e x i g l a s s and aluminum
r e s p e c t i v e l y , and the to p , bottom, and r e a r o f th e e n c lo s u re were th e
body o f th e e le c tr o d e assembly.
A removeable door was lo c a te d a t th e
f r o n t of th e e n c lo s u re to allo w a c c e ss to th e furnace and a fused
s i l i c a window was mounted in th e door f o r th e purpose o f flu o r e s c e n c e
c o lle ctio n .
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Figu re 29:
Furnace Enclosure fo r I n e r t Atmosphere.
4.2.4.5
Optical Baffling of Furnace Balsaion
Furnace blackbody emission was o f major co ncern i n th e desig n o f th e
carbon fu r n a c e atom c e l l f o r t h i s work.
T his i s because f lu o r e s c e n c e
was being d e te c te d from w ith in th e tube as opposed to above the
ato m iz er a s i s th e c a s e w ith cups or ro d s .
I t i s common p r a c t i c e in
th e d e sig n o f atomic a b s o r p tio n in s tr u m e n ta tio n to p re v e n t as much of
th e fu rn a c e em issio n a s p o s s ib le from re a c h in g th e d e t e c t o r by th e use
of b a f f l e s .
This i s done p r im a r i ly to minimize n o is e .
Furnace
em ission may be an even more s e r i o u s problem f o r LEAFS d e t e c t i o n .
Not
only does fu rn ac e emission c o n t r i b u t e to th e n o is e l e v e l , i t may be of
s u f f i c i e n t i n t e n s i t y to cause th e PMT t o s a t u r a t e .
T h is o ccurs
e s p e c i a l l y i f very h ig h PMT v o lta g e s a re used f o r measurements c lo s e
to th e d e t e c t i o n l i m i t .
In th e p r e s e n t in stru m e n t, th e amount o f fu rn a c e em issio n re a c h in g
th e d e t e c t o r was reduced by a s e t o f o p t i c a l b a f f l e s ( f i g u r e 30)
lo c a t e d in th e f r o n t door o f th e fu rn a c e e n c lo s u re .
These blocked a
la r g e p o r t i o n o f th e furn ace e m ission from re a c h in g th e d e t e c t o r w hile
a llow ing a la rg e s o lid angle (18°) f o r f lu o r e s c e n c e c o l l e c t i o n .
18° was th e acc e p tan c e angle o f th e monochromator used f o r t h i s work.
102
F ig ure 30:
O p tic a l B a f f le s f o r Furnace Em ission.
4.2.5
Furnace Sample Introduction
Three methods f o r in tr o d u c in g the sample in t o th e carbon tube fu rn a c e
were i n v e s t i g a t e d .
The O bjective h e re was to determ ine th e b e s t
c o n d itio n s f o r fu rn a c e sample in t r o d u c t i o n , fo r m e ta ls o f d i f f e r e n t
v o l a t i l i t y , and to compensate by optim ized sample in tr o d u c tio n , fo r
p o s s ib ly non-optimum fu rn a c e h e a ti n g .
The HGA-2000 fu rn ac e power
supply used f o r t h i s work was not equipped w ith an o p t i c a l sen so r t h a t
would have allowed feedback c o n tr o l o f fu rn a c e h e a tin g .
A lso, the
HGA-2000 power supply does not have the "maximum power" f e a t u r e t h a t
i s in c lu d e d in th e more modern power s u p p lie s such a s th e Perk in-Elmer
HGA-500.
This f e a t u r e a llo w s maximum power to be used to r a p id ly h e a t
th e fu r n a c e to th e d e s ir e d f i n a l te m p e ra tu re .
Once t h i s tem p eratu re
has been ach iev ed , a d d i t i o n a l c i r c u i t r y p ro v id e s the
fe e d b a c k -re g u la te d power to m ain tain t h a t te m p e ratu re.
The HGA-500
system a llo w s fu rn a c e h e a tin g r a t e s in e xcess o f 1500 °C /sec.
Rapid
h e a tin g r a t e s such as t h i s a re e s s e n t i a l to minimizing v a p o r iz a ti o n
in te rfe re n c e s.
4.2.5.1
Furnace Wall Sampling
The sample could be d e p o s ite d , using a m ic ro p ip e t, on th e in s id e w all
o f th e fu rn a c e .
This was done in a manner s i m i l a r to t h a t used in
fu rn a c e atomic a b s o r p tio n w ith th e e x c e p tio n t h a t th e sample was
in tro d u ce d through th e f r o n t a x i a l fu rn ac e h o le as opposed to
in t r o d u c t i o n through a sample p o r t in th e tube w a ll.
T his method
re q u ire d th e removal o f th e f r o n t door o f th e fu rn ac e e n c lo s u re w ith
each sample in t r o d u c t i o n .
A d isad v a n ta g e o f t h i s approach was t h a t
each time th e f r o n t door was removed t o in tro d u c e a sample, some a i r
e n te re d the furnace e n c lo s u re and argon f lu s h in g p e rio d s o f up to one
minute were necessary to purge oxygen frcm th e e n c lo s u re .
Also, the
lo c a t i o n o f th e furnace tube w ith in th e magnet p o le gap made t h i s
method o f sample i n tr o d u c tio n cumbersome.
4.2.5.2
Probe Atomization
As an a l t e r n a t i v e to fu rn a c e w all sam pling, a fu rn ac e accesso ry f o r
probe a to m iz a tio n (54-56) was c o n s tr u c te d ( f i g u r e s 31 and 3 2 ).
Probe
a to m iz a tio n prov ides a means by which th e sample can be va p o riz ed in t o
a fu rn ace t h a t i s a lre a d y h o t r e l a t i v e to th e s u rfa c e o f th e probe t i p
from which th e sample i s v a p o riz e d .
V a p o riz atio n o f a n a ly te i n t o an
a lre a d y h o t furn ace has been shown (57,58) to help m itig a te vapor
phase i n t e r f e r e n c e s .
The b a s ic o p e r a tin g p r i n c i p l e o f th e probe
a to m iz er i s a s fo llow s:
The sample d r o p l e t was p laced on the probe
t i p i n a r e c e s s t h a t was machined in t o i t s end to r e t a i n th e d r o p l e t .
The sample was then d rie d by i n s e r t i n g th e probe i n t o th e furnace tube
which was h e ate d to about 500 °C.
P r io r t o th e a to m iz a tio n s te p , th e
probe was m echanically removed from th e fu rn ac e.
The fu rnace was then
heated t o maximum te m p e ratu re, and th e probe t i p was quickly
r e - i n s e r t e d by so len o id a c t i o n , i n t o th e h o t furn ace tu b e .
The probe
t i p was h e a te d r a d i a t i v e l y by th e fu rn a c e , r e s u l t i n g in v a p o r iz a tio n
o f th e sample.
The tem perature o f th e probe t i p was probably about
200-300 °K lower than t h a t o f th e
fu rn a c e tub e (5 6 ).
Probe
a to m iz a tio n h as been used in both
fu rn ace AAS (54,56) and AES (55) a s
a s t r a t e g y to overcome v a p o r iz a ti o n i n t e r f e r e n c e s .
4 .2 .5 .3
C o n s tru c tio n
of
th e
P ro b e
A c c e sso ry
F ig ure 31 i l l u s t r a t e s th e probe a c c e sso ry and i t s r e l a t i o n s h i p to th e
fu rn a c e assem bly.
The req u ire m e n ts f o r th e probe accesso ry fo r use
w ith th e LEAFS fu r n a c e , were t h a t
th e probe t i p must
be i n s e r t e d
r a p id ly and re p ro d u c ib ly i n t o th e
h o t fu rn a c e tu b e . P re v io u s ly , t h i s
has been accomplished u s in g a so le n o id (55) and a pneumatic c y lin d e r
(5 6 ).
Rapid i n s e r t i o n o f th e probe in s u r e s t h a t th e a n a ly te atoms a re
v a p orized a s a " p lu g ", and a l s o guards a g a in s t re c o n d en satio n o f th e
a n a ly te on th e c o o le r p a r t s o f th e probe t i p .
For th e p r e s e n t work,
th e probe t i p was a tta c h e d to a g ra p h ite rod 8 mm in d iam eter and
40 mm long ( f i g u r e 3 1 ).
The rod t r a v e l e d back and f o r t h in an
aluminum tube vhich served as a guide f o r th e movement o f the probe
( f i g u r e 3 1 ).
The probe mechanism was designed f o r two d i s t i n c t probe
m otions. The f i r s t was a s h o r t - r a n g e (10 mm) motion f o r ra p id
i n s e r t i o n o r removal o f th e probe t i p from th e fu rn ace tu b e , and th e
second, a long range (150 mm) motion, f o r withdrawl o f th e probe t i p
to a p o in t o u ts id e th e fu rn a c e assembly where a sample could be
d e p o site d on th e probe t i p .
The probe mechanism was mounted on a
150 mm l i n e a r t r a n s l a t i o n s ta g e (Daedal, H a rriso n C ity , PA) to p rovide
th e lo n g -ran g e motion.
The l i n e a r t r a n s l a t i o n s ta g e could be moved
e i t h e r manually o r by a s te p p e r m o to r-d riv e n c h ain and s p ro c k e t
system.
At one extreme o f th e lo n g -ran g e motion o f the probe
mechanism, th e probe t i p was lo c a t e d b e n e a th a sample in tr o d u c tio n
p o r t in the aluminum guide tu b e .
Here, th e sample d r o p le t was
d e p o site d w ith a m ic ro p ip e t.
At th e o th e r extreme o f i t s lo n g -ran g e t r a v e l , th e probe mechanism
was p o s itio n e d such t h a t the s h o r t range motion, a c tu a te d by a
s o le n o id , could i n s e r t or withdraw th e probe t i p from th e fu rn a c e
tube.
The working a x is o f the s o le n o id was a c t u a l l y a t a r i g h t angle
to th e d i r e c t i o n o f t r a v e l o f th e probe ( f i g u r e 3 1 ).
The a c t i o n o f
th e so le n o id was t r a n s l a t e d by a l e v e r arm to th e s h a f t t h a t
c o n t r o l l e d th e motion o f the g r a p h ite rod which h e ld th e probe t i p .
The le v e r arm was sp rin g loaded to c o u n te r th e a c t i o n o f th e s o le n o id .
The c l o s i n g o f th e so len o id was used t o withdraw th e probe from th e
furnace tu b e .
When c u r r e n t to the 120 V A.C. so le n o id was switched
o f f using a r e l a y , the s o len o id opened and th e te n s i o n in th e s p r in g
caused th e probe t i p to be r a p id ly i n s e r t e d in t o th e fu rn ace tu b e.
An
a d j u s t a b l e " s to p " was b u i l t i n t o th e l e v e r arm t o allow f i n e tu n in g o f
th e probe r e s t p o s i t i o n in th e fu rn ac e tube.
The p o s i t i o n i n g o f th e
probe t i p in th e fu rn a c e was c r i t i c a l in two r e s p e c t s .
F i r s t , i t was
im portant t h a t the end o f th e probe t i p which h e ld th e sample d r o p l e t
be re p ro d u c ib ly p o s itio n e d d i r e c t l y below th e l a s e r p o r t s in th e
furnace tu b e .
Secondly, th e probe t i p had to be be p o s itio n e d low
enough in th e fu rn a c e tube so t h a t i n c i d e n t l a s e r l i g h t would n o t be
s c a t t e r e d o f f the probe t i p and blackbody em issio n from the probe t i p
would be blo cked by th e o p t i c a l b a f f l e s .
I t was p a r t l y f o r th e s e
re a so n s t h a t th e 8 mm fu rn ac e tub es were abandoned in fa v o r o f th e
10 mm tu b e s .
A ll pro b e t i p s were made from s o l i d p y r o l y t i c g r a p h ite ( P f i z e r ,
E asto n, PA).
F ig u re 32 shows th e d e t a i l s o f th e probe t i p s and t h e i r
p o s i t i o n in th e fu rn a c e tu b e .
wide, and 1 mm t h i c k .
The probe t i p s were 50 mm lon g, 2 mm
The r e c e s s in th e end o f th e probe t i p was
app ro xim ately 0 .5 mm deep, 5 mm lo n g , and 1 mm wide, and was c r e a te d
by u s in g a sm all m illin g b i t .
4.2.5.4
Probe-as-Platform Atomization
In the p resen t work, probe atom ization was simply more convenient than
w all sampling because the probe tip could a lso be used in a manner
analogous to the L'vov platform (4 9 ,5 2 ,5 3 ) .
Here, the probe t ip (w ith
dried sample) remained in the furnace a f t e r the drying ste p .
I t was
then heated r a d ia tiv e ly by the furnace tube and because o f the thermal
mass o f the probe t ip , i t s temperature lagged th a t o f the tube.
This
r e su lte d in a delay in atom ization such th a t the sample was vaporized
in to a high i f not constant temperature environment.
This was in
co n tra st to furnace w all sampling where the sample was vaporized w hile
the furnace tube was s t i l l h ea tin g , and i t s temperature was not
co n sta n t.
FURNACE
108
]
Fig ure 31:
Probe Accessory fo r LEAFS in a Carbon Tube Furnace.
109
F ig u re 32:
D e ta il s o f Probe Tip and P o s i t i o n in Furnace Tube.
4.2.6
Detection and Signal Processing
The d e t e c t i o n system used f o r th e fu rn a c e LEAFS work was s i m i l a r to
t h e flame LEAFS d e t e c t i o n t h a t was d e s c rib e d i n d e t a i l in Chapter I I I .
A f a s t monochromator ( f / 3 . 5 ) was used t o maximize l i g h t c o l l e c t i o n .
The p h o to m u lti p lie r tube was o p e ra ted w ith a tap ered dynode ch ain
s p e c i f i c a l l y designed f o r h ig h p u ls e d c u r r e n t s i g n a l s (2,121) (see
a l s o Chapter I I I ) .
The advantages o f o p e ra tin g th e p h o to m u lti p lie r
tub e in a g a ted mode f o r flame LEAFS was p re s e n te d in Chapter I I I and
t h i s f e a t u r e was employed in th e p r e s e n t in s tru m e n t.
a v e ra g e r was o p e ra te d in a s i n g l e channel mode.
The boxcar
The 50 ohm impedance
in p u t o f the boxcar i n t e g r a t o r modules was used as a load fo r th e PMT
o u tp u t.
E x p o n e n tia l a v e ra g in g w ith a 10 ys i n t e g r a t o r time c o n s ta n t
and a 10 ns boxcar g a tew id th were used f o r a l l measurements.
A 10 ms
time c o n s t a n t was used f o r th e boxcar mainframe o u tp u t.
The boxcar i t s e l f was tr i g g e r e d by em ission from th e excimer l a s e r
a s d e te c te d by th e f i b e r o p t i c / p h o t o m u l t i p l i e r system which was
d e s c rib e d in Chapter I I I .
A 50 mm f o c a l le n g th bi-convex le n s was
used to c o l l e c t th e flu o r e s c e n c e from in s id e th e fu rn ace and image i t
onto th e e n tra n c e s l i t o f th e monochromator.
This involved forming a
s h a rp image of the r in g of fu rn ac e em issio n on th e p la n e o f th e
e n tr a n c e s l i t .
slit.
The r i n g o f e m ission surrounded b u t did n o t e n t e r th e
C a re fu l imaging here was c r i t i c a l , d e s p it e th e use o f o p t i c a l
b a f f l e s , to minimize th e amount o f fu rn a c e em ission re a c h in g th e
d e tec to r.
4.2.7
Data Collection
A la b o r a to r y c o n s tr u c te d computer i n t e r f a c e (141) was used w ith a DEC
11/03 minicomputer fo r d a ta c o l l e c t i o n from the boxcar i n t e g r a t o r .
The t r a n s i e n t flu o r e s c e n c e s ig n a l s were d i g i t i z e d and th e n tem p o ra rily
s to r e d i n memory.
Data f i l e s were then r e c a l l e d and c a l c u l a t i o n s of
peak h e ig h t and peak a re a were performed.
In a d d itio n to i t s d a ta c o l l e c t i o n and m anipulation f u n c tio n s , the
computer was a ls o used t o c o n t r o l v a rio u s components o f th e
in s tru m e n t.
These in clu d ed th e excimer l a s e r , a c h a r t r e c o r d e r , and
th e fu rn ace probe a c c e ss o ry .
These f u n c tio n s a s w ell a s d a ta
a c q u i s i t i o n , were i n i t i a t e d by th e te rm in a tio n of softw are loop
d e la y s .
The e n t i r e sequence o f a to m iz a tio n , l a s e r f i r i n g , and d ata
c o l l e c t i o n , was tr i g g e r e d by a r e la y in th e HGA-2000 fu rn ace power
su pp ly .
The computer so ftw are used f o r d a ta a c q u i s i t i o n and
in strum en t autom ation was designed and w r itt e n by Joseph P. Dougherty.
4.3
RESULTS AND DISCUSSION
4.3.1
Evaluation of Furnace Heating Performance
Furnace te m p e ratu res and h e a tin g r a t e s were ob tained u s in g an o p t i c a l
pyrometer ( I r c o n , Sk ok ie, I L ) .
The o u tp u t s ig n a l s o f th e pyrometer
were a m p lifie d and fed through an i n t e r f a c e , to a minicomputer.
The
s i g n a l s were s to r e d and th e n s e n t through th e i n t e r f a c e to an analog
c h a r t re c o rd e r where a c t u a l measurements were made.
Furnace tubes o f
8 and 10 mm O.D. were e v a lu a te d in terms o f h e a t i n g r a t e through a
s e le c te d ran g e, maximum f i n a l tem p e ratu re, and r e p r o d u c i b i l i t y of
f i n a l te m p e ratu re.
Table 8 shows a comparison o f th e t y p i c a l h e atin g
r a t e s and maximum f i n a l te m p e ra tu re s f o r 8 and 10 mm O.D. fu rn ace
tu b e s.
Heating r a t e s were determ ined f o r th e te m p e ratu re range 1600 -
2000 °C.
TABLE 8
Heating Rates and F in a l Temperatures o f 8 mm and 10 mm Tubes
Furnace O.D.
F in a l
Temperature
H eating Rate
Temperature
Range
8 mm
2500 °C
300 °C/sec
1600-2000 °C
10 mm
2200 °C
250 °C/sec
1600-2000 °C
Furnace h e a tin g r a t e d a ta was o b tain ed in c o n ju n c tio n with
Joseph P. Dougherty.
S e v e ra l measurements were made a t th e maximum power s e t t i n g o f th e
fu rn ace power supply to compare th e r e p r o d u c i b i l i t y o f th e maximum
f i n a l te m p e ra tu re s o f th e 8 and 10 mm fu rn a c e tu b e s .
Table 9 l i s t s
th e r e s u l t s o f t h i s comparison.
TABLE 9
R e p r o d u c ib ility o f Maximum F in a l Temperature o f 8 mm and 10 iran Furnace
Tubes
Furnace O.D.
F in a l
Temperature
Standard
D e v iatio n
Number o f
Measurements
8 mm
2500 °C
50 °C
20
10 mm
2200 °C
30 °C
25
Furnace h e a tin g r e p r o d u c i b i l i t y d a ta was o b ta in e d in
c o n ju c t io n w ith Joseph P. Dougherty.
The h e a tin g r a t e s and maximum f i n a l te m p e ra tu re s t h a t were observed
a r e s i g n i f i c a n t l y i n f e r i o r to v a lu e s t h a t a r e t y p i c a l f o r commercial
atomic a b s o r p tio n furnace a to m iz e r s .
tw o -fo ld :
The d is a d v a n ta g e s o f t h i s were
The u se o f th e p r e s e n t fu rn a c e system was l i m i t e d t o th e
a n a l y s i s o f e lem ents of high to medium v o l a t i l i t y .
Also, the
advantages o f ra p id fu rn a c e h e a ti n g f o r m i t i g a t i o n o f v a p o r iz a tio n
i n t e r f e r e n c e s may not have been e v id e n t.
I t was p o s tu la te d t h a t th e poor fu rn a c e h e a tin g perform ance was a
r e s u l t of b o th an in a d e q u a te power supply, and l e s s th an optimum
e l e c t r i c a l c o n ta c t between th e fu r n a c e tube and th e g r a p h ite
e l e c t r o d e s t h a t conduct th e e l e c t r i c a l c u r r e n t to th e tube.
Two
ex p erim en ts were c a r r i e d o u t t o e v a lu a te th e r e l a t i o n s h i p between th e
c h o ice o f fu rn ace power supply and th e r e s u l t i n g fu rn ac e h e a tin g
perform ance.
A Perkin-Elm er HGA-500 fu rn ac e power supply was u sed ,
i n s t e a d o f th e HGA-2000, to power th e la b o r a to r y c o n s tr u c te d fu r n a c e .
The HGA-500 power supply employs a more s o p h i s t i c a t e d c i r c u i t d e s ig n
t h a t a llo w s ra p id fu rnace h e a tin g r e l a t i v e to t h e l e s s r e c e n t l y
m anufactured HGA-2000.
H e atin g r a t e s and f i n a l te m p e ra tu re s f o r b o th
8 and 10 mm fu rn a c e s were measured f o r th e HGA-500 power supply f o r
th e purpose o f comparison w ith th e HGA-2000.
Both power s u p p lie s
produced h e a tin g r a t e s and f i n a l te m p e ratu res t h a t d i f f e r e d by amounts
t h a t were l e s s than th e stan d a rd d e v i a t i o n o f fu r n a c e te m p e ra tu re s
l i s t e d i n Table 9 .
te m p e ra tu re s .
Table 10 l i s t s th e s e h e a tin g r a t e s and f i n a l
TABLE 10
Heating Rates f o r th e Laboratory C o n stru cted Furnace System
HGA-500 or HGA-2000 Power S u p p lie s
Furnace O.D.
Temperature
Range
Heating Rate
H ig h e st F in a l
Temperature
2300 °C
10
mm
1300-1900 °C
400 °C/sec
10
mm
1600-2200 °C
250 °C/sec
8
mm
1600-2200 °C
300 °C/sec
2500 °C
Furnace h e a tin g r a t e d a ta was o b ta in e d in c o n ju n c tio n w ith
Joseph P. Dougherty.
The r e s u l t s o f th e above comparison seem to i n d i c a t e t h a t th e poor
h e a tin g r a t e s were not a r e s u l t of an inadequate power supply but may
have been r e l a t e d t o th e fu rn a c e i t s e l f .
A second exp erim ent was
performed tfiich in v o lv e d measuring th e h e a tin g r a t e s and f i n a l
tem peratu re o f a commercial fu rn a c e ato m iz er (from th e Perkin-Elm er
Zeeman 5000) which was powered by e i t h e r th e HGA-500 or th e HGA-2000
power sup ply .
Table 11 l i s t s th e r e s u l t s o f t h i s comparison.
TABLE 11
H eating Rate and Temperature Measurements f o r a Commercial Furnace
Using th e HGA-500 or HGA-2000 Power Supply
P a r t a: Maximum F in a l Temperature
Power Supply
HGA-2000
HGA-500
F in a l
Temperature
H eating Rate
1300-1600 °C
3000 °C
1200 °C/sec
1900-2900 °C
3000 °C
600 °C/sec
1300-2900 °C
3000 °C
1300 °C/sec
1900-2900 °C
3000 °C
500 °C/sec
Temperature
Range
P a r t b : In te rm e d ia te F in a l Temperature
HGA-2000
HGA-500
1300-1600 °C
1700 °C
71 °C/sec
1600-2400 °C
2400 °C
236 °C/sec
1300-1600 °C
1700 °C
1300 °C/sec
1600-2400 °C
2400 °C
1300 °C/sec
Furnace h e a tin g r a t e d a ta was o b ta in e d in c o n ju n c tio n w ith
Joseph P. Dougherty.
The d a ta l i s t e d in Table 11 in d ic a te s t h a t r a p id h e a tin g r a t e s and
high f i n a l te m p e ra tu re s can be ob tain ed w ith e i t h e r power supply, when
a commercial fu rn a c e i s used.
This f u r t h e r v in d ic a te d th e HGA-2000
power supply as th e cause o f th e poor performance o f th e la b o ra to ry
c o n s tr u c te d fu rn a c e .
I f th e power s u p p lie s were n o t th e l i m i t i n g
f a c t o r i n d eterm inin g th e h e a tin g r a t e and f i n a l tem perature o f the
la b o ra to ry c o n s tr u c te d fu rn ac e, i t can be assumed t h a t the e l e c t r i c a l
p r o p e r t i e s o f th e fu rn ace system were the cause o f th e poor
perform ance.
Both power s u p p lie s , when used with th e la b o ra to ry
c o n s tr u c te d fu rn a c e , produced n e a rly i d e n t i c a l h e a tin g r a t e s and f i n a l
te m p e ratu res.
T his was ir o n ic in view o f th e f a c t t h a t the two power
s u p p lie s have d i f f e r e n t power c a p a b i l i t i e s , and seemed to i n d i c a t e
t h a t th e c u r r e n t flow through th e fu rn ac e may have been lim it e d by the
e x tr a e l e c t r i c a l r e s i s t a n c e c re a te d by poor c o n ta c t between th e
fu rn a c e and e l e c t r o d e s .
I t was subsequently determ ined t h a t th e poor
e l e c t r i c a l c o n ta c t was due to weaknesses in th e d e sig n o f the
e le c tr o d e assembly and a lack o f c lo s e to le r a n c e i n th e machining of
i t s p a r t s , p r im a r ily th e hinge p o r ti o n .
Slack in th e hinge p o r ti o n of
th e e le c tr o d e assembly allowed l a t e r a l movement o f th e b r a s s e le c tr o d e
h o ld e rs w ith r e s p e c t t o each o th e r , which r e s u l t e d in a l o s s of
p o s i t i v e c o n ta c t between th e fu rn ace and e l e c t r o d e s .
C ontact could be
improved by in c r e a s in g th e p re s s u re on th e e l e c t r o d e s , b u t e x c e s siv e
e le c tr o d e p r e s s u r e caused p h y s ic a l f a i l u r e of th e fu rn ace tu b e s.
4.3-1 -1
Furnace Tube Lifetime
I t was d i f f i c u l t to m aintain an i n e r t atmosphere in th e fu rnace
e n c lo s u re .
This was due p r im a r i ly , to th e use o f fu rn ace wall
sampling which re q u ire d t h a t th e f r o n t door o f th e e n c lo s u re be
removed f o r each sample in tr o d u c tio n .
Although tim e was allowed fo r
the e n c lo su re to be purged o f a i r a f t e r each time th e door was opened,
a p p a re n tly enough a i r remained in th e e n c lo s u re to h a s te n the
o x id a tiv e d e g ra d a tio n o f th e fu rn a c e tu b e and e le c t r o d e s .
P y ro litic
c o a tin g o f th e fu rnace tu b e s and e l e c t r o d e s helped to reduce th e r a t e
o f o x id a tiv e d e g ra d a tio n , b u t did n o t m itig a te t h i s problem e n t i r e l y .
I t was p o s s i b l e to r e - c o a t th e fu rn ace tu b e s a f t e r s e v e ra l f i r i n g s and
th u s extend t h e i r u s e f u l l i f e t i m e s , b u t th e r e s u l t s were no t
re p ro d u c ib le .
The fu rn ace tubes t y p i c a l l y l a s t e d fo r between 30 and
50 f i r i n g s w ith th e o r i g i n a l p y r o l i t i c c o a tin g , and a r e - c o a t in g in
some c a s e s , allow ed s e v e ra l more f i r i n g s .
D espite th e l e s s than
optimum performance of the fu rn ac e system, some p re lim in a ry s tu d ie s of
LEAFS i n a carbon tube fu rn ac e were c a r r i e d o u t.
4.3.2
Evaluation of Sample Introduction Methods
Because of th e furnace problems d e s c rib e d above, the methods of sample
in tr o d u c tio n d e sc rib e d in th e Experim ental s e c tio n , were not
u n iv e rsally a p p lic ab le .
In ord er fo r probe a to m iz a tio n to be
f e a s i b l e , i t i s necessary to h e a t th e fu rn a c e tube r a p id ly to a f i n a l
tem peratu re 250-300 °K h ig h e r than th e a to m iz a tio n appearance
tem perature o f a given a n a ly te s p e c ie s .
This i s because th e probe
u s u a lly reach ed a f i n a l tem perature 200-300 °K l e s s than t h a t o f th e
furnace i n which the probe was being h e a te d (5 6).
The u se o f probe a to m iz a tio n f o r t h i s work, was li m i t e d t o very
v o l a t i l e s p e c ie s such as th a lliu m .
The o r i g i n a l rea so n in g t h a t
supported probe a to m iz a tio n , was t h a t th e r e was an advantage in
v a p o riz in g th e sample in to a c o n s ta n t tem peratu re environment (5 4 -5 8 ).
The slow h e a tin g r a t e s observed f o r t h i s fu rn a c e system r e q u ir e d t h a t
th e furnace be heated f o r as much a s 10 seconds b e fo re a c o n s ta n t
tem p eratu re was reached and th e probe could be i n s e r t e d .
Long h e a tin g
p e rio d s such as t h i s caused a ra p id o x id a tiv e d e g ra d a tio n o f th e
fu rn a c e tubes and e l e c t r o d e s .
The a l t e r n a t i v e was to u se th e probe as
a p la tfo rm , (see E xperim ental s e c tio n ) and use s h o r t e r h e a tin g p eriod s
a t h ig h e r h e a tin g power.
T h is seemed t o h elp extend th e l i f e t i m e of
th e fu rn ace tu b e and e l e c t r o d e s .
4.3.3
Atomization Conditions for Selected Elements
What fo llow s i s a summary o f th e a to m iz a tio n c o n d itio n s and sample
in t r o d u c t i o n methods t h a t were in v e s t ig a te d f o r th e t e s t elem ents
th a lliu m , le a d , iro n , and c o b a l t .
I t i s recog nized t h a t th e s e
elem ents cover a range o f v o l a t i l i t i e s and t h a t th e c o n d itio n s t h a t
were s u c c e s s fu l f o r th e more v o l a t i l e s p e c ie s were not n e c e s s a r i l y
a p p lic a b le to th e l e s s v o l a t i l e elem ents.
These elem ents were chosen
p r im a r i ly f o r t h e i r non-resonance flu o re s c e n c e t r a n s i t i o n s and a l s o
b ecause the e x c i t a t i o n w avelengths were e a s i l y o b ta in a b le u s in g e i t h e r
d i r e c t l a s e r outp ut or frequency doubled l a s e r o u tp u t.
Table 12
summarizes an e v a lu a tio n o f th e r e p r o d u c i b i l i t y o f sample in tr o d u c tio n
f o r s e le c te d elem en ts, by th e t h r e e methods d e sc rib e d above.
4.3.3.1
Thallium
Thallium i s a r e l a t i v e l y v o l a t i l e elem ent t h a t i s an aly z e d in fu rn a c e
AA by w all sampling w ith an a to m iz a tio n te m p e ratu re o f 1400 °C (1 42 ).
For the p r e s e n t s tu d y , th a lliu m was atomized by w all sam pling, probe
a to m iz a tio n , and p ro b e -a s - p la tfo rm a to m iz a tio n .
For w a ll sam pling,
10 yl a l i q u o t s o f t h a lliu m s o l u t i o n were p i p e t t e d o n to th e bottom
furnace w all d i r e c t l y below th e l a s e r beam p o r t s .
The samples were
d rie d by h e a tin g th e fu rn a c e t o 150 °C f o r 60 seconds.
was used f o r any o f th e aqueous a n a ly te s o l u t i o n s .
No c h a r step
For th e
a to m iz a tio n s te p , th e fu r n a c e power supply was s e t t o a power l e v e l
t h a t was known to g iv e a f i n a l tem p e ratu re o f 1400 °C.
A f te r th e
a to m iz a tio n c y c le was c o m p le te, th e fu rn ace was k e p t a t a maximum
tem p eratu re f o r a few seconds, a s a c lea n -u p s te p .
For probe a to m iz a tio n o f th a lliu m , 5 pi a l i q u o t s o f t h a lliu m
s o l u t i o n were p i p e t t e d onto th e probe t i p as i t s a t in
p o s i t i o n o u ts id e
th e fu rn a c e e n c lo s u re .
its rest
The probe t i p was c a r e f u l l y
r e - i n s e r t e d i n t o th e fu rn a c e tu b e , and th e sample was d r ie d by h e a tin g
th e tube to about 500 °C f o r 120 sec .
A fte r d ry in g , and p r i o r t o th e
a to m iz a tio n h e a tin g s te p , th e probe t i p was a u to m a tic a lly removed by
computer c o n t r o l o f a r e l a y and s o le n o id .
The power supply was s e t to
h e a t th e fu rn a c e to a te m p e ra tu re o f ab o u t 1700 °C.
A f te r a
programmable d e la y , th e probe t i p was q u ic k ly r e - i n s e r t e d i n t o th e h o t
fu rn a c e tu be and th e atomic flu o r e s c e n c e s i g n a l was re c o rd e d a s th e
a n a ly te atoms appeared.
A c le a n -u p s te p , in which th e probe t i p
remained in th e f u r n a c e , follow ed th e a tc m iz a tio n s t e p .
The fu rn a c e
tu b e was k e p t a t maximum tem p erature f o r f i v e seconds to v a p o riz e and
remove any r e s i d u a l a n a l y t e .
F ig u re 33 i l l u s t r a t e s th e c h a r t re c o rd e r
t r a c i n g s o f t r a n s i e n t fu rn a c e LEAFS s i g n a l s from probe a to m iz a tio n of
r e p l i c a t e 500 pg th a l l i u m samples.
For th e p ro b e -a s - p la tfo rm a tc m iz a tio n o f th a lliu m , a 5 pi sample
was p la c e d on th e probe t i p and the t i p was i n s e r t e d i n t o th e fu rnace
tu be where th e sample was d r i e d .
The probe t i p remained in th e
fu rn a c e tube during a to m iz a tio n ( a t 1700 °C f o r 4 s e c . ) , and th e
sample was v a p o riz e d i n t h e same delayed manner a s in c o n v e n tio n a l
p la tfo rm a to m iz a tio n .
A c le a n -u p step s i m i l a r t o t h a t used f o r probe
in t r o d u c t i o n followed th e a to m iz a tio n s t e p .
122
F igu re
33:
Probe A tcm ization o f R e p lic a te 500 pg Thallium Samples.
123
4.3.3.2
Lead
Lead i s commonly analyzed by f u r n a c e AA u s in g p la tfo rm a to m iz a tio n
with a tube te m p e ratu re o f 1900 °C (14 2 ).
Furnace LEAFS sample
in t r o d u c t i o n f o r le a d was c a r r i e d out by p ro b e -a s - p la tfo rm o nly, and
in th e same manner a s f o r t h a lliu m which i s o f s i m i l a r v o l a t i l i t y .
A
fu rn a c e a to m iz a tio n te m p e ratu re o f ab o ut 2100 °C was used f o r
p ro b e -a s - p la tfo rm i n t r o d u c t i o n o f le a d .
F ig ure 34 i l l u s t r a t e s c h a r t
re c o rd e r t r a c i n g s o f le a d flu o r e s c e n c e s i g n a l s o b ta in e d using
p ro b e -a s - p la tfo rm sample i n t r o d u c t i o n .
The sample s i z e s and boxcar
in p u t s e n s i t i v i t i e s a r e a ls o shown.
4.3-3-3
Iron and Cobalt
Iro n and c o b a lt a r e elem ents o f medium t o low v o l a t i l i t y .
Both
elem ents a re commonly analyzed by furnace AA u sin g p la tfo rm
a to m iz a tio n with a tu be tem p e ratu re o f 2400 °C (1 4 2 ).
Because th e
furnace system used h e re was l i m i t e d in terms o f i t s maximum f i n a l
te m p e ratu re, probe a tc m iz a tio n or p ro b e -a s - p la tfo rm sample
in t r o d u c t i o n d id no t appear to be f e a s i b l e f o r iro n and c o b a l t .
n e c e s s i t a t e d t h a t w all sampling be used f o r th e s e m e ta ls .
This
For b o th
iro n and c o b a l t , th e fu rn ac e power supply was s e t to i t s maximum power
o u tp u t which r e s u l t e d in a fu rn a c e tube tem p eratu re o f a b o ut 2200 °C.
Although t h i s tem p erature was l e s s th a n t h a t normally used fo r AA
fu rn a c e a tc m iz a tio n , i t did allow li m i t e d a tc m iz a tio n o f th e s e m e ta ls.
Incomplete a to m iz a tio n was suggested as th e cause o f a sev e re memory
e f f e c t f o r ir o n d e s p it e extended fu rnace c le a n up s t e p s .
12M
\
Figure 3^:
P ro b e-as-P latfo rm Atom ization o f Lead Samples.
TABLE 12
R e p r o d u c ib ility o f Sample I n tr o d u c ti o n f o r Wall Sampling, Probe, and
P ro b e-as-P latfo rm Atomization
Element
Method
R e la tiv e
Standard D e v iation
Sample S iz e
30%
100 pg
P
8%
500 pg
T1
P-A-P
10%
500 pg
Fe
W
12 %
10 ng
Co
W
14%
10 ng
T1
W
T1
W = w all sampling; P = probe; P-A-P = p ro b e -a s-p la tfo rm
R e p ro d u c ib ility d a ta was ob tained in c o n ju n c tio n w ith
Joseph P. Dougherty.
4.3.4
Measurement of Detection Limits
D e te c tio n l i m i t s f o r LEAFS in th e carbon tube furn ace were o b ta in e d by
c a l c u l a t i n g the s tan d a rd d e v ia t io n o f 16 measures o f th e blank (peak
a re a ) and u s in g t h i s v a lu e a s th e n o is e in s i g n a l - t o - n o i s e
c a lc u la tio n s.
T y p ic a lly , d e t e c t i o n l i m i t s (SNR=3) were e x tr a p o la t e d
from c a l i b r a t i o n curves t h a t were l i n e a r w ith a s lo p e o f one, w ith
a n a ly te c o n c e n tra tio n s t h a t were one to two o rd e rs o f magnitude above
th e d e te c tio n l i m i t .
This was in accordance w ith th e method d e s c rib e d
by Long and Winefordner Table 13 i s a summary o f th e p re lim in a ry
d e t e c t i o n l i m i t s o b ta in e d f o r LEAFS in a carbon tu be fu rn a c e .
Also in c lu d e d in Table 13 a re th e method o f sample i n t r o d u c t i o n , th e
e x c i t a t i o n and flu o r e s c e n c e d e t e c t i o n w avelengths, and d e t e c t i o n
l i m i t s f o r fu rn a c e AA and fu rn a c e LEAFS t h a t were p re v io u s ly r e p o r te d
i n th e l i t e r a t u r e .
TABLE 13
Carbon Tube Furnace LEAFS D e tec tio n L im its
CONDITIONS
Element
Method
DETECTION LIMITS* (pg)
Wavelength (nm)
E x c i t . F lu o r .
This Work
GFAAS3
Furnace
LEAFS
Fe
W
296.7
373.5
500
2
0.1 b
Co
W
304.4
340.5
0.8
2
0.0 6 b
T1
P-A-P
377.6
535.0
20
10
0.025 c
Pb
P-A-P
283.3
405.7
3
5
0.001 d
* SNR=3
a r e f . 142
b r e f . 15
c r e f . 17
d r e f . 14
W = w all sam pling, P-A-P = p ro b e - a s - p la tf o r m
Furnace LEAFS d e t e c t i o n l i m i t s were o b ta in e d in c o n ju n c tio n w ith
Joseph P. Dougherty.
The carbon tu be fu rn ac e LEAFS d e t e c t i o n l i m i t s l i s t e d in t a b l e 13
r e p r e s e n t a p re lim in a ry a tte m p t t o e v a lu a te th e s e n s i t i v i t y o f t h i s
approach to LEAFS a tc m iz a tio n .
The d e t e c t i o n l i m i t s o b ta in e d f o r t h i s
work a re not as good as th o se p r e v io u s ly r e p o rte d f o r LEAFS in
g r a p h i t e cup fu rn ac es (14-16) and a g r a p h i t e rod a to m iz er (17).
However, th e carbon tube LEAFS d e t e c t i o n l i m i t s a re comparable to
g r a p h ite fu rn a c e atomic a b s o r p tio n v a lu e s (142).
The d e t e c t i o n l i m i t
o b ta in e d fo r ir o n i n the p r e s e n t work (500 p g ) , r e p r e s e n t s a
l i m i t a t i o n caused by co n tam in atio n o f th e fu rn ac e tu bes and th e
fu rn ac e e n c lo s u re .
This may have been due to th e c l o s e p ro x im ity o f
th e magnet p o le p ie c e s t o th e fu rn a c e tu b e .
The magnet p o le gap i n
which the fu rn ace was p o s itio n e d was a b o u t 12 mm a c r o s s .
Since th e
fu r n a c e tu bes were 10 mm in d ia m e te r, t h i s l e f t a 1 mm c le a r a n c e on
e i t h e r s id e o f th e furn ace tu b e .
ir o n tra n s fo rm e r core p l a t e s .
The magnet c o re
was c o n s tr u c te d from
An a d d i t i o n a l problem w ith iro n
c o n ta m in a tio n was due to th e i n a b i l i t y o f th e fu rn ace system to
complete an e f f i c i e n t " c le a n -u p " s te p a f t e r a to m iz a tio n o f an i r o n
sample.
4.3.5
Linear Dynamic Ranges
Some p re lim in a ry experim ents were conducted t o e v a lu a te th e l i n e a r
dynamic range (LDR) o f LEAFS in th e carbon tube fu rn ace.
LEAFS
s i g n a l s were measured over a wide range o f a n a ly te c o n c e n tr a tio n s and
th e r e s u l t s a re shown in f i g u r e 35 f o r i r o n and le a d .
o b ta in e d in c o n ju n c tio n w ith Joseph P. Dougherty.
experim ent was no t completed due t o l a s e r problems.
This d a ta was
The le a d LDR
The LDR's in
g e n e r a l, were d i f f i c u l t t o o b ta in due t o se v e re memory e f f e c t s in th e
fu rn a c e tu b e a f t e r th e a to m iz a tio n o f h ig h a n a ly te c o n c e n t r a t i o n s .
It
was v i r t u a l l y im p o ssib le to work n e a r th e d e t e c t i o n l i m i t a f t e r u s in g
a fu rn a c e tu be f o r an LDR experim ent because o f th e memory e f f e c t .
However, th e se c a l i b r a t i o n c urv es a re l i n e a r , w ith a slope o f one, as
a r e s u l t o f ju d i c i o u s o p tim iz a tio n o f th e p h o to m u ltip lie r v o lta g e ,
s l i t s i z e , and use o f n e u t r a l d e n s ity f i l t e r s to a t t e n u a t e the
f lu o r e s c e n c e s i g n a l s .
A s a l i e n t f e a t u r e o f th e s e l i n e a r dynamic
rang es was t h e i r le n g th .
T h eir l i n e a r i t y over s e v e ra l o rd e rs o f
magnitude o f a n a ly te c o n c e n tr a tio n seemed to in d ic a te th e absence o f
s i g n i f i c a n t p o s t - f i l t e r e f f e c t s vdiich would have caused a n e g a tiv e
d e v ia t io n from l i n e a r i t y a t h ig h e r c o n c e n tr a tio n s .
These l i n e a r
dynamic ran ges are s u p e r io r to th o se o b ta in a b le by atomic a b s o r p tio n
spectro m etry under th e b e s t c o n d itio n s .
Long l i n e a r dynamic ran g es
a re t y p i c a l fo r LEAFS i n fla m e s, plasm as, and fu r n a c e s .
129
SIGNAL
O .
O.
o
-
RELATIVE
FLUORESCENCE
m
o .
CONCENTRATION (pg)
Figure 35:
Linear Dynamic Ranges fo r Iron and Lead
4.3*6
Scatter and Resonance Detection of LEAFS In a Carbon Tube
Furnace
L ig h t s c a t t e r i n g i s a s i g n i f i c a n t i n t e r f e r e n c e in atomic resonance
flu o re s c e n c e spectrom etry .
The e f f e c t i s sev ere i n flame AFS where
i n c id e n t r a d i a t i o n i s s c a t t e r e d by n o n - v o l a t i l i z e d p a r t i c l e s .
The
s c a t t e r i n g in t e r f e r e n c e i s most pronounced in LEAFS where th e in c id e n t
l i g h t i n t e n s i t y i s o rd e rs o f magnitude g r e a t e r than t h a t o f
c o n v en tio n al so u rc e s .
This became even more s e r io u s when high l a s e r
powers were used in an a tte m p t to a ch iev e s a t u r a t i o n in e x c i t a t i o n .
The use of d i r e c t l i n e or non-resonance flu o r e s c e n c e circumvented th e
s c a t t e r problem, b u t re q u ire d th e presen ce o f a p p r o p r ia te energy l e v e l
tra n sitio n s.
In th e p r e s e n t work, a resonance f lu o r e s c e n c e t r a n s i t i o n fo r
cesium (4 5 5 .5 /4 5 5 .5 nm) was used to e v a lu a te th e e f f e c t s o f s c a t t e r on
the d e t e c t i o n o f th e cesium f lu o r e s c e n c e s i g n a l .
A number o f s te p s
were taken to minimize the amount o f l i g h t s c a t t e r e d by the in stru m ent
and i t s components.
These included th e fu rn a c e b a f f l e s , the box t h a t
s h ie ld e d th e beam expansion o p t i c s , and th e use of a v a r i a b l e l a s e r
a t t e n u a t o r t o reduce th e l a s e r i n t e n s i t y and th u s , the amount of
s c a tte r e d l i g h t .
I t was observed t h a t th e l e v e l o f s c a t t e r in th e
fu rn ac e i t s e l f was so s e v e re , t h a t i t p re v e n te d th e d e t e c t i o n of
f lu o re s c e n c e from a l l b u t very h igh cesium c o n c e n t r a t i o n s .
I t was
subsequently determ ined t h a t th e s c a t t e r oc cu rred a t th e s e l e v e l s only
when th e fu rn ace was heated to h ig h tem p eratu re and not when i t was
c o ld .
A lso, the le v e l o f s c a t t e r was observed to in c re a s e
d ra m a tic a lly w ith s u c c e ss iv e a to m iz a tio n s .
Table 14 summarizes the
r e s u l t s o f an experiment to e v a lu a te th e e f f e c t of s c a t t e r in th e
resonance d e t e c t i o n o f cesium.
TABLE 14
S c a t t e r and th e Resonance D e te c tio n of Cesium
C ondition
R e la tiv e Signal*
( f lu o re s c e n c e p lu s background)
Furnace o f f
0.01
Furnace on, beam blocked
0.76
Furnace on, 100 ng/ml Cs
5.31
Furnace on, no sample
7.56
Furnace on, 100 ng/ml Cs
10.70
Furnace on, no sample
11.76
Furnace on, 100 ng/ml Cs
12.16
Furnace on, no sample
13.04
measurements were made s e q u e n t i a l l y w ith th e same
fu rn ace tu b e .
S c a t t e r d a ta was o b ta in e d in c o n ju n c tio n w ith
Joseph P. Dougherty.
The o b s e r v a tio n t h a t s c a t t e r o ccu rred most s tr o n g ly in a h o t fu rn a c e
and in c re a s e d w ith s u c c e ss iv e f i r i n g s r e g a r d l e s s o f whether or not a
sample was p r e s e n t , seemed to i n d i c a t e t h a t th e s c a t t e r was n o t due to
u n v o l a t i l i z e d a n a ly t e s p e c ie s , b u t p o s s ib ly due to o th e r p a r t i c u l a t e s
p r e s e n t in th e h o t fu r n a c e .
These p a r t i c u l a t e s were probably f in e
b i t s o f g r a p h ite t h a t were eroded from th e w a lls o f th e furnace tube
a s a r e s u l t o f s u c c e s s iv e f i r i n g s .
These p a r t i c l e s could r o u ti n e ly be
observed w ith th e naked eye du ring a fu rn ac e f i r i n g .
The o b s e rv a tio n
t h a t th e s c a t t e r became worse w ith s u c c e s s iv e f i r i n g s was an
i n d i c a t i o n o f fu rn a c e d e g r a d a tio n .
Furnace d e g ra d a tio n not only
caused poor a to m iz a tio n r e p r o d u c i b i l i t y , i t a ls o p re c lu d ed th e u s e of
resonance f lu o r e s c e n c e d e t e c t i o n .
4.4
CONCLUSION
The p re lim in a ry i n v e s t i g a t i o n s r e p o rte d h e r e , dem onstrated th e
f e a s i b i l i t y of LEAFS i n a carbon tube f u r n a c e .
In g e n e r a l, th e
a n a l y t i c a l s e n s i t i v i t y o f t h i s approach was found to be comparable to
g ra p h ite fu rn ac e atomic a b s o r p tio n spectrom etry (GFAAS).
The carbon
tube LEAFS l i n e a r dynamic ranges were c h a r a c t e r i s t i c o f th e long
l i n e a r dynamic ra n g e s r o u t i n e l y o b ta in a b le by LEAFS in flam es,
plasm as, and carbon rod and cup a to m iz e rs.
The l i n e a r dynamic ranges
o b tain ed i n th e p r e s e n t work were f a r s u p e r io r as expected, to those
r o u ti n e ly o b ta in a b le f o r GFAAS.
L i n e a r ity in GFAAS i s a f f e c t e d by a
number o f p r o c e s s e s b u t i s u l t i m a t e l y lim it e d by a s t r a y l i g h t
s p e c tra l in te rfe re n c e .
T hat i s , i f more th an one s p e c t r a l l i n e
em itted by th e l i g h t source f a l l s w ith in th e s p e c t r a l bandwidth o f the
s p e c tro m e te r, th e a n a l y t i c a l curve w i l l be b e n t, u n le s s a l l r a d i a t i o n
i s absorbed by a n a ly t e to th e same e x te n t (143).
This problem i s
i r r e v e l a n t in atomic f lu o r e s c e n c e spectrom etry due t o th e n a tu re of
th e flu o r e s c e n c e measurement as compared to an a b s o r p tio n measurement.
The long l i n e a r dynamic ran ges o b ta in e d fo r th e p r e s e n t work were a ls o
d ia g n o s t ic evidence o f th e s u c c e s s fu l d e sig n o f th e carbon tube
fu rn a c e in k eep in g w ith th e design re q u ire m e n ts mentioned e a r l i e r in
t h i s c h a p te r.
That i s , th e tube furnace desig n allow ed f o r e f f i c i e n t
i llu m i n a ti o n o f th e a n a ly t e atom cloud and e f f i c i e n t c o l l e c t i o n of
flu o r e s c e n c e from w ith in th e tu b e .
The l i n e a r i t y over s e v e ra l decades
o f a n a ly te c o n c e n tr a tio n su p p o rts th e assum ption t h a t p o s t - f i l t e r
e f f e c t s in th e tube fu rn ac e were not e v id e n t.
The u se o f probe a to m iz a tio n f o r fu rn a c e LEAFS was dem onstrated
fo r th e f i r s t time and was found to be a co n v en ien t a l t e r n a t i v e to
fu rn a c e w all sampling.
The well known u t i l i t y o f probe a to m iz a tio n
f o r m itig a tin g v a p o r iz a ti o n i n t e r f e r e n c e s was no t e x p lored in t h i s
LEAFS a p p l i c a t i o n b u t f u t u r e experim ents w ith r e a l samples may
dem onstrate t h i s .
The proposed advantages o f a carbon tube ato m iz er
f o r LEAFS, over cup or rod a to m iz e rs, were not dem onstrated because of
th e poor d e t e c t i o n l i m i t s o b tain ed w ith th e p r e s e n t system .
However,
the fu rn a c e system used h e re was no t optimum and d i r e c t comparisons
w ith p re v io u s ly r e p o rte d LEAFS fu rnace work may n o t be v a l i d .
The s e n s i t i v i t y o b ta in e d fo r t h i s te c h n iq u e a s w ell a s a l l
a n a l y t i c a l te c h n iq u e s , depends on both s ig n a l s iz e and r e p r o d u c i b i l i t y
o f o b ta in in g and r e c o r d in g t h a t s i g n a l .
In th e p r e s e n t work, b o th
f a c t o r s s tro n g ly in flu e n c e d th e outcome o f a tte m p ts to ach iev e high
se n sitiv ity .
These two f a c t o r s were d i r e c t l y r e l a t e d t o th e h e a ti n g
performance o f th e fu rn ac e system used f o r t h i s work.
The furnace
system was shown to be inadequ ate in term s o f p ro v id in g r a p id h e a tin g
and h igh a to m iz a tio n te m p e ra tu re s.
At b e s t , t h i s l i m i t e d i t s use to
the a to m iz a tio n o f r e l a t i v e l y v o l a t i l e elem ents.
At w o rs t, i t
r e s u l t e d i n an in com plete a to m iz a tio n o f th e l e s s v o l a t i l e elem ents
which l e d to memory e f f e c t s in th e fu rn a c e .
Sample i n t r o d u c t i o n in t o
th e fu rn a c e , by th e th r e e methods t h a t were i n v e s t i g a t e d , was
c h a r a c te r iz e d by l e s s th a n optimum r e p r o d u c i b i l i t y in a to m iz a tio n and
poor p r e c is io n .
The com bination o f incom plete a to m iz a tio n and poor
p r e c i s i o n was undoubtedly th e primary c au se o f th e h ig h d e te c tio n
l i m i t s r e l a t i v e to fu r n a c e LEAFS work p re v io u s ly r e p o r te d in the
lite ra tu re .
The sample in t r o d u c t i o n methods them selves were probably
r e l i a b l e , b u t th e h e a ti n g o f the fu rn ac e was th e l i m i t i n g f a c to r in
determ inin g p r e c i s i o n .
The most obvious approach to r e c t i f y i n g th e s e problems i s to
re d e s ig n th e fu rn ace a to m iz er system to in s u re r a p i d , re p ro d u c ib le
h e a tin g o f th e fu r n a c e .
Improvements in th e m echanical i n t e g r i t y of
the fu rn ace e l e c t r o d e assembly a re needed to a llo w optimum e l e c t r i c a l
c o n ta c t between th e fu rn a c e and e l e c tr o d e s to maximize th e h e a ti n g of
the fu rnace t u b e .
In a d d itio n , inprovements in sample in t r o d u c t i o n
such as th e im plem entation o f an automated sam pling d e v ice may a ls o
r e s u l t i n inproved p r e c i s i o n and th u s , b e t t e r s e n s i t i v i t y .
The
u ltim a te improvement would be th e a d a p ta tio n o f a commercial carbon
tu b e furnace system f o r LEAFS.
Conmercial fu rn ace a to m iz e rs such as
those c u r r e n t l y used in atomic a b so r p tio n sp ec tro m e try , have been
optim ized in terms o f h e a tin g r a t e , tem perature s t a b i l i t y , furnace
m a t e r i a l s , and sample i n tr o d u c tio n (L’vov p la tfo rm w ith an auto
san p le r).
The a d a p ta tio n o f a commercial furn ace system would allow
th e same r e l i a b l e and e f f i c i e n t a to m iz a tio n f o r LEAFS t h a t i s
c u r r e n tly a v a i l a b l e f o r atomic a b s o r p tio n sp ec tro m e try .
Note: Some s i g n i f i c a n t improvements have r e c e n tly been in c o rp o ra te d
i n t o th e furnace LEAFS work in t h i s la b o r a to r y , by Joseph P. Dougherty
and Frank R. P r e l i .
F i r s t , the LEAFS fu rn ac e e le c tr o d e assembly was
re -d e sig n e d and a new device was f a b r ic a te d .
This new d esig n was
machined to c lo s e to le r a n c e s which has in su re d mechanical s t a b i l i t y
and optimum e l e c t r i c a l c o n ta c t.
This has allow ed more ra p id furnace
h e a tin g to h ig h e r te m p e ra tu re s.
Secondly, a new sou rce o f g ra p h ite rod stock fo r fu rn a c e tube
f a b r i c a t i o n was found.
This has r e s u l t e d in more d u ra b le furn ace
tu b e s which in combination w ith th e new e le c tr o d e assembly desig n ,
have provided very r e p ro d u c ib le h e a tin g and a to m iz a tio n .
LEAFS
d e te c tio n l i m i t s o b ta in e d w ith th e new fu rn ace atom izer d e sig n ,
approached or surpassed th e b e s t furnace LEAFS d e te c tio n l i m i t s
re p o rte d to d a te .
Chapter T
FABRY-PEROT INTERFEROMETRY FOR MEASUREMENT OF PULSED
LASER SPECTRAL BANDWIDTH
5.1
INTRODUCTION
The o u tp u t of a p u lsed dye l a s e r i s h ig h ly monochromatic.
I t i s o f te n
d e s ir a b le to have knowledge o f th e degree o f monochrom aticity o f th e
l a s e r o u tp u t e s p e c i a l l y in s i t u a t i o n s where i t has a d i r e c t b e a r in g on
experim ental r e s u l t s .
In l a s e r e x c ite d atomic flu o r e s c e n c e
sp ectro m etry , measurement o f the s p e c t r a l bandwidth o f th e l a s e r used
fo r e x c i t a t i o n i s f o r s e v e ra l re a so n s , e s s e n t i a l in fo rm a tio n .
5.1.1
Laser Spectral Bandwidth and Spectral Resolution In LEAFS
The s p e c t r a l s e l e c t i v i t y o f l a s e r e x c ite d atomic flu o r e s c e n c e
spectrom etry depends on th e s p e c t r a l bandwidth o f th e e x c i t a t i o n
source and not upon th e s p e c t r a l bandwidth o f th e monochromator ( 5 ) .
The w ell known gallium-manganese s p e c t r a l i n t e r f e r e n c e (5) in AFS can
be e a s i l y m itig a te d by u s in g o p t i c a l l y narrowed l a s e r o u tp u t to
improve th e s p e c t r a l r e s o l u t i o n .
In ICP-LEAFS work, Human e t a l . (12)
used an i n t r a c a v i t y e t a l o n to produce a l a s e r s p e c t r a l bandwidth t h a t
was narrower than th e wavelength d i f f e r e n c e between th e i n t e r f e r i n g
- 137 -
manganese and g a lliu m l i n e s , 403.307 nm and 403.298 nm r e s p e c t i v e l y .
The a u th o rs were a b le t o v i r t u a l l y e l i m i n a t e th e i n t e r f e r e n c e in t h i s
manner, b u t a ls o p o in te d o u t th e im portance o f reducing th e l a s e r
power to avoid s a t u r a t i o n broad en in g o f th e a b s o r p tio n p r o f i l e s o f th e
a n a ly t e and i n t e r f e r e n t .
I t i s t h e r e f o r e u s e f u l to be ab le t o measure
the s p e c t r a l bandwidth o f th e l a s e r , w ith or w ith o u t i n t r a c a v i t y
lin e - n a r r o w in g , when s p e c t r a l s e l e c t i v i t y i s c r i t i c a l .
5.1.2
Spectral Scans of Excitation and Fluorescence Profiles
In LEAFS work, i t i s o f t e n d e s i r a b l e to o b ta in s p e c t r a l scan d a ta in
o rd e r t o e l u c i d a t e th e e x c i t a t i o n and f lu o r e s c e n c e p r o f i l e s o f an
a n a ly t e s p e c ie s i n a g iv e n atom c e l l .
This may be done fo r th e
p urpose o f background c o r r e c t i o n (6) o r f o r d ia g n o s t ic measurements,
and u s u a l l y in v o lv e s scanning th e wavelength o f th e dye l a s e r by
e i t h e r moving th e g r a t i n g , or tu n in g an i n t r a c a v i t y e t a l o n (3 ,1 2 ) ,
vtfiile s im u lta n e o u sly re c o rd in g th e flu o r e s c e n c e s i g n a l .
The
in fo rm a tio n c o n ta in e d in t h e s e s p e c t r a l sca n s i s a c o n v o lu tio n o f th e
em ission p r o f i l e o f th e l a s e r and th e a b s o r p tio n p r o f i l e of th e
a n a ly te atoms.
The s i t u a t i o n i s s i m i l a r to t h a t in which
i n t e r f e r o m e t r i c measurements o f atomic l i n e p r o f i l e s a r e made
(144-146).
Here, the e x p e rim e n ta l measurement o f l i n e p r o f i l e s i s
c o m p licated by in s tru m e n t b ro ad en in g due to th e f i n i t e r e s o lv in g power
o f th e in s tru m e n t (14 4 ).
The e f f e c t o f in s tru m e n t broadening i s a
d i s t o r t i o n o f th e e x p e rim e n ta lly o b ta in e d p r o f i l e s w ith r e s p e c t to th e
actu al lin e p ro f ile .
I t i s p o s s i b l e to c o r r e c t f o r th e e f f e c t o f
in s tru m e n t bro ad en in g on s p e c t r a l measurements and c e r t a i n
d e -c o n v o lu tio n a lg o rith m s e x i s t f o r th e purpose of norm alizin g such
d a ta (1 47 ).
When th e p r o f i l e o f th e s p e c t r a l sou rce to be measured
can be approximated by a Gaussian f u n c tio n and th e in strum en t
broadening i s r e p re s e n te d by a L o re n tz ia n fu n c tio n , the con voluted
ex p erim en tal r e s u l t i s a Voigt p r o f i l e which i s d e sc rib e d by th e
fo llo w in g e q u a tio n (146).
A*act
where :
l n 2 ( A^exp “ 5*instr^
A*act = a c t u a l h a l f - w i d t h o f s p e c t r a l p r o f i l e
AXeXp = e x p e rim e n ta lly measured h a l f - w i d t h
and
5* i n s t r = in s tru m e n t s p e c t r a l bandwidth
When th e in s tru m e n t s p e c t r a l bandwidth i s known, i t s broadening e f f e c t
can be c o r r e c t e d f o r .
However, when th e in stru m e n t s p e c t r a l bandwidth
i s sm all w ith r e s p e c t to th e h a lf - w id th o f th e atomic l i n e p r o f i l e ,
th e e f f e c t o f th e in s tru m e n t s p e c t r a l bandwidth may be n e g le c te d and
the w idth o f th e atomic l i n e p r o f i l e can be determined d i r e c t l y (144).
In th e c a s e where a l a s e r s p e c t r a l scan i s used t o e l u c i d a t e an
atom ic a b s o r p tio n ( e x c i t a t i o n ) or flu o r e s c e n c e p r o f i l e , both th e l a s e r
e m issio n p r o f i l e and th e atomic l i n e p r o f i l e , under c e r t a i n
c irc u m s ta n c e s , can be approximated by Gaussian f u n c tio n s .
The
e x p e rim e n ta l p r o f i l e o b ta in e d by t h i s method r e s u l t s from th e
c o n v o lu tio n o f th e a c t u a l atomic l i n e p r o f i l e and the Gaussian p r o f i l e
o f th e l a s e r em ission .
T his c o n v o lu tio n can be d e s c rib e d by th e
q u a d r a tic sum o f th e h a lf - w id th s o f th e two p r o f i l e s .
A*exp
where:
A* a ct + A* la s e r
^act
= ac1:ua^ h a l f - w i d t h o f p r o f i l e
AXexp
= e x p e rim e n ta lly measured h a l f - w i d t h
AAiager
= l a s e r s p e c t r a l bandwidth
I f th e l a s e r s p e c t r a l bandwidth (AAlager>) i s sm all with r e s p e c t to th e
a c t u a l h a lf-w id th (AXa c t ) o f th e atomic l i n e being measured, i t can be
seen t h a t th e e x p e rim e n ta lly measured h a l f - w i d t h (AAeXp) w i l l c lo s e ly
approxim ate th e a c t u a l h a lf - w id th o f th e s p e c t r a l l i n e .
I f th e l a s e r
s p e c t r a l bandw idth i s not small w ith r e s p e c t to th e h a l f - w i d t h o f th e
s p e c t r a l l i n e , i t then must be c o n sid e re d and c o r r e c t e d f o r .
In
e i t h e r c a s e , i t i s necessary to be a b le t o measure th e s p e c t r a l
bandwidth o f th e l a s e r e s p e c i a l l y i f t h i s in fo rm atio n i s needed fo r
th e d e -c o n v o lu tio n p ro c e ss or a s a p r i o r i knowledge which would
p re c lu d e th e need fo r d e -c o n v o lu tio n .
5.1.3
Measurement of Spectral Bandwidth
V arious te ch n iq u es allo w th e measurement o f th e s p e c t r a l bandwidth of
a l i g h t so u rc e.
Some methods employ a h ig h r e s o l u t i o n g r a t i n g
monochromator (14 8,1 49 ).
F a b ry -P e ro t in te r f e r c m e tr y (150) has become
th e p r e f e r r e d method fo r measurement o f th e s p e c t r a l bandwidth o f a
v a r i e t y o f c o n v e n tio n a l l i n e so u rces (108,151,152) as w ell a s s p e c t r a l
em ission from atom c e l l s (146,153*154).
has a number o f advantages (1 55 ).
F ab ry -P erot in te rfe ro m e try
The most o u tsta n d in g advantage i s
t h a t th e l i g h t g a th e r in g power, or entendue, fo r a g iven r e s o l u t i o n
w ith a F a b ry -P e ro t e t a l o n , i s much g r e a t e r than f o r a monochromator
(156).
There a r e two b a s ic approaches to re c o rd in g and measuring s p e c tr a
by F a b ry -P e ro t in te r f e r c m e tr y (157).
The f i r s t in v o lv e s th e use o f a
s i n g l e l i g h t d e t e c t o r which m onitors th e passage o f i n t e r f e r e n c e
f r i n g e s w hile the o p tic a l p a th o f th e in te r f e r o m e te r element ( e ta lo n )
i s v a r ie d .
T h is v a r i a t i o n in o p t i c a l p a t h may be accomplished through
p r e s s u r e v a r i a t i o n s (158-160) which in tu rn cause changes in th e
r e f r a c t i v e in d e x , or by o th e r m echanical systems (1 61 ,1 6 2 ).
In th e
second method, the l i g h t flu x tr a n s m itte d by th e in te r f e r o m e te r i s
focused onto an image d e t e c t o r such as a photographic p l a t e (155) o r
an a rra y o f d e t e c t o r s d i s t r i b u t e d in space and d e te c tin g
sim u lta n e o u sly (157).
Photographic p l a t e s , which were perhaps th e
f i r s t m u ltic h a n n el d e t e c t o r s , p r e s e n t inconveniences such as poor
quantum e f f i c i e n c y , poor l i n e a r i t y in l i g h t re sp o n se , and d i f f i c u l t
i n t e r p r e t a t i o n o f th e d a ta (1 57 ).
The advent o f s e lf -s c a n n e d
photodiode a r r a y s (160,163) and o p t i c a l m u ltich an nel a n a ly z e r s (164)
have allowed s p e c tro s c o p ic a p p l i c a t i o n c h a r a c te r iz e d by b o th high
s e n s i t i v i t y and s p a t i a l r e s o l u t i o n .
McIntyre and Dunn (149) r e p o r te d
a d e v ic e f o r measurement o f l a s e r s p e c t r a l bandwidth in which a
photodiode a rra y was mounted in th e f ilm p lane of a camera.
The
camera assembly was mounted a t th e e x i t s l i t o f a 1 m f o c a l le n g t h
monochromator.
The a u th o rs were a b le to measure l a s e r s p e c t r a l
bandwidth down to 0.007 nm.
monochromator.
The r e s o l u t i o n was li m i t e d by th e
Since th e s p e c t r a l bandwidths o f many l a s e r s in
c u r r e n t u se exceed t h i s r e s o l u t i o n l i m i t , a d e v ice such a s t h i s f in d s
li m i t e d a p p l i c a t i o n .
Laser s p e c t r a l bandwidth can be c o n v en ie n tly measured u sin g th e
techn iqu e o f F abry-P ero t i n te r f e r o m e tr y (150).
M orris and M c llra th
(165) have re p o rte d a d ev ice which in c o rp o ra te d F abry -P ero t
in te r f e r o m e tr y w ith m u lti- c h a n n e l, photodiode a rra y d e te c tio n of the
i n t e r f e r e n c e f r in g e s f o r th e measurement of l a s e r s p e c t r a l bandwidth.
The in stru m e n t re p o rte d by M orris and M cllrath (165) was designed f o r
use w ith b o th CW and pu lsed o u tp u t l a s e r s .
A s i m i l a r d e v ice which has
been optim ized f o r use w ith pulsed l i g h t from an excimer pumped dye
l a s e r , i s re p o rte d in t h i s d i s s e r t a t i o n .
E le c tr o n ic c i r c u i t r y was
designed f o r the p r e s e n t d e v ic e , to f i r e a s i n g le l a s e r p u lse and
t r i g g e r th e d e te c tio n , re c o rd in g , and subsequent measurement of
i n t e r f e r e n c e f r i n g e s produced by t h a t p u ls e .
5.1.4
Principles of Operation of the Fabry~Perot Interferometer
The F abry -P erot i n t e r f e r a n e t e r i s th e s im p le st of a l l in te r f e r o m e te r s
(1 55 ), u s u a lly c o n s i s t i n g o f two p a r t i a l l y t r a n s m i t t i n g m ir r o r s fa c in g
each o th e r and s e p a ra te d by some fix e d d is ta n c e (166).
The o u ts id e
( n o n - r e f l e c t i n g ) fa c e s o f th e m ir r o r s o f te n have s l i g h t wedge an g les
to help r e j e c t "ghost" r e f l e c t i o n s (168).
from th e r e f l e c t i o n o f e x tra n e o u s l i g h t .
"Ghosts" i n e ta l o n s a r i s e
The F a b ry -P e ro t m ir r o r s form
an o p t i c a l c a v ity in which s u c c e s s iv e r e f l e c t i o n s c r e a t e m u ltip le beam
in te rfe re n c e frin g e s.
One v e r s io n o f th e F a b ry -P e ro t in te r f e r o m e te r
employs a s o lid e ta l o n in s te a d o f th e two p a r a l l e l m irro rs used in th e
c l a s s i c a l d e v ice .
Solid e t a l o n s a r e made from a p ie c e o f o p t i c a l l y
homogeneous m a te ria l such a s fused q u a r tz .
Opposite fa c e s are
p o lis h e d f l a t and p a r a l l e l to a h ig h degree and co ated w ith a
d i e l e c t r i c m a te ria l to th e d e s ir e d r e f l e c t i v i t y (166).
When an e t a l o n
i s illu m in a te d by a n o n -c o ilim a te d monochromatic l i g h t so urce o f eq u al
i n t e n s i t y in a l l d i r e c t i o n s , i t tr a n s m i t s an i n t e r f e r e n c e p a t t e r n of
l i g h t i n t e n s i t y maxima and minima, which may be imaged by a l e n s to
form a p a t t e r n o f c o n c e n tric rin g s (165).
F igu re 36 i l l u s t r a t e s th e s e
c o n c e n tric r i n g s and a t y p i c a l p r o f i l e o f the l i g h t i n t e n s i t y
d i s t r i b u t i o n i n th e i n t e r f e r e n c e p a t t e r n .
A re c o rd o f t h i s i n t e n s i t y
d i s t r i b u t i o n i s c a l l e d an in te r f e r o g r a m and i s u s e f u l f o r making
m easuranents o f th e s p e c t r a l bandwidth o f th e l i g h t source used to
produce th e in te rfe ro g r a m .
mu
O
m
Figure
36:
F a b ry -P e ro t I n t e r f e r e n c e F rin g es and I n t e n s i t y P r o f i l e .
5.1.5
Theory of the Interference Fringes Produced by the Fabry-Perot
Interferometer
F ig ure 37 i l l u s t r a t e s th e p ro c e s s by which m u ltip l e r e f l e c t i o n s w ith in
a s o lid e t a l o n le a d t o t h e fo rm atio n o f i n t e r f e r e n c e f r i n g e s .
An
i n c id e n t monochromatic l i g h t ra y , R, e n t e r s th e s o l i d e t a l o n and
acc o rd in g to S n e l l ' s law, i s r e f r a c t e d tow ards th e norm al.
th en i n c i d e n t upon th e o p p o s ite fa c e ( p o i n t A).
The ray i s
Since th e o p p o s ite
fa c e s o f an e t a l o n a re c o a te d s p e c i f i c a l l y to p a r t i a l l y r e f l e c t
c e r t a i n w av elen g ths, a p o r t i o n o f th e i n c i d e n t ra y i s r e f l e c t e d back
i n t o th e e t a l o n w hile th e rem ainder i s tr a n s m i t t e d th rough th e
r e f l e c t i v e c o a tin g i n t o th e a i r .
The r e f l e c t e d p o r t i o n o f th e ra y i s
then i n c i d e n t upon th e o p p o s ite fa c e (p o in t B) where th e p ro c e s s i s
r e p e a te d .
The r e f l e c t e d p o r t i o n o f t h i s ra y i s i n c i d e n t upon the
o p p o s ite fa c e (p o in t C) where p a r t i a l r e f l e c t i o n and tr a n s m is s io n
again ta k e s p l a c e .
The r a y s e x i t i n g th e e t a l o n a t p o i n t s A, C, and E
a r e propagated p a r a l l e l to each o t h e r .
The wave t r a i n s o f th e ra y s
e x i t i n g a t th e s e p o i n t s a re c o h e r e n t and can i n t e r f e r e (155).
The
i n t e r f e r e n c e p a t t e r n s w i l l be s h a r p e s t a t i n f i n i t y or a t th e f o c a l
p lan e o f an image-forming o p t i c a l system (155).
37;
Mux
Pit
a* ri
e° ti
°t]a
Of
Sht
K th
$n
5.1.5.1
Geometry of Fabry-Perot Interference Fringes
Each l i g h t ray t h a t i s tra n s m itte d by th e e ta l o n ex p erien c e s an
o p t i c a l p a th o f d i f f e r e n t le n g th due to the m u ltip le r e f l e c t i o n s of
ra y s w ith in th e e t a l o n .
I f th e path d i f f e r e n c e s o f th e v a rio u s ra y s
d i f f e r s by i n t e g r a l m u ltip le s o f A, the wavelength o f th e l i g h t , the
wave t r a i n s o f th e s e ra y s w i l l a r r i v e in phase a t sane p o in t i n space,
and c o n s tr u c ti v e ly i n t e r f e r e cau sin g an i n t e n s i t y maximum.
For a
g iven wavelength o f l i g h t , the l i g h t i n t e n s i t y maxima w i l l occur fo r
an g les 0 acc o rd in g to th e eq u atio n :
mA = 2tncos0
where:
m = i n t e r f e r e n c e o rd e r number
A = wavelength of l i g h t
n = r e f r a c t i v e index o f th e spacing m a te ria l
t = e ta lo n spacing (o r th ic k n e s s , i f s o lid e ta lo n )
and
e = angle of in c id e n ce of th e l i g h t beam
in the spacing m a te ria l
C o n stru c tiv e i n t e r f e r e n c e occurs f o r i n te g e r v a lu es o f m, the
i n t e r f e r e n c e o rd e r number.
The i n t e n s i t y o f th e i n t e r f e r e n c e f r in g e s
decreases w ith d e crea sin g o rd e r number.
Consequently, the b r i g h t e s t
f r i n g e s occur near th e c e n te r o f th e i n t e r f e r e n c e p a t t e r n .
5.1.6
Characteristics of Etalons
The perform ance o f an e t a l o n depends on a number o f c h a r a c t e r i s t i c
p a ra m ete rs.
Knowledge o f th e se param eters i s e s s e n t i a l to s e l e c t i n g
an e t a l o n f o r a g iven a p p l i c a t i o n and making measurements w ith t h a t
e ta l o n .
5.1.6.1
Free Spectral Range
The f r e e s p e c t r a l range (FSR) o f an e ta l o n r e p r e s e n t s th e range of
wavelengths t h a t can be d is p la y e d in th e same s p e c t r a l o rd e r m w ith o u t
ov erlapping a d ja c e n t o rd e rs (166).
The FSR s e t s th e l i m i t f o r th e
maximum bandwidth o f wavelengths w ith which th e e t a l o n should be
illu m in a te d to avoid o v e r la p .
The FSR a l s o d eterm in e s th e r e l a t i v e
spacing between r i n g s in th e F ab ry -P ero t i n t e r f e r e n c e p a t t e r n .
The
f r e e s p e c t r a l range o f an e ta l o n can be c a l c u l a t e d as fo llo w s:
A2
FSR = ----2tn
where:
A = wavelength o f source l i g h t
n = r e f r a c t i v e index o f e t a l o n medium
and
t = m ir ro r spacing (o r th ic k n e s s f o r s o lid e ta lo n )
5.1.6.2
Finesse
The f i n e s s e o f an e t a l o n i s a measure o f th e i n t e r f e r o m e t e r 's a b i l i t y
to re s o lv e c l o s e l y spaced l i n e s .
The f i n e s s e o f an e ta l o n determ in es
th e r a t i o o f th e s e p a r a tio n and th ic k n e s s o f th e F ab ry -P ero t
in te rfe re n c e f r in g e s .
A h ig h degree o f f i n e s s e , which r e s u l t s in a
h igh s e p a r a tio n t o th ic k n e s s r a t i o , i s d e s i r a b l e (166).
The n e t
f i n e s s e o f an i n t e r f e r o m e t e r depends on a number o f c o n t r i b u t i n g
fa c to rs.
For th e a p p l i c a t i o n d e s c rib e d h e r e , i t i s s u f f i c i e n t to
d e s c rib e th e n e t f i n e s s e i n terms o f r e f l e c t i v i t y f i n e s s e and th e
f l a t n e s s f i n e s s e o n ly .
The r e f l e c t i v i t y f i n e s s e f o r an e t a l o n w ith
re fle c tiv ity R is :
tt/R
r
1-R
Some v a lu e s o f r e f l e c t i v i t y f i n e s s e as a f u n c tio n o f e t a l o n
r e f l e c t i v i t y , a r e shown below.
R eflec tiv ity
Fp
.70
.75
.80
.85
.90
.95
.99
8 .8
11
14
19
30
61
314
The degree o f o p t i c a l f l a t n e s s o f th e e t a l o n c o n t r i b u t e s to th e t o t a l
f i n e s s e and i s r e f e r r e d to a s flatness finesse.
The f l a t n e s s f i n e s s e
i s giv en by:
M
where
M i s t h e f r a c t i o n a l wavelength d e v ia t io n from
X
t r u e f l a t n e s s ( — a t 5M6 nm). T his f i g u r e r e f e r s to f l a t n e s s over
M
an 80? a p e r tu r e . The f l a t n e s s over s m a lle r a p e r t u r e s , near th e c e n te r
o f th e e t a l o n u s u a lly exceeds t h i s v a lu e (167).
The ne t ( t o t a l ) f i n e s s e due t o f l a t n e s s and r e f l e c t i v i t y i s c a ll e d th e
instrument finesse (166).
1
The in s tru m e n t f i n e s s e (F^) i 3 given by:
1
1
+
5.1.6.3
Minimum Resolvable Bandwidth
The minimum re s o lv a b le bandwidth 6A o f an e ta l o n i s a f u n c tio n o f both
f r e e s p e c t r a l range and f i n e s s e and i s given by:
FSR
6 A = ----Fi
The minimum r e s o lv a b le bandwidth o f an e ta l o n r e p r e s e n t s th e s m a l le s t
wavelength d i f f e r e n c e t h a t can be d i s t i n g u i s h e d by in t e r f e r o m e t r i c use
o f t h a t e ta l o n .
5.1.7
Calculation of Spectral Bandwidth
When an e t a l o n i s p la ce d in th e p a th o f monochromatic l i g h t and th e
i n c i d e n t l i g h t has a d i s t r i b u t i o n in wavelength t h a t i s sm all compared
to th e f r e e s p e c t r a l range o f th e e t a l o n y e t la rg e compared to i t s
r e s o l u t i o n l i m i t , th e e f f e c t w ill be a broadening c i r c u l a r
i n t e r f e r e n c e f r i n g e s r e l a t i v e t o th o s e f o r an i d e a l , a b s o l u t e l y
monochromatic sou rce (1 6 5 ).
The e t a l o n can be used t o measure th e
s p e c t r a l bandw idth o f a l i g h t s o u rc e , when th e v a lu e o f t h a t s p e c t r a l
bandwidth f a l l s i n a range bounded on one end by th e r e s o l u t i o n l i m i t
and on th e o th e r by th e f r e e s p e c t r a l rang e.
By m easu rin g th e
r e l a t i v e spacing o f th e r in g s t r u c t u r e ( f i g . 3 6 ), and th e p o s i t i o n s of
th e h a l f power p o i n t s on a s e l e c t e d r i n g , the s p e c t r a l bandw idth
can be c a l c u l a t e d a c c o rd in g to th e method d e sc rib e d by M orris and
M c llra th (165) which in v o lv e s th e fo llo w in g e x p re s s io n :
AX = 2 *
tn
d m+1A rm+1
d2 - d 2 .
m m+1
where th e fo llo w in g p a ra m e te rs a re used:
AX =
d ( m+i )=
d ia m e ter o f in n e r r i n g used
d ( m)
d ia m e ter o f a d j a c e n t o u te r r in g
=
Ar(m+i)=
and
l a s e r s p e c t r a l bandw idth (FW»0
h a lf - w id th o f in n e r rin g used
t
=
n
= r e f r a c t i v e index o f the e t a l o n s u b s t r a t e
X =
th ic k n e s s of e t a l o n
w avelength o f th e l a s e r l i g h t
AA
152
An a l t e r n a t e method o f c a l c u l a t i n g th e s p e c t r a l bandw idth based on a
m easurenent o f F a b ry -P e ro t i n t e r f e r e n c e f r i n g e s has been su gg ested by
th e m anu facturer o f th e M olectron dye l a s e r used f o r t h i s work.
This
method in v o lv e s m easuring th e d i s t a n c e between th e f i r s t and t h i r d
r i n g s o f th e i n t e r f e r e n c e p a t t e r n , and th e w id th (FWHM) o f th e second
rin g .
The s p e c t r a l bandwidth can th e n be c a l c u l a t e d a c c o rd in g to th e
form ula:
X
AA= FSR Y
FSR = f r e e s p e c t r a l range o f e t a l o n o r ----2 tn
where
Y = 1/2 d i s t a n c e between 1 s t and 3rd r i n g s
and
X = w id th o f 2nd r i n g (FWIW)
For example, a t a wavelength o f 600 nm, u s in g an e t a l o n w ith a
f r e e s p e c t r a l range o f 0.040 nm, the fo llo w in g measurements o f th e
r i n g s t r u c t u r e were made:
Y = 24 mm
X = 2.5 mm
th e s p e c t r a l bandwidth i s th e n c a l c u l a t e d a s fo llo w s :
AA
2 .5
= 0.0 40 nm —— * 0.0042 nm
24
I f th e minimum re s o lv a b le bandwidth o f th e e t a l o n i s very small in
comparison to th e s p e c t r a l bandwidth o f th e l a s e r outpu t being
measured, the e f f e c t o f broad enin g by th e e ta l o n can be ignored (169).
However, i f th e bandwidth o f th e e ta l o n i s not small in comparison to
th e s p e c t r a l bandwidth o f th e l a s e r , the e x p e rim e n ta lly measured l a s e r
p r o f i l e may be e rro n e o u sly l a r g e .
This i s a n o th e r example o f a
c o n v o lu tio n between a G aussian f u n c tio n ( l a s e r p r o f i l e ) and a
L o re n tz ia n fu n c tio n (in s tru m e n t bandwidth, in t h i s case an e t a l o n ) .
T h is r e l a t i o n s h i p i s s i m i l a r to t h a t d e sc rib e d e a r l i e r f o r the
c o n v o lu tio n of an e ta l o n bandwidth and a s p e c tr a l p r o f i l e .
The a c tu a l
l a s e r s p e c t r a l bandwidth, c o r r e c te d f o r d i s t o r t i o n due to th e
bandwidth of th e e t a l o n , can be approximated in t h a t same manner.
5.1.8
Instrumental Requirements
The F abry -P erot in te r f e r o m e te r and s i g n a l p ro c e s s in g system system
d e sc rib e d here were designed to be used p r im a r i ly w ith a pulsed l a s e r
system .
I t was d e s i r a b l e to f i r e th e l a s e r once, a t th e b e g in nin g o f
an a rra y because th e l a s e r pulsew id th was s h o rt (10 ns) compared to
th e minimum i n t e g r a t i o n time o f th e R eticon a r r a y (1 ms), scan.
It
was advantageous to use a s i n g le l a s e r p u ls e f o r s p e c t r a l bandwidth
measurement because th e p u l s e - t o - p u l s e power r e p r o d u c i b i l i t y and
s p a t i a l s t a b i l i t y of th e l a s e r system was p o o r.
The d e t a i l s o f th e
e l e c t r o n i c s r e q u ire d to sy n ch ro n ize th e f i r i n g o f th e l a s e r t o the
a rra y scan are d e sc rib e d in th e experim ental s e c t i o n .
15H
5.2
EXPERIMENTAL
Figure 38 i l l u s t r a t e s th e design o f a F abry -P ero t in te r f e r o m e te r t h a t
was c o n s tr u c te d f o r th e purpose o f m easuring th e s p e c t r a l bandwidth of
a pu lsed dye l a s e r .
The in te r f e r o m e te r was based on a d e sig n s im i l a r
to th a t re p o rte d by M orris and M cllrath (165).
The p r e s e n t instrum ent
employs b o th s o lid and fix e d a i r - g a p e t a l o n s , and a R eticon photodiode
a rra y f o r imaging th e i n t e r f e r e n c e p a t t e r n .
Table 15 i s a l i s t of th e
components o f th e in te r f e r o m e te r system and t h e i r m a n u fa ctu rers.
Figure 39 i s a block diagram o f th e components t h a t make up th e
d e te c tio n system f o r th e i n te r f e r o m e te r .
F ig u re HO i l l u s t r a t e s the
a d a p te r fo r th e e ta l o n gimbal t h a t was c o n s tr u c te d to a llo w th e use of
th e f i x e d - a i r - g a p e t a l o n s normally used f o r i n t r a c a v i t y l i n e narrowing
i n th e dye l a s e r .
155
O-
o
O
,8 .
Fig ure 38:
Design o f Laboratory C o n stru cted Fabryj^ g p fe rc m e te r.
P e ro t
TABLE 15
I n te r f e r o m e te r System Components
Component
Model Number
M anufacturer
Laser O p tic s,
Danbury, CT
S o lid e ta l o n s
F ix e d -a ir- g a p e t a l o n s
DL 226
M olectron,
Cooper Laser Sonics
S anta C la ra , CA
He Hum-Neon Laser
ML-810
M etrologic
Bellmawr, NJ
Esco C o rp .,
Oak Ridge, NJ
Fused s i l i c a le n s e s
E talon gimbal
22-2109
E aling O p tic a l
N atick, MA
Photodiode Array
102UG
EG&G R eticon
Sunnyvale, CA
Array motherboard
100B
ii
ii
Waves aver
Epic In s tru m e n ts ,
F o ste r C ity , CA
T rig g erin g c i r c u i t r y
la b o r a to r y c o n s tr u c te d
Helium-Neon Laser
ML810
M etro lo g ic , I n c . ,
Bellmawr, NJ
5.2.1
Optics
I t i s n e c e s sa ry t h a t th e l i g h t e n t e r i n g th e e t a l o n be s l i g h t l y
d iv e r g e n t in o rd e r to o b ta in optimum performance o f th e e t a l o n .
The
o u tp u t beam o f the tu n a b le dye l a s e r used h e re was h ig h ly c o llim a te d
so a 50 mm fo c a l le n g th fused s i l i c a c y l i n d r i c a l le n s was used to
enhance th e div e rg e n ce o f th e l a s e r l i g h t and a ls o t o c o n c e n tra te th e
i n t e n s i t y i n t o the narrow c r o s s - s e c t i o n o f th e i n t e r f e r e n c e p a t t e r n
t h a t was sampled by th e photodiode a r r a y ( f i g u r e 3 6 ).
The l e n s was
mounted such t h a t th e c y l i n d r i c a l a x is was p a r a l l e l to th e t a b l e .
This was done because th e o u tp u t beam o f th e dye l a s e r was more
s p a t i a l l y homogeneous i n th e v e r t i c a l d i r e c t i o n th an in th e h o r i z o n t a l
d ire c tio n .
A 300 mm f o c a l le n g th fu s e d s i l i c a biconvex le n s was used
to p r o j e c t th e i n t e r f e r e n c e f r i n g e s onto th e photodiode a r r a y which
was o r i e n t e d with i t s lo n g a x is p e r p e n d ic u la r to th e t a b l e .
The l e n s
pla y ed no p a r t i n forming th e f r i n g e s , b u t i t served to image th e
sou rce in th e same p la n e as th e f r i n g e s thereby i n c r e a s i n g th e
i n t e n s i t y o f i llu m i n a ti o n (168).
A tu b u la r b a f f l e was i n s t a l l e d to
s h i e l d th e photodiode a r r a y from ambient l i g h t and to p re v e n t th e
p o s s i b l e s a t u r a t i o n o f th e a r r a y .
The b a f f l e allow ed only l i g h t t h a t
was p ro p a g a ted along th e o p t i c a l a x i s o f th e i n te r f e r o m e te r to
il l u m i n a t e th e a r r a y .
~T
>”
= cc
CHART
RECORDER
PHOTODIODE
ARRAY
QC »-
F ig u re 39:
<
CD
ir cr
OC UJ
< 5
o
UJ
Block Diagram o f I n te r f e r o m e te r D e te c tio n System.
Figure 40:
Gimbal Adapter f o r Fixed-A ir-G ap E ta lo n s .
5.2.2
Selection of Etalons
S o lid e ta l o n s f o r t h i s p r o j e c t were s e le c te d a c c o rd in g to c r i t e r i a
t h a t re p re s e n te d a compromise between r e s o l u t i o n , l i g h t throughput
(e n te n d u e), and economy.
The g e n e r a l s p e c i f i c a t i o n s o f th e s o lid
e ta l o n s used h ere a re as fo llo w s:
re fle c tiv ity :
fla tn e ss:
p a ra llelism :
th ic k n e s s :
90$ b o th s id e s
X/20
l e s s than
one second o f
arc
9.52 mm
Three s e p a ra te s o l i d e t a l o n s , coated f o r c e n t r a l w avelengths of
286 nm, 407 nm, and 572 nm, were o b ta in e d .
These wavelengths were
chosen because o f t h e i r proxim ity to th e e x c i t a t i o n l i n e s o f a number
o f e le m e n ts.
For example:
286 nm:
Mg 285 nm
407 nm:
Ca 422 nm
Pb 283
nm
V
411 nm
Mn 279
nm
In
410 nm
Pb 405 nm
Mn 403 nm
572 nm:
Na 589 nm
and fundamental w avelengths f o r
frequency doubled r a d i a t i o n
example: frequency doubled, 283.3 nm; fundam ental, 566.6 nm
An e ta l o n f l a t n e s s o f X/20 i s a compromise between economy and
p ra c tic a l lim ita tio n s.
Although e ta l o n s can be obtained t h a t a re f l a t
to w ith in X/100 or X/200, the expense may be p r o h i b i t i v e .
Also, i t i s
d i f f i c u l t to o b ta in a s o lid e ta lo n w ith t h i s degree o f f l a t n e s s on
both s id e s and a ls o having h igh p a r a l l e l i s m .
I t i s more common to
have f ix e d - a ir - g a p e ta l o n s with f l a t n e s s to X/100 or b e t t e r , w ith
p a r a l l e l i s m determined by sp ac e rs r a t h e r than o p t i c a l machining.
The
X/20 f l a t n e s s o f th e e t a l o n s chosen f o r t h i s work c o n tr ib u te s a
f l a t n e s s f i n e s s e o f 10.
The r e f l e c t i v i t y f i n e s s e , based on th e 90$
r e f l e c t i v i t y c o a tin g s , was c a lc u la te d to be 2 9 .8 .
T h e re fo re , the
in stru m e n t f i n e s s e o f the s o lid e t a l o n s , based on th e f l a t n e s s and
r e f l e c t i v i t y c o n tr ib u tio n s , was 9 .5 .
The X/20 f l a t n e s s o f th e s o lid
e ta l o n s was th e l im it in g f a c t o r in e s t a b l i s h i n g th e f i n e s s e .
r e f l e c t i v i t y was secondary i n t h i s r e s p e c t .
The
An in c r e a s e in the
r e f l e c t i v i t y may improve th e r e f l e c t i v i t y f i n e s s e .
However, because
o f th e q u a d r a tic n a tu re o f th e f i n e s s e r e l a t i o n s h i p , an a p p re c ia b le
improvement in in stru m e n t f i n e s s e would n o t be e v id en t and a
c o n s id e ra b le l o s s in l i g h t throughput would r e s u l t .
I t i s im portant
to note t h a t , s in c e i n th e p r e s e n t work, a r e l a t i v e l y small e ta l o n
a p e r tu r e (30-40$) was b e ing illu m i n a te d by l a s e r l i g h t , the a c tu a l
f l a t n e s s over t h i s a p e r tu re may have been a s good as X/50.
C onsequently, th e f i n e s s e value based on t h e f l a t n e s s should be h ig h e r
than t h a t c a lc u la te d fo r an 80$ a p e r tu r e .
The f r e e s p e c t r a l range, f i n e s s e , and minimum re s o lv a b le
bandwidths o f the s o lid e ta l o n s were c a l c u l a t e d and a r e as l i s t e d in
Table 16.
TABLE 16
Performance Param eters f o r S o lid E ta lo n s
w avelength
FSR
*
F in e s se
*
*
6X
286 nm ±10/1
2.86x10"3 nm
9.5
3.0 x 10"1* nm
407 nm ±10%
5.80x10-3 nm
9.5
6.1 x
572 nm ±10$
1.14x10 " 2 nm
9.5
1 .2 x 10“ 3 nm
#
nm
C a lc u la te d f o r c e n t r a l w avelength only.
The r e f l e c t i v e c o a tin g s on e t a l o n s perform over a l im it e d wavelength
ran g e.
C oatin g s a re a v a i l a b l e however, t h a t perform over a b ro a d e r
range o f wavelengths b u t th e in c re a s e d th ic k n e s s o f the c o a ti n g s
r e q u ir e d f o r t h i s may in c o r p o r a te f l a t n e s s e r r o r s .
Maximum
r e f l e c t i v i t y and hence, maximum f i n e s s e i s ob tained a t th e c e n t r a l
wavelength o f th e range f o r which an e t a l o n i s co ated and e t a l o n
performance d e crea se s th e f u r t h e r from th e c e n t r a l wavelength t h a t one
works.
Because o f the r e l a t i o n s h i p between r e f l e c t i v i t y and f i n e s s e
d is c u s s e d e a r l i e r , a small d e crea se in r e f l e c t i v i t y t r a n s l a t e s in to a
s i g n i f i c a n t d e crea se in f i n e s s e and a subsequent l o s s in r e s o l u t i o n .
The performance param eters o f th e f i x e d - a i r - g a p e t a l o n s (M olectron)
t h a t were used, were c a l c u l a t e d based on th e f r e e s p e c t r a l range and
f i n e s s e v a lu e s quoted by th e m an u factu rer, and a re l i s t e d in Table 17.
TABLE 17
Performance Param eters f o r Fixed-A ir-G ap E ta lo n s
wavelength
range
FSRa
F in e sse
6 Ac
20
9.0 X
440-570 nm
2.8 X 10-2 nm
20
1 .4 X 10_3 nm
560-730 nm
4 .6 X
nm
20
2.3 X 10-3 nm
720-950 nm
7.7 X 10 2 nm
20
00•
on
<=r
i
o
1 .8 X 10-2 nm
3
CM
1
O
360-450 nm
nm
X 10-3 nm
“1
Based on m a n u f a c tu r e r 's v a lu e o f 1.1 cm , and
c a lc u la te d f o r c e n t r a l wavelength o n ly .
b F in e s s e v a lu e quoted by m anu facturer and i s v a l i d fo r
c e n t r a l wavelength on ly .
Q
Minimum r e s o l v a b l e bandwidth was e s tim a te d from
th e c e n t r a l wavelength FSR and f i n e s s e v a lu e s .
5.2.3
Retlcon Photodiode Array
M orris and M c llra th (165) used a R eticon 1024G p h oto diod e a r r a y in
t h e i r l a s e r m o n o c h ro m a to r-in te rfe ra n e te r to image t h e in t e r f e r e n c e
frin g e s.
The vid eo o u tp u t of th e photodiode a r r a y was viewed on a
s to ra g e o s c il lo s c o p e .
These a u th o rs used a d i g i t a l p u ls e -c o u n tin g
c i r c u i t t o measure th e d i s t a n c e between i n t e n s i t y peaks in th e a r r a y
v id eo o u tp ut d is p la y as viewed on th e o s c i l l o s c o p e .
A R eticon 1024G photodiode a r r a y w ith a q u a rtz window was employed
i n th e p r e s e n t in s tr u m e n t.
in the U.V.
The q uartz window allow ed l i g h t d e t e c t i o n
The photodiode a r r a y measured th e l i g h t i n t e n s i t y
d i s t r i b u t i o n a lo n g th e diam eter o f th e c i r c u l a r i n t e r f e r e n c e p a t t e r n .
The a rra y and a s s o c ia te d c i r c u i t r y produced a v id e o v o lta g e o u tpu t
c o rre sp o n d in g t o , and l i n e a r w ith r e s p e c t to , th e i n t e n s i t y
d i s t r i b u t i o n o f th e f r i n g e s t h a t f e l l on th e s e n s i t i v e p o r ti o n of the
array.
The vid eo o u tp u t was synchronized t o th e i n t e r n a l clock
(200 KHz) of th e a rra y motherboard.
An o s c i l l o s c o p e w ith a s to ra g e
f u n c tio n was n o t a v a i l a b l e f o r t h i s work, so a "Wavesaver" d i g i t a l
waveform s to r a g e d e v ic e (Epic In s tru m e n ts , F o s te r C ity , CA) was used
to s t o r e th e vid eo v o lta g e o u tp u t o f th e photodiode a r r a y .
The s to r e d d a ta p o i n t s could th e n be s e n t to a c o n v en tio n al
o s c i l l o s c o p e f o r m o n ito rin g , or t o an analog c h a r t re c o rd e r f o r a
hardcopy and subsequent measurement o f t h e i n t e r f e r e n c e f r in g e
in te n sity p ro f ile .
The a r r a y was i n t e r n a l l y clocked a t a r a t e of
200 KHz w ith an i n t e g r a t i o n tim e o f 10 ms.
These param eters were
chosen p r im a r i ly to acccmodate t r i g g e r i n g o f th e l a s e r and th e d i g i t a l
waveform s to ra g e system (se e n ex t s e c t i o n ) .
5.2.4
Triggering Electronics
Fig ure 41 shows a schem atic drawing o f th e d i g i t a l c i r c u i t used to
sy nchro nize th e f i r i n g o f th e l a s e r to th e a rra y scan.
This c i r c u i t
p ro vid ed a 10 ms wide p u ls e to t r i g g e r th e l a s e r and a 1 ms p u ls e to
t r i g g e r a d i g i t a l waveform s to ra g e d e v ic e .
The c i r c u i t used the START
p u ls e s g e n e r a te d by th e a r r a y motherboard (170) to d e r iv e th e l a s e r
t r i g g e r p u ls e .
The i n t e r v a l between th e s e START p u ls e s was u s e r
s e l e c t e d and f o r t h i s a p p l i c a t i o n was 3 e t t o 10 ms.
This was
accomplished by s e t t i n g a s e r i e s o f DIP sw itch e s on th e a rra y
motherboard t o a count o f 2048.
The p ro d u c t o f 2048 c o u n ts, times 5ys
per c o u n t, gave a t o t a l o f 10 ms between START p u ls e s .
When th e
c i r c u i t had been enabled by a u s e r a c t i v a t e d b o u nceless sw itch (555
tim e r) and (74121 monostable m u l t i v i b r a t o r ) , th e START p u ls e s were
g a ted t o a f l i p - f l o p (7473).
The f l i p - f l o p was c o n fig u re d t o to g g le
on a d ja c e n t START p u ls e s , producing a 10 ms p u lse a t i t s o u tp u t.
This
10 ms p u ls e was v o lta g e a m p lifie d to 15 v o l t s by a 2N2222 t r a n s i s t o r
and used to t r i g g e r th e l a s e r .
The f a l l i n g edge o f th e l a s e r t r i g g e r
p u ls e was used to t r i g g e r a second 74121 which provided a 1 ms t r i g g e r
p u ls e f o r th e " Wavesaver".
F ig u re 42 i s a tim in g diagram showing th e r e l a t i o n s h i p o f th e
t r i g g e r i n g f u n c tio n s of t h i s c i r c u i t to th e scanning sequence o f th e
photodiode a r r a y .
( f i g . 42a).
The b o u n c ele ss sw itch t r i g g e r e d th e enable p u ls e
This enable p u lse was long enough to g a te two subsequent
START p u ls e s ( f i g . 42b) to th e f l i p - f l o p .
The o u tp u t o f th e f l i p - f l o p
was a 10 ms l a s e r t r i g g e r p u lse ( f i g . 42c).
A 1 ms t r i g g e r p u ls e
( f i g . 42d) f o r th e d i g i t a l waveform s to ra g e d e v ic e c o in c id e d w ith th e
f a l l i n g edge o f th e l a s e r t r i g g e r p u ls e .
A c tiv a tin g th e b o u n c ele ss
sw itch caused th e l a s e r to f i r e one p u ls e a t th e b e g in ning o f an a r r a y
scan.
The "Wavesaver" was s im u lta n e o u sly t r i g g e r e d to c a p tu re th e
o u tp u t scan o f th e a r r a y .
The "Wavesaver" a c q u ire d 1K o f d a ta p o in ts
a t a u s e r s e l e c t e d r a t e o f 5 us p e r p o i n t .
166
O
O
CO
eg
m
in
m
in
to
o
Li­
to
en
to eg
to
eg
>
in
m
v£>
>
in
o
Figure 41:
T rig g e rin g C i r c u i t r y f o r S ynchronizing D e te c tio n of Laser
I n t e r f e r e n c e F rin g es by th e Photodiode Array.
a. E nable
b S ta rt
pulses
ji
c. L aser trigger
d/'W avesaver* trigger
Figure 42:
ji
10 m s
A
Timing Diagram f o r T rig g e rin g C i r c u i t r y .
5.2.5
Procedure for Obtaining Fabry-Perot Interfprograms
The p roced ure f o r o b ta in in g a F a b ry -P e ro t in te rfe ro g ra m to measure
l a s e r s p e c t r a l bandwidth i s o u tlin e d a s fo llo w s .
1) S e le c t th e p rop er e ta l o n f o r th e l a s e r wavelength o f i n t e r e s t .
2) D ire c t th e l a s e r beam through i n te r f e r o m e te r o p t i c s so t h a t
photodiode a rra y i s illu m in a te d .
3) Place an i r i s (4 mm) in p a th o f th e l a s e r beam (b e fo re
in te rfe ro m e te r).
This i s to r e j e c t l a s e r background
and in s u re t h a t only monochromatic l a s e r l i g h t
il l u m i n a t e s th e e t a l o n .
4)
With l a s e r ru n n in g on i n t e r n a l t r i g g e r i n g (10-20 Hz),
observe i n t e r f e r e n c e f r i n g e s (on f l u o r e s c e n t t a r g e t )
w hile a d ju s t in g e t a l o n gimbal to o b ta in optimum p a t t e r n .
5)
A djust p o s i t i o n o f 300 mm fo c a l le n g th le n s to c e n te r
th e in t e n s e p o r ti o n o f th e p a t t e r n on th e s e n s i t i v e
r e g io n o f the photodiode a r r a y .
A tte n u ate th e l a s e r
i f n e c e s s a ry , to prevent saturation of the array.
8)
Place l a s e r t r i g g e r i n g s e l e c t o r on EXT mode and connect
t r i g g e r c a b le from t r i g g e r i n g c i r c u i t to th e
"d ata l i n k t r a n s m i t t e r " .
169
7) The "Wavesaver" t r i g g e r o u tp u t o f th e t r i g g e r i n g c i r c u i t
should be connected to th e wavesaver t r i g g e r in p u t (p in
#15 in r e a r ) .
The photodiode a r r a y o u tp u t should be connected
to th e "Wavesaver" s ig n a l in p u t.
The o u tp u t o f th e "Wavesaver"
i s con nected t o e i t h e r an o s c i l l o s c o p e o r an analog r e c o r d e r .
8)
The "Wavesaver" should be s e t to a c q u ir e d a ta a t th e r a t e
o f 2 ys p e r p o i n t .
O ther "Wavesaver" c o n d i t i o n s a re
a u to arm; e x t. t r i g g e r ; and DC i n p u t .
9)
The t r i g g e r i n g sequence i s i n i t i a t e d by p r e s s i n g th e t r i g g e r
b u tto n on th e f r o n t o f th e box t h a t housed th e t r i g g e r
c irc u itry .
When c i r c u i t i s e n ab le d , a re d LED w i l l l i g h t .
The LED w i l l be e x tin g u is h e d in c o in c id e n c e w ith th e f i r i n g
o f th e l a s e r and t r i g g e r i n g o f th e "Wavesaver".
10)
When a s a t i s f a c t o r y looking in te rfe ro g r a m h a s been s to re d
by the "Wavesaver", a hardcopy can th e n be o b ta in e d by
c o n n e c tin g th e outp ut o f the "Wavesaver" to a c h a r t r e c o r d e r .
A d a ta o u tp u t r a t e o f 10 Hz seemed t o work w e ll, b u t t h i s
cou ld be v a r ie d alo ng w ith th e c h a r t speed to o b ta in th e
d e s ir e d r e s u l t .
5.3
RESULTS AND DISCUSSION
In te rf e ro g r a m s were o b ta in e d a c c o rd in g to th e pro ced u re d e sc rib e d
above.
In each c a s e , i t was n e ce ssa ry t o o ptim ize th e p o s i t i o n o f the
o p t i c s to o b ta in th e b e s t r e s u l t .
Each in te rfe ro g r a m was s e l e c t e d
a f t e r s e v e ra l t r i a l a tte m p ts to observe a s a t i s f a c t o r y i n t e r f e r e n c e
p a tte rn .
T h is was because o f v a r i a b i l i t y i n th e s h o t - t o - s h o t
i n t e n s i t y and beam q u a lity of th e excimer pumped, dye l a s e r system.
5-3-1
Spectral Bandwidth of Dye Laser at 377-6 nm
The l a s e r dye BBQ (4 ,4 '- B is b u ty lo c ty lo x y - p - q u a te r p h e n y l) was used to
produce l a s e r r a d i a t i o n a t 377.6 nm which was th e e x c i t a t i o n
wavelength f o r t h a lliu m (377.6 nm/535.0 nm).
The s p e c t r a l bandwidth
o f th e l a s e r o u tp u t a t t h i s wavelength was measured using a Molectron
f i x e d - a i r - g a p e t a l o n (DL 226).
5.3-1-1
Fixed-Air-Gap Etalon
F igu re 43 shows an in te rfe ro g r a m o b ta in e d f o r l a s e r r a d i a t i o n a t
377.6 nm u s in g a f i x e d - a i r - g a p e ta l o n .
The f i x e d - a i r - g a p e t a l o n used
f o r t h i s measurement had a f r e e s p e c t r a l range
of
1.1 cm- ^ or 0.016 nm
a t 377.6 nm and a minimum r e s o lv a b le bandwidth
of
approxim ately
0.0008 nm.
The minimum r e s o lv a b le bandwidth was probably s l i g h t l y
worse than t h i s because t h i s f i g u r e r e p r e s e n t s
an
e s tim a te o f th e
r e s o l u t i o n a v a i l a b l e a t th e c e n t r a l wavelength
of
th e e t a l o n where th e
f i n e s s e i3 h i g h e s t .
Eleven r e p l i c a t e in te rfe ro g r a m s were o b ta in e d and
th e s p e c t r a l bandw idth was c a lc u la te d by using b o th o f th e num erical
methods o u tlin e d above, r e s u l t i n g in a mean v a lu e o f 0.0034 nm.
r e l a t i v e sta n d a rd d e v ia t io n based on 0
The
was c a l c u l a t e d to be 10 .3 ?.
The co n fid e n ce i n t e r v a l based on th e t d i s t r i b u t i o n and a 95$
co n fid en ce l e v e l was between 0.0032 and 0.0036 nm.
172
z ?
z >
:>
Figure 43:
In te rfero g ram fo r Laser R adiation a t 377.6 nm.
173
5-3-2
Spectral Bandwidth of Dye Laser at 566.6 nm
The ouput o f th e dye l a s e r o p e ra tin g w ith one o f th e
Rhodamine dyes i s
commonly used t o p ro v id e fundamental r a d i a t i o n f o r co n v ersio n by
frequency d o u b lin g , to tu nab le u l t r a - v i o l e t r a d i a t i o n .
In th e p r e s e n t
a p p l i c a t i o n , t h i s tu n a b le U.V. r a d i a t i o n i s used t o e x c it e th e atomic
flu o r e s c e n c e t r a n s i t i o n s f o r s e v e ra l e le m e n ts .
I t i s t h e r e f o r e of
i n t e r e s t t o know th e s p e c t r a l bandwidth o f th e fundamental r a d i a t i o n
before c o n v e rsio n .
5.3-2.1
Fixed-Air-Gap Etalon
Fig ure 44 i s th e in te rfe ro g r a m o b ta in e d f o r th e dye l a s e r o u tp u t a t
566.6 nm u s in g a f i x e d - a i r - g a p e t a l o n .
The f r e e s p e c t r a l range of th e
e ta l o n a t t h i s w avelength was 1.1 cm- ^ , or 0.035 nm.
re s o lv a b le bandwidth 61 was e stim a te d t o be 0.002 nm.
The minimum
Sixteen
r e p l i c a t e in te r f e r o g r a m s were ob tain ed and th e s p e c t r a l bandwidth was
c a l c u l a t e d y i e l d i n g a mean v a lu e o f 0.0074 nm w ith a
d e v ia t io n o f 8 .2 $ .
r e l a t i v e s tan d a rd
The co n fid en ce i n t e r v a l based on th e t
d i s t r i b u t i o n and a 95$ co nfidence le v e l was betw een 0.0071 and
0.0077 nm.
Fig ure 44:
In te rfe ro g ra m o f Laser Output a t 566.6 nm Using a FixedAir-Gap E talo n .
5.3.2.2
Solid Etalon
F ig u re 45 i s th e in te rfe ro g r a m o b ta in e d f o r th e dye l a s e r o u tp u t a t
566.6 nm u s in g a s o l i d e t a l o n c o ate d f o r a c e n t r a l wavelength o f
572 nm.
The f r e e s p e c t r a l range o f th e s o l i d e t a l o n a t 566.6 nm was
0.011 nm and th e minimum r e s o lv a b le bandwidth was e s tim a te d to be
0.001 nm.
Seven r e p l i c a t e in te rfe ro g r a m s were o b ta in e d u s in g th e
s o l i d e ta l o n and th e mean l a s e r s p e c t r a l bandwidth was c a lc u la te d to
be 0.0017 w ith a r e l a t i v e stan d a rd d e v ia t io n o f 5$.
i n t e r v a l based on th e
t
The co n fid en ce
d i s t r i b u t i o n and th e 95$ c o n fid e n c e l e v e l was
between 0.0016 and 0.0018 nm.
The l a s e r s p e c t r a l bandwidth o b tain ed
w ith th e s o l i d e t a l o n i s no d i f f e r e n t than th e e stim a te d minimum
r e s o lv a b le bandwidth o f th e e t a l o n .
I t can t h e r e f o r e be assumed t h a t
th e s p e c t r a l bandwidth was a t l e a s t t h a t good, b u t may have been
b e tte r.
F ig ure 45:
I n te rf e ro g r a m o f L aser Output a t 566.6 nm Using a S o lid
E ta lo n .
The obvious d iscrep an cy between th e s p e c t r a l bandwidths o b ta in e d with
th e f i x e d - a i r - g a p and s o l i d e t a l o n was probably due to th e d i f f e r e n c e
in 6A, th e minimum r e s o lv a b le bandwidth, of th e two e t a l o n s .
The
l a s e r s p e c tr a l bandwidths measured were in c lo s e p rox im ity to t h e
e s tim a te d r e s o l u t i o n l i m i t s o f th e e t a l o n s .
However, the s p e c t r a l
bandwidth t h a t was measured u s in g th e f i x e d - a i r - g a p e ta l o n was
c o n sid e ra b ly l a r g e r th an th e minimum re s o lv a b le bandwidth e stim a te d
f o r th e e ta l o n a t a wavelength o f 566.6 nm.
I t was p o s tu la te d t h a t th e 566.6 nm l i n e was f a r enough from the
c e n t r a l wavelength of th e e ta l o n t h a t th e f i n e s s e v alue may have been
a p p re c ia b ly lower r e s u l t i n g in a h ig h e r minimum r e s o lv a b le bandwidth.
The tra n s m is s io n spectrum o f th e e ta l o n was recorded between 500 and
800 nm u s in g a Carey 17 U.V.-VIS sp ec tro p h o to m ete r.
I t was determ ined
upon exam ination o f the spectrum, t h a t th e tra n s m is s io n a t 566.6 nm
was about 85$ of the tra n s m is s io n n ear th e c e n t r a l wavelength o f
645 nm.
I t can be assumed t h a t t h e f i n e s s e and r e s o l u t i o n o f th e
e t a l o n were a ls o l e s s optimum a t 5 66 .6 nm s in c e th e perform ance o f an
e ta l o n was d i r e c t l y r e l a t e d to th e e f f i c i e n c y w ith which the
r e f l e c t i v e c o a tin g s promoted m u ltip l e r e f l e c t i o n s o f a given
w avelength of l i g h t .
The e t a l o n f i n e s s e may have been degraded by a
f a c t o r o f two o r more, by using th e e ta l o n a t 566.6 nm a s opposed to
645 nm which was th e approxim ate c e n t r a l wavelength o f th e r e f l e c t i v e
c o a ti n g s .
The f a c t o r o f two re d u c tio n in f i n e s s e was an e s tim a te
based on the above assumption and th e r e l a t i o n s h i p between
r e f l e c t i v i t y , r e f l e c t i v i t y f i n e s s e , and in stru m e n t f i n e s s e .
A f a c t o r o f two or more lo s s in f i n e s s e r e s u l t e d in a p r o p o r tio n a l
in c r e a s e i n th e minimum re s o lv a b le bandwidth of an e t a l o n .
Consequently, the measurement o f l a s e r s p e c t r a l bandwidth u s in g the
f ix e d - a ir - g a p e ta l o n a t 566.6 nm was pro bably lim it e d by th e f i n e s s e
o f the e ta l o n and th e only c o n clu sio n t h a t can be made about t h i s
measurement i s t h a t the l a s e r s p e c tr a l bandwidth was a t l e a s t as good
a s 0.007 nm b u t may have been b e t t e r .
This was confirmed by u s in g the
s o lid e ta l o n .
The in te rfe ro g ra m ob tained w ith th e s o lid e ta l o n ( f i g u r e 45)
appeared to be l e s s well defin ed than t h a t obtained w ith the
f ix e d - a ir - g a p e ta l o n ( f ig u r e 4 4 ).
This was a r e s u l t o f l e s s than
optimum c o n t r a s t between th e l i g h t and dark bands o f th e i n t e r f e r e n c e
p a tte rn .
The c o n t r a s t observed in an i n t e r f e r e n c e p a t t e r n i s a
fu n c tio n of th e f i n e s s e o f th e e ta l o n t h a t c r e a t e s th e p a t t e r n .
The
s o l i d e ta lo n had a nominal f i n e s s e o f about 9 .5 as opposed to th e
f i n e s s e of th e f ix e d - a i r - g a p e ta l o n which was 20.
Consequently, th e
f i x e d - a i r - g a p e t a l o n produced a n i c e r looking i n t e r f e r e n c e p a t t e r n
which was e a s i e r to view w ith th e naked eye th an t h a t produced by th e
s o lid e ta lo n .
The f i x e d - a i r - g a p e ta l o n s had wedge-shaped m ir ro rs
which helped d isp ose of secondary or "ghost" r e f l e c t i o n s t h a t could
reduce the c o n t r a s t and r e s o l u t i o n o f the i n t e r f e r e n c e p a t t e r n .
The
s o lid e t a l o n s , because o f t h e i r one-piece c o n s tr u c ti o n , cannot have
t h i s f e a tu r e and a r e s u b je c t to s t r a y r e f l e c t i o n s .
The s o l i d e ta l o n
re q u ire d c o n sid e ra b ly more e f f o r t to qptim ize than d id the
f i x e d - a i r - g a p e ta l o n , however, the s o lid e ta l o n produced s i m i l a r
r e s u l t s and was much l e s s expensive than th e f ix e d - a i r - g a p e t a l o n .
5.3.3
Spectral Bandwidth of Frequency Doubled Radiation
The n o n - lin e a r o p t i c a l p ro c e ss o f second harmonic g e n e r a tio n p ro v id e s
a means by which tu n a b le , frequency doubled r a d i a t i o n in th e U.V. can
be r e a d i l y o b ta in e d .
Tunable U.V. l a s e r r a d i a t i o n a llo w s th e
e x c i t a t i o n o f atomic t r a n s i t i o n s f o r s e v e r a l elem ents t h a t a r e in many
c a s e s th e most s e n s i t i v e t r a n s i t i o n s o f th e s e e le m e n ts.
Because th e
u se o f a frequency do u b lin g in s tru m e n t r e q u i r e s c o n s id e ra b le
o p tim iz a tio n , i t i s u s e f u l to be a b le to measure th e s p e c t r a l
bandwidth o f th e o u tp u t r a d i a t i o n as w ell a s th e o u tp u t power.
5.3.3.1
Solid Etalon
A s o l i d e ta l o n was used to o b ta in a measurement o f th e s p e c t r a l
bandwidth of frequency doubled U.V. r a d i a t i o n a t 283.3 nm.
Because o f
th e r e l a t i v e l y low l i g h t i n t e n s i t y e x i t i n g th e frequency d o u b le r, i t
was d i f f i c u l t to observe th e i n t e r f e r e n c e f r i n g e s even on a
flu o resc en t ta rg e t.
I t was t h e r e f o r e not p o s s ib le t o a d j u s t th e
i n te r f e r o m e te r i n the same f a c i l e manner as w ith i n t e n s e , v i s i b l e
la se r lig h t.
T his p re s e n te d a problem in th r e e s t e p s o f th e
measurement p ro c e d u re .
F i r s t , a d i r e c t o p tim iz a tio n o f th e angle of
in c id e n c e o f l a s e r l i g h t on th e e t a l o n , by gimbal a d ju stm e n t, was not
p o ssib le.
manner.
This had to be done i t e r a t i v e l y i n a t r i a l - a n d - e r r o r
Secondly, a c c u r a te p r o j e c t i o n o f th e i n t e r f e r e n c e f r i n g e s onto th e
s e n s i t i v e p o r ti o n o f th e photodiode a rra y was a d i f f i c u l t ta sk even
w ith th e rocm l i g h t s sw itched o f f .
T h ird ly , a sharp focus o f th e
f r i n g e s onto th e a rra y was no t a c h ie v e d , which r e s u l t e d in a l o s s in
c o n t r a s t and r e s o l u t i o n .
An i n t e n s i t y p r o f i l e o f th e F ab ry -P ero t f r i n g e s was o b ta in e d a f t e r
s e v e ra l i t e r a t i v e a d ju stm en ts o f th e o p t i c s .
F u rth e r a tte m p ts to
improve upon t h i s in te r f e r o g r a m were no t s u c c e s s f u l.
F ig u re 46 i s an
in te rfe ro g r a m o b tain ed w ith a s o l i d e ta l o n o f th e frequency doubled
l a s e r o u tp u t.
Figure 46:
In te rf e ro g r a m o f Frequency Doubled Laser Output.
The in te rfe ro g ra m shown in f i g u r e 46 i s n o t a s w ell d e fin e d a s
those ob tained fo r v i s i b l e r a d i a t i o n from th e dye l a s e r .
This i s a
d i r e c t r e s u l t o f th e d i f f i c u l t i e s encountered in o p tim izin g th e
in te r f e r o m e te r o p t i c s .
A measurement o f th e s p e c t r a l bandw idth was
obtained from th e in te r f e r o g r a m shown in f i g u r e 46.
This r e q u ire d th e
g raph ic c o n s tr u c ti o n o f a b a s e l i n e from which peak h e i g h t could be
measured i n an a tte m p t to determ ine th e f u l l width a t h a l f maximum of
th e a p p ro p ria te peak.
The f r e e s p e c t r a l range f o r th e s o l i d e t a l o n a t
283.3 nm was c a l c u la te d to be 0.003 nm and th e minimum r e s o lv a b le
bandwidth was e stim a te d to be 0.0003 nm, which seemed more than
adequate to re s o lv e th e freq u en cy doubled r a d i a t i o n .
The s p e c t r a l
bandwidth was c a l c u l a t e d t o be 0.0012 nm which was s i m i l a r t o t h a t
c a lc u la te d f o r th e fundamental r a d i a t i o n (0.0017 nm a t 566.6 nm) used
to i r r a d i a t e th e KDP (p otassium dihydrogen phosphate) dou bling
c ry sta l.
T y p ic a lly , a l i n e narrow ing phenomenon accompanies th e frequency
doubling p ro cess (171).
Since th e frequency doubling c r y s t a l i s
o p tim a lly phase-matched f o r th e c e n t r a l wavelength o f a given l a s e r
output bandwidth, r a d i a t i o n a t t h i s wavelength i s co n v erted w ith a
g r e a t e r e f f i c i e n c y than w avelengths a t th e wings o f th e l a s e r l i n e .
The r e s u l t i s a d e crea se i n th e f u l l w idth a t h a l f maximum o f t h e U.V.
r a d i a t i o n r e l a t i v e t o th e fundam ental.
The observed r e d u c tio n in
bandwidth t y p i c a l l y ap pro ach es 50% bu t l a s e r l i g h t d i f f r a c t i o n in the
dou blin g c r y s t a l l i m i t s th e degree o f l i n e narrowing t o 20-40% (171).
T h e re fo re , the e f f e c t o f l a s e r l i n e narrowing a s s o c ia te d w ith th e
frequency doubling p ro c e s s , i s n o t very a p p re c ia b le .
However, th e re
h as been a r e c e n t r e p o r t of l i n e narrowing by a KDP c r y s t a l where th e
s p e c t r a l bandwidth a t 295 nm (0.21 nm) was observed to be seven times
as narrow a s the fundamental a t 590 nm (1.4 nm) (172).
In th e p r e s e n t
work, i t could n o t be concluded t h a t l i n e narrow ing o ccu rred d u rin g
th e frequency doubling p r o c e s s .
This was due to th e f a c t t h a t th e
sm all d i f f e r e n c e in th e s p e c t r a l bandwidths o f th e fundamental and
frequency doubled r a d i a t i o n , a s measured w ith th e i n t e r f e r o m e t e r
d e sc rib e d h e re , may have been l e s s than th e e r r o r a s s o c ia te d w ith
th e s e measurements.
5.3.4
Spectral Bandwidth of Hellui-Weon CW Laser
Although th e in t e r f e r o m e t e r system d e sc rib e d here was optim ized fo r
use w ith p u lsed l a s e r s , i t can c o n v en ie n tly be used to measure th e
s p e c tr a l bandwith of a c o n tin u o u s wave (CW) l a s e r .
Low power
helium-neon l a s e r s a re a v a i l a b l e in most l a b o r a t o r i e s f o r u se in th e
o p t i c a l alignm ent of l a r g e r l a s e r s or spectroscopy in s tru m e n ta tio n .
Helium-neon l a s e r s produce a c h a r a c t e r i s t i c re d l i g h t o u tp u t a t
632.8 nm, a lth o ug h o th e r t r a n s i t i o n s a re c u r r e n tly a v a il a b le .
The
atomic n a tu re o f th e gaseous l a s e r medium le n d s i t s e l f to narrow
s p e c t r a l outpu t w itho ut th e use o f s o p h i s t i c a t e d i n t r a - c a v i t y l i n e
narrow ing o p t i c s .
The He-Ne l a s e r used fo r th e p r e s e n t work,
a ccord ing to th e m anufacturer ( M e tro lo g ic ), has an a x ia l mode
bandwidth o f 1.5 GHz, or 0.002 nm a t 632.8 nm (173).
5.3-4.1
Fixed-Air-Gap Etalon
A f ix e d - a ir - g a p e t a l o n was used to measure th e s p e c t r a l bandwidth
o f th e He-Ne l a s e r and t h e in te rfe ro g ra m obtained i s shown in f i g u r e
47.
The ap p aren t s p e c t r a l bandw idth was c a lc u la te d to be 0.005 nm.
The a c tu a l s p e c t r a l bandwidth was probably narrower than t h i s , b u t th e
measurement was l im it e d because o f i t s proxim ity to th e mininum
r e s o lv a b le bandwidth o f th e e t a l o n .
This in te rfe ro g ra m was
p a r t i c u l a r l y easy to o b ta in due t o th e CW n a tu re o f the l i g h t so u rc e.
This made o p tim iz a tio n of th e e t a l o n and a s s o c ia te d o p tic s very
sim ple.
Figu re 47:
In te rfe ro g ra m o f Helium-Neon Laser Output Obtained With a
Fixed-Air-Gap E ta lo n .
5.3.4.2
Solid Etalon
F ig u re 48 i s an in te rfe ro g r a m o b ta in e d fo r th e He-Ne l a s e r a t 632.8 nm
u s in g a s o lid e ta l o n c o ate d fo r a c e n t r a l wavelength o f 572 nm.
The
f r e e s p e c t r a l range and th e minimum r e s o lv a b le bandwidth o f th e s o l i d
e t a l o n a t 632.8 nm, were c a lc u la te d to be 0.014 nm and 0.001 nm
resp ec tiv e ly .
The o u tp u t wavelength o f th e He-Ne l a s e r i s f a r enough
removed from the c e n t r a l wavelength o f th e e t a l o n (572 nm) t h a t th e
f i n e s s e and r e s o l u t i o n may have been degraded r e l a t i v e to perform ance
a t 572 nm.
The a p p aren t s p e c t r a l bandwidth o f th e He-Ne l a s e r was
c a l c u l a t e d , using measurements made w ith a s o lid e t a l o n , to be
0.002 nm which i s th e value o f th e a x i a l mode bandwidth quoted by t h e
l a s e r m an ufacturer (173).
F ig ure 48:
In te rfe ro g ra m f o r He-Ne Laser Output Obtained W ith S olid
E talo n .
5.3.5
De-convolutlon of Laser Spectral Bandwidth Measurements
Of th e th r e e measurements of s p e c t r a l bandwidth f o r th e excimer-pumped
dye l a s e r system l i s t e d in Table 18, the measurements a t 283.3 nm and
377.6 nm appeared to be th e only ones t h a t were not li m i t e d by th e
r e s o l u t i o n o f th e e t a l o n , i . e . , th e measurements were c o n sid e ra b ly
l a r g e r (by a f a c t o r o f 4) th an th e minimum r e s o lv a b le bandwidth o f th e
e t a l o n s used and t h e r e f o r e r e p r e s e n te d th e l a s e r p r o f i l e s and n o t th e
in s tru m e n t p r o f i l e s .
These d i f f e r e n c e s were not s u f f i c i e n t l y la r g e to
allow th e bandwidths o f th e e t a l o n s t o be d is re g a r d e d .
T h e re fo re ,
th e s e measurements should be c o n sid e re d fo r d e -c o n v o lu tio n .
The
measurement a t 56 6.6 nm however, was l i m i t e d t o a measurement o f th e
e t a l o n bandwidth because th e f i n e s s e was no t adequate to re s o lv e a
narrow er p r o f i l e and th e r e s u l t could n o t be d e -c o n v o lu ted .
The minimum r e s o lv a b le bandwidth 6Ae ^ , of th e s o l i d e ta l o n used
f o r th e frequency doubled r a d i a t i o n a t 283.3 nm was e s tim a te d to be
.0003 nm.
The e x p e rim e n ta lly measured l a s e r s p e c t r a l bandw idth AAeXp ,
a t t h i s wavelength was .0012 nm.
“a c t ‘ l n 2 ( s 4 p
According to th e follow in g form ula:
- “ eta>
The a c t u a l s p e c t r a l bandwidth, A*a c t , was .00097 nm.
T his f i g u r e i s
not s i g n i f i c a n t l y d i f f e r e n t from th e e x perim ental h a l f - w i d t h , which
s u g g e s ts t h a t th e bandwidth o f th e e t a l o n could have been ig n o re d .
The f i x e d - a i r - g a p e t a l o n used t o measure th e l a s e r s p e c t r a l
bandwidth a t 377.6 nm had an e s tim a te d minimum r e s o lv a b le bandwidth of
0.0008 nm.
Although t h i s measurement was made a t a w avelength
s l i g h t l y removed from th e c e n t r a l wavelength o f th e e t a l o n , i t was
determ ined t h a t th e f i n e s s e , hence th e r e s o l u t i o n , was comparable a t
b o th wavelengths.
A tr a n s m is s io n spectrum o f t h i s e t a l o n re v e a le d a
l e s s than 10$ l o s s in tra n s m is s io n a t 377.6 nm r e l a t i v e to th e c e n t r a l
wavelength (405 nm), th u s th e s i m i l a r performance a t both w avelengths.
The e x p e rim e n ta lly measured l a s e r bandwidth a t 377.6 nm was 0.0034 nm.
The l a s e r s p e c t r a l bandwidth, c o r r e c t e d fo r e ta l o n b ro a d e n in g , was
0.0028 nm.
E r r o r s a s s o c ia te d w ith measurement of l a s e r s p e c t r a l bandw idth and
th e bandwidths th em selv es, a re sm all r e l a t i v e to th e h a lf - w id th s o f
th e atomic p r o f i l e s encountered in th e p r e s e n t fu rn a c e LEAFS work,
t y p i c a l l y around 0.01 nm.
T h e re fo re , th e l a s e r s p e c t r a l bandwidth can
be n e g le c te d in LEAFS s p e c t r a l - s c a n d e te rm in a tio n s o f atom p r o f i l e s .
5.3.6
Summary of Results
The u t i l i t y o f th e i n te r f e r o m e te r r e p o r te d h e re was dem onstrated and
i t s performance was e v a lu a te d .
I n t e r f e r e grams and measurements of th e
s p e c t r a l bandw idths f o r a number o f d i f f e r e n t l a s e r o u tp u t w avelengths
fo r both pulsed and CW l a s e r s were o b tain ed .
Table 18 l i s t s th e
r e s u l t s o f th o se measurements as w ell as th e e t a l o n c o n d itio n s
a s s o c ia te d w ith th e measurement o f l a s e r s p e c t r a l bandwidth f o r the
LEAFS e x c i t a t i o n l i n e s o f s e l e c t e d elem ents.
Table 18 can be used a s
a r e f e r e n c e f o r f u t u r e measurements o f l a s e r s p e c tr a l bandwidth.
It
can be s a f e ly assumed t h a t l a s e r l i n e s t h a t a r e in c l o s e proxim ity to
th o s e a lre a d y measured w i l l e x h i b i t s i m i l a r s p e c tr a l bandwidths.
For
example, frequency doubled l a s e r r a d i a t i o n a t 279.5 nm r e q u i r e s the
same l a s e r dye (R575) and doubling c r y s t a l (KDP-B) as t h a t a t
283*3 nm.
I t can t h e r e f o r e be a n t i c i p a t e d t h a t th e s p e c t r a l bandwidth
a t 279.5 nm be very s i m i l a r to t h a t measured a t 283*3 nm.
For tho se
l i n e s n o t y e t measured, th e miminum s p e c t r a l bandwidth o f th e e ta l o n
6A d e fin e s th e a b s o lu te lower l i m i t a t which th e s e p r o f i l e s can be
m easured.
Because th e s e f i g u r e s were e stim a te d based on th e optimum
v a lu e s of f i n e s s e , a t th e c e n t r a l w avelength o f th e e t a l o n , the
r e s o l u t i o n a t th e wavelength of i n t e r e s t may be degraded r e l a t i v e to
th e c e n t r a l w avelength, r e s u l t i n g in a h ig h e r minimum r e s o lv a b le
bandwidth.
191
TABLE 18
Summary o f L aser S p e c tra l Bandwidth Measurements
Element
Aexc.
E talon
Ag
328.1 nm
d
Ba+
614.0 nm
226 C
.002
nm
Co
304.4 nm
S olid A
.004
nm
Cu
324.7 nm
Fe
296.7 nm
S o lid A
.0003 nm
In
410.4 nm
S o lid B
.0006 nm
Mn
279.5 nm
S o lid A
.0003 nm
Pb
283.3 nm
S o lid A
.0003 nm
S o lid C
.0012 nm
226 C
.0008 nm
d
(fundamental 566.6 nm)
T1
377.6 nm
T1
Excimer-Pumped
Dye Laser
Bandwidth AA
E talon
Bandwidth 61
276.8 nm
S o lid A
.0012 nmb (.00097 nm)c
.0017 nmb
.0034 nmb (.0028 nm)c
.0003 nm
e stim a te d frcm f r e e s p e c t r a l range and f i n e s s e
v a lu e s fo r c e n t r a l wavelength.
measured v a lu es c a l c u l a t e d from in te rfe ro g r a m s .
v alu es a f t e r c o r r e c t i o n f o r e ta l o n broadening.
e ta l o n not c u r r e n tly a v a il a b le in t h i s la b o r a to r y .
S o lid
E talon
S o lid A
S o lid B
S o lid C
c en tral
w avelength
286 nm
407 nm
572 nm
Fixed-Air-Gap
E talon
226
226
226
226
A
B
C
D
wavelength
range
360-450
440-570
560-730
720-950
nm
nm
nm
nm
5.4
PRACTICAL CONSIDERATIONS
The in stru m e n t d e sc rib e d h e re has been shown to allow ra p id and f a c i l e
measurement o f the s p e c t r a l bandwidth of both a CW and pulsed l a s e r .
The accuracy o f the measurements depended on a number o f f a c t o r s
in c lu d in g o p tim iz a tio n of e ta l o n and o p t i c s a lign m en t, p ro xim ity to
th e r e s o l u t i o n l i m i t of the e ta l o n , and th e imaging o f th e
i n t e r f e r e n c e p a t t e r n on th e photodiode a r r a y .
I t i s probably
b e n e f i c i a l t o have a rough e stim a te o f th e s p e c t r a l bandwidth o f th e
source to be measured in o rd e r to p ro p e rly s e l e c t an e t a l o n of
s u ffic ie n t re so lu tio n .
O therw ise, measurements a re l im it e d to th e
"in stru m e n t bandwidth" imposed by th e l i m i t a t i o n s of th e
i n te r f e r o m e te r .
In view o f th e l im it e d wavelength coverage o f a g iven
e ta l o n , i t would be advantageous to have a s e r i e s of e ta l o n s on hand,
w ith o v e rlap pin g wavelength ra n g e s, to in s u re optimum performance a t
a l l wavelengths o f i n t e r e s t .
I t i s advantageous to magnify th e i n t e r f e r e n c e p a t t e r n , thereby
illu m in a tin g a g r e a t e r number of a rra y elem ents with o p t i c a l
in fo rm a tio n .
For th e work re p o rte d h e r e , t h i s r e s u l t e d in h ig h e r
d e te c tio n r e s o l u t i o n which allow ed th e e l u c i d a t i o n o f f i n e r d e t a i l s in
th e i n t e r f e r e n c e p a t t e r n .
This a ls o promoted b e t t e r b a s e l i n e
r e s o l u t i o n o f th e i n t e n s i t y peaks, which f a c i l i t a t e d measurement.
A predominant source o f e r r o r in th e measurement o f s p e c t r a l
bandwidth u s in g t h i s in s tru m e n t, was s a t u r a t i o n o f th e photodiode
array.
I f th e i n t e n s i t y peaks of th e in te rfe ro g ra m were n o t w ithin
th e l i n e a r range o f th e a r r a y , then measurements o f f u l l w id th a t h a l f
maximum were e rro n e o u s and in a c t u a l i t y , larger than th e t r u e
h a lf - w id th o f th e peak.
This s i t u a t i o n was b e s t avoided by c a r e f u l l y
a t t e n u a t i n g th e l a s e r l i g h t u n t i l s a t u r a t i o n was no lo n g e r e v id e n t .
S a t u r a t i o n o f th e a rra y was i n d ic a te d by th e a p p a r e n t t r u n c a t i o n o f
i n t e n s i t y peaks on th e in te r f e r o g r a m .
U n fo rtu n a te ly , a t t e n u a t i o n of
th e l a s e r l i g h t r e s u l t e d in a l o s s o f a l l b u t th e most i n te n s e ( in n e r )
in te rfe re n c e frin g e s.
I f s a t u r a t i o n o f th e a r r a y could n o t be
avo id ed , measurements could th e n be made on th e l e s s i n t e n s e lower
o rd e r f r i n g e s , f u r t h e r from th e in t e n s e c e n te r o f th e in te r f e r o g r a m .
This may i n i t s e l f have been a so urce o f e r r o r s in c e th e ap pro x im a tio n
on which th e s e measurements a r e based i s most v a l i d f o r th e in n e r
frin g e s.
5.4.1
High Performance Fabry-Perot Interferometry
When a simple i n t e r f e r o m e t e r such as th e one d e s c rib e d above, i s not
s u f f i c i e n t to a c c u r a te ly measure l a s e r s p e c t r a l bandwidth, due t o
r e s o l u t i o n l i m i t a t i o n s imposed by t h e t h e i n t e r f e r o m e t e r 's
s p e c i f i c a t i o n s , a more s o p h i s t i c a t e d d e v ice may be n e c e s s a ry .
E ta lo n s
a r e a v a i l a b l e w ith f l a t n e s s up to A/200 and c o a te d f o r 99$
re fle c tiv ity .
An e t a l o n w ith such s p e c i f i c a t i o n s would have an
in s tru m e n t f i n e s s e c l o s e t o 100.
T h e o r e t i c a l l y , a minimum r e s o lv a b le
bandwidth o f 0.0001 nm would be p o s s i b l e .
This i s more th a n a f a c t o r
o f te n lower than t h a t a chieved w ith th e s o l i d e t a l o n s i n v e s t i g a t e d
f o r th e p r e s e n t work.
There a re however, a number o f c o m p lic a tio n s
involved in working with a h ig h perform ance e t a l o n .
A ll e t a l o n s a re
s u b je c t to b oth therm al and m echanical i n s t a b i l i t y .
The o p t i c a l
e f f e c t s o f t h i s i n s t a b i l i t y may n o t b e e v id e n t f o r e t a l o n s o f lower
r e s o l u t i o n , b u t th e performance o f h ig h r e s o l u t i o n e t a l o n s can be
s e v e re ly degraded i f s t e p s a r e n o t tak en t o minimize th e s e e f f e c t s by
a d d re ss in g t h e i r c a u se s.
5.4.1.1
Mechanical Instability
The two sou rces o f m echanical i n s t a b i l i t y a r e v i b r a t i o n and mechanical
creep (174).
In th e c l a s s i c a l E ab ry -P ero t i n t e r f e r o m e t e r , th e
F a b ry -P e ro t p l a t e s o r m ir r o r s , must not move any more th a n ab o u t 10 A
( f o r A/200 p l a t e s ) in re sp o n se t o normal v i b r a t i o n s (174) and must
r e t u r n to t h e i r o r i g i n a l p o s i t i o n s w ith in ab o ut 10 A.
f i n e s s e w i l l change enough t o degrade perform ance.
O th erw ise, the
M echanical d e v ice s
and th e m a t e r i a l s from which th e y a r e made tend to "cre e p " a f t e r b e in g
p e rtu rb e d (174).
U su a lly , a tim e i n t e r v a l up t o s e v e r a l h o u rs may be
n e ce ssa ry f o r e q u i l i b r a t i o n .
Modern h ig h perform ance e t a l o n s employ
p i e z o e l e c t r i c (PZT) p o s i t i o n i n g d e v ic e s as an a c t i v e a l t e r n a t i v e to
th e more p a s s iv e m echanical e q u i l i b r a t i o n p ro c e ss (17 4 ).
H i g h - l i n e a r i t y , low therm al expansion PZT m a t e r i a l s a llo w p r e c i s e and
remote (h a n d s -o ff) p o s i t i o n i n g o f e t a l o n p l a t e s .
A system f o r
m onitoring and a p p r o p r i a t e feedback could m aintain a n e a r ly p e rp e tu a l
alignm ent o f the p l a t e s t o c o u n te r th e e f f e c t s o f m echanical
in sta b ility .
5.4.1.2
Thermal Instability
The m a t e r i a l s from which an o p t i c a l d e v ice such a s a F ab ry -P ero t
i n t e r f e r o m e t e r i s c o n s tr u c te d , a re s u b je c t to therm al s t r e s s e s .
U n fo rtu n a te ly , each m a t e r i a l has unique therm o-m echanical p r o p e r t i e s
such as therm al i n e r t i a and c o e f f i c i e n t o f th erm al expansion.
When
s u b j e c t to a th erm al p e r t u r b a t i o n such as a te m p e ra tu re change, each
m a te ria l w i l l e q u i l i b r a t e a t a d i f f e r e n t r a t e u n t i l a g e n e ra l
e q u ilib riu m i s a t t a i n e d .
The r e s u l t i s a d e - tu n in g o f th e
in t e r f e r o m e t e r and subsequent d e g ra d a tio n in perform ance.
One
s o l u t i o n to t h i s problem i s to c o n ta in th e i n t e r f e r o m e t e r elem ent in a
th e rm o s ta tte d environm ent.
The p r e f e r a b l e s o l u t i o n however, i s to
c o n s t r u c t th e in s tru m e n t w ith m a t e r i a l s o f b o th h ig h therm al i n e r t i a
and low c o e f f i c i e n t o f therm al expansion.
Examples o f m a te r ia ls such
a s th e se a r e In v a r a l l o y and low expansion c e ra m ic s.
Modern r e s e a r c h
grade F a b ry -P e ro t in te r f e r o m e te r s employ t h e s e advanced m a te r ia ls as
w ell a s PZT m ir r o r p o s i t i o n i n g to allo w h ig h -p erfo rm an ce o p e ra tio n
c h a r a c te r iz e d by v ery h ig h r e s o l u t i o n and in s tru m e n ta l s t a b i l i t y .
Appendix A
GATED INTEGRATOR
A.1
PURPOSE
A d i g i t a l l y g a te d i n t e g r a t o r was c o n s tr u c te d f o r th e purpose o f
sampling D.C. a n a l y t i c a l s i g n a l s such as th o s e o u tp u t by a boxcar
a v erag e r o r a l o c k - in a m p l i f i e r .
The re q u ire m e n ts o f th e i n t e g r a t o r
were t h a t i t must have a 1 second and a 10 second time c o n s ta n t and
g a te w id th , and be a b le to handle in p u t s i g n a l s over th e range o f ± 10
v o lts.
A.2
DESIGN AND CONSTRUCTION
The g a ted i n t e g r a t o r was c o n s tr u c te d using commercial c i r c u i t boards
f o r th e i n t e g r a t o r c i r c u i t r y and th e d i g i t a l tim e delay
th e i n t e g r a t o r c i r c u i t .
used to
These bo ard s were o b ta in e d from Evans
E l e c t r o n i c s , B erk eley , CA and a re as fo llo w s:
4130
I n t e g r a t o r module
4145-2
D i g i t a l delay
4146
D i g i t a l d e la y e x te n d e r
- 1 96 -
g a te
F ig u re 49 i s a block diagram and p a r t i a l schematic o f th e g ated
in te g ra to r c irc u itry .
The 4130 g a te d i n t e g r a t o r module has a high
impedance (10 Mfl) v o lta g e fo llo w e r in p u t which can handle v o lta g e
swings up to ± 3 v o l t s .
T his re q u ir e d t h a t
a v o lta g e d i v i d e r ( f i g u r e
49) be used on th e in p u t to reduce th e inp ut s ig n a l s to w ith in th e
a c c e p ta b le range o f th e i n t e g r a t o r .
The v o lta g e d i v i d e r c i r c u i t
a llo w s t h e in p u t s ig n a l to be reduced by u s e r s e le c te d amounts from
100$ to 22$ of th e t o t a l s i g n a l .
G ating o f th e i n t e g r a t o r i s
accomplished by an in p u t p u lse g e n e ra te d by
th e d i g i t a l delay b o a rd s.
The g a te p u ls e i s a ls o u s e r s e l e c t e d and i s e i t h e r 1 or 10 seconds in
d u r a tio n .
The i n t e g r a t o r was c o n fig u re d w ith two u s e r s e l e c t e d time
c o n stan ts.
This was accomplished by allo w in g th e r e s i s t a n c e in th e
i n t e g r a t o r RC c i r c u i t to be switched between th e two v a lu e s ,
approxim ately 450 Kft and 4.5 MO.
These r e s i s t a n c e s were o b ta in e d by
u sin g a 380 k r e s i s t o r and a 100 k p o t in s e r i e s , and a 3 .8 M r e s i s t o r
and a 2 M p o t in s e r i e s , r e s p e c t i v e l y .
RC time c o n s ta n ts to be trimmed.
2.2 yF low leakage ty p e .
The v a r ia b le p o ts allowed th e
The i n t e g r a t i n g c a p a c ito r used was a
These f e a t u r e s a re i l l u s t r a t e d in f i g u r e 50.
The o u tp u t p u lse from th e d i g i t a l delay boards t h a t was used to
g a te th e i n t e g r a t o r was s e l e c t e d by BCD programming th e d elay c i r c u i t
fo r th e d e s ir e d d elay .
The c i r c u i t bo ard s were wired in to th e c i r c u i t
by u s in g ed ge-p in c o n n e c to rs .
A l i q u i d c r y s t a l d i s p la y d i g i t a l p anel
m eter (D atel I n t e r s i l , M ansfield, MA) was used to prov ide a re a d o u t o f
th e i n t e g r a t o r o u tp u t.
A 1.6 v o l t c a l i b r a t i o n v o lta g e , su p p lie d by
v a r i a b l e r e s i s t o r (R72) on th e 4130 i n t e g r a t o r b o ard , could be used
t e s t th e accuracy and r e p r o d u c i b i l i t y o f th e g ated i n t e g r a t o r .
199
u j P=
*-
ou
111
>-<=>
-2 ; 2Ui2I -
o 0_i (/>
'r Q X
INPUT
«o
<=>
2 0.
02
F ig u re 49:
Block Diagram and P a r t i a l Schem atic o f Gated I n te g r a t o r .
200
or
o
»—
<
ct
o
UJ
oc
in
LU >
IS >
< ° •”
£ -! =>
-J ^ 0 .
Op
> U. z
±
Figure
50: S im p lifie d I l l u s t r a t i o n o f M o d ific a tio n s t o I n t e g r a t o r .
A 1. o r 10 second tim e c o n s ta n t can be s e l e c t e d by
changing th e v a lu e o f th e in p u t r e s i s t o r .
A.3
OPERATION OF THE GATED INTEGRATOR
An in p u t s i g n a l can be fe d i n t o th e i n t e g r a t o r by co n n ec tio n t o a BNC
ja c k on th e f r o n t p a n e l .
The in p u t s i g n a l can be a t t e n u a t e d , as
needed, by s e l e c t i n g th e d e s ir e d " ^ s ig n a l " v a lu e u sin g th e r o ta r y
sw itch on th e f r o n t panel.
Although t h e maximum in p u t v o lta g e fo r th e
i n t e g r a t o r board i s ±3 v o l t s , th e LCD r e a d o u t has a maximum re a d o u t
v o lta g e o f ±1 .999 v o l t s .
H igher v o lta g e e x c u r s io n s th an t h i s r e s u l t
i n a b la n k in g o f th e d i s p l a y .
Before a s ig n a l i n t e g r a t i o n , th e
i n t e g r a t o r must be r e s e t by p r e s s i n g th e r e s e t b u t t o n , a SPST sw itch
la b e l e d as such, on th e f r o n t p a n e l.
To i n i t i a t e th e g a tin g o f th e
i n t e g r a t o r , th e g a te w id th (1 second o r 10 seconds) i s s e l e c t e d by a
DPDT sw itch on th e f r o n t p a n el.
This sw itch a l s o s e l e c t s th e
a p p r o p r ia te r e s i s t a n c e f o r th e i n t e g r a t o r RC c i r c u i t .
i s t r i g g e r e d upon p r e s s i n g th e ' T r i g g e r ' b u t t o n .
The i n t e g r a t i o n
A red LED i s
e x tin g u is h e d f o r th e d u r a tio n o f th e i n t e g r a t i o n g a te w id th and i s
r e - l i g h t e d a t th e end o f t h i s i n t e r v a l .
The i n t e g r a t e d v o lta g e then
a p p e a rs on th e LCD re a d o u t.
The power supply re q u ire m e n ts f o r th e g a te d i n t e g r a t o r a r e a s f o llo w s :
VOLTAGE
CURRENT
+5 v o l t s
+15 v o l t s
-15 v o l t s
1 .2 A
250 mA
125 mA
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