EXCIMER LASER EXCITED ATOMIC FLUORESCENCE
DETECTION OF ARSENIC, AND THE DESIGN AND CHARACTERIZATION
OF A FLOWING ELECTROLYTIC HYDRIDE GENERATOR FOR ARSENIC
AND SELENIUM ANALYSIS
DENNIS MICHAEL HUEBER
A DISSERTATION PRESENTED TO THE GRADUATE SCHOOL
OF THE UNIVERSITY OF FLORIDA IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
DOCTOR OF PHILOSOPHY
UNIVERSITY OF FLORIDA
I would like to thank my parents.
As if giving me life were not enough,
also gave me their love,
guidance, support and patience.
Thanks also go to my sisters,
for everything they have done for me, and everything they didn't do to me.
get me hooked on che.
the list of acknowledgements must start with Dr. Leo Bares who
mistry. I would like to acknowledge my research advisor, Dr.
)r. Winefordner has always found a way to give me the guidance
I needed while allowing me to make the mistakes it takes to really learn.
I will be
eternally grateful for the chance he has given me to pursue my own ideas and for his
patience and humor.
My thanks go to Ben Smith for all his assistance with my research.
always available to point me in the right direction.
I would also like to thank the whole
I could not have completed this work if I did not enjoy coming to work
each morning with this great group of people.
I can't mention all their names here, but
I can't go without listing some of them.
My special thanks go to:
Tye Barber, Mike
If I never work with a better group of people,
I'll consider myself lucky.
TABLE OF CONTENTS
KEY TO ACRONYMS USED
* 3 3 3 3 3
3 . 3 3 3 9 3 .
General Introduction .
The Scope of This Dissertati
Suggested General Texts on
Notes Concerning Atomic
Solution Nebulization for
in Atomic Spectrometry
Chemistry of Hydri
Methods Used to G
Method of Detectio.
Light Sources for Atomic i
Incoherent Line Sot
Dye Laser Sources
The Nature of ArF Laser I
FAAS, ICP-AES AND AFS
Atomizers, Emission Sources
The FAA Spectrometei
The ICP-AE Spectrom
The ICP-LEAF Spectre
Electronics Systems .
on .............. 2
Atomic Spectroscopy .. .. 4
oectrometric Terminologv .. 5
3 6 S S 3 5 3 5 5 5 S 3 3 3 3 3 3 5
* 5 5 . S 3 3 3 . S 9
* 3 S 3 3 3 . .9
de Generation 10
generate Hydrides . . 12
n for Hydride Generation ...... 14
...... ................ 14
e Generation 17
Fluorescence Spectrometry ... 20
Jrces * * 20
. . 22
radiation . . . 24
and Optics . . . 28
r .................. 28
. . ........ 39
Electronics for ICP-AES
Electronics for LEAFS
Performance and Comparison to
FAA Spectrometers .
ICP-AE Spectrometry .
.* . . .. .. . 43
* . . . . 43
* C . . . 57
. .. .. . ..... 57
. ............... 59
DESIGN OF THE CONTINUOUS
ELECTROLYTIC HYDRIDE GENERATOR
Introduction . . . . .
A Detailed Walk Through and Around thi
Voltage Regulation . . . .
The Cell Voltage and Overpotential .
Contributions to the Cell Potential
Resistance Terms .
Overpotential . . .
. . U.61
....... ..... 61
e Generator . .62
. . . . 74
. . 745
CHOICE OF ELECTRODE MATERIALS
Introduction . . .
Cathodes . .
Reagents and Stock Solu
Preparation of the FAA
Spectrometer . . 85
Flow Rate . . .
Cathode and Electrolyte Tests
. S S .
* . 88
ANALYTICAL PERFORMANCE OF CEHG
WITH FAAS AND ICP-AES DETECTION
Introduction . . . . .
Experimental. . . . .
Reagents and Stock Solutions .
General Procedure . . .
Procedure used for the FAAS
Procedure Used for the ICP-AES
Calibration Curves .
Sample Preparation and Procedure
for the Analysis of SRMs .
Procedure for the Estimation of
* . 101
* *. S S S 105
Results and Discussion
Analytical Figures of Merit
Concomitant Effects .
Results of the Analysis of the SRMs
Estimation of Efficiency .
. . . C110
* a a a 110
* a a a .. 115
. a a a a a a 115
* a . 126
. a a S S 126
Effect of Flow Rates with ICP-AES Detection
A Simple Mathematical Model
Conclusions .. ... .. .
ARGON FLUORIDE LASER EXCITED ATOMIC
.. .... ...... 137
.. .. . . . 137
Measurement of the ArF Laser Spectral Profile
Experimental . .
Results and Discussion
U SS S 1410
* . C S . 140
a a a a a a a a 141
Comparison of two Atomizers for Arsenic LEAFS
* .*S S U S U a a a a a a .. . a a a a 144
a a 144
Results and Discussion
a a S 55 S S S C SC 5 14~7
. S a a a a a a a a S a a a a 5 170
Sample Introduction Using the
Electrolytic Hydride Generator
Sa a a a a a a a a 170
* a a a a a a a a a a a a a a a 170
* a a a a a a a a a A 170
Results and Discussion
* a a a a a a a a a .1'72
S. a a a 182
CONCLUSIONS AND POSSIBLE DIRECTIONS
FOR FUTURE RESEARCH
A Summation of the LODs
Observations on ArF Fluorescer
Observations on CEHG
Two Suggested Future Noveltie
KEY TO ACRONYMS USED
The following is a list of acronyms used in this dissertation.
lower case "s" is added to an acronym in the text to pluralize the acronym (i.
is used for hollow cathode lamps).
In contrast, a capital "S" added to the end of an
for atomic absorption,
for atomic absorption
In addition two or more of these acronyms may be combined with a
hyphen to create a new acronym.
For example, ICP-AES, ICP-LEAFS or HCL-AF.
Acronyms ending in
for spectrometry are included in this list.
acronyms are not.
AAS: Atomic absorption spectrometry.
AE: Atomic emission.
AES: Atomic emission spectrometry.
AF: Atomic fluorescence.
AFS: Atomic fluorescence spectrometry.
CEHG: Continuous electrolytic hydride generations.
DCP: Direct coupled plasma atomic.
DIN: Direct injection nebulizer.
Electrodeless discharge lamp.
Flame atomic absorption.
S: Flame atomic absorption spectrometry.
i: Flame atomic emission spectrometry.
Flow injection analysis.
M: Full width at half maximum.
Hollow cathode lamp.
: High performance liquid chromatography.
Inductively coupled plasma.
: Laser excited atomic fluorescence.
S: Laser excited atomic fluorescence spectrometry.
Laser enhanced ionization.
Linear dynamic range.
Laser induced fluorescence.
Limit of detection.
Microwave induced plasma.
Parts per million.
Parts per billion.
OES: Optical emission spectrometry.
Relative standard deviation.
Abstract of Dissertation Presented to the Graduate School of the
University of Florida in Partial Fulfillment of the Requirements
for the Degree of Doctor of Philosophy
EXCIMER LASER EXCITED ATOMIC FLUORESCENCE
ARSENIC, AND THE DESIGN AND CHARACTERIZATION
A FLOWING ELECTROLYTIC HYDRIDE GENERATOR FOR ARSENIC
AND SELENIUM ANALYSIS
Dennis M. Hueber
Currently there is great interest in the detection of arsenic and selenium at trace
methods have been assessed in
The first is a novel method of hydride
generation for sample introduction into an atomic spectrometer, and the second is a
method for the detection of arsenic.
Hydride generation is a popular alternative to solution nebulization in analytical
atomic spectrometry for elements that easily form volatile hydrides (i.e., As, Se, Sb,
Presently, the common methods involve the reduction of these
elements by sodium tetraborohydride.
A continuous electrolytic hydride generator was
4~~r C C
1 1 1
solutions containing trace aqueous concentrations of arsenic,
selenium or antimony.
This new hydride generator can be used for discrete or continuous sample introduction.
This method has the advantage of requiring no chemical reducing agent, and thus the
potential for sample contamination is reduced.
Laser excited atomic fluorescence spectrometry (LEAFS) can be an extremely
absorption lines below 200 nm.
Dye laser systems capable of the resonance excitation
of arsenic are complicated and excessively expensive.
By coincidence, a simple fixed
frequency argon fluoride (ArF) excimer laser produces a broad band output centered at
193.0-193.2 m that overlaps with the arsenic absorption line at 193.7 nm.
The use of
an ArF excimer laser as a source for the LEAFS detection of arsenic in a flames and
evaluated when coupled with sample introduction using the flowing electrolytic hydride
Arsenic and selenium have been known poisons since ancient times.
arsenic has long been a poison of choice, both in fiction and reality.
So for obvious
reasons, long before trace methods of analysis were developed for most elements they
were available for arsenic.
The classic Marsh test dates back to 1836, and the Gutzeit
test dates to
While selenium is poisonous in large amounts, it is a vital nutrient
in small concentrations.
Arsenic and selenium have been
great interest to both
In fact, this research was funded in part by
the United States Environmental Protection Agency (EPA
- CR 8188 48-01-0).
EPA has set maximum contaminant levels for arsenic and selenium in drinking water.
The maximum contaminant level for As is 50 pg/mL, and it is 10 pg/mL for Se.
These elements have ground state absorption lines only below
compared to lower energy transitions.
Hollow cathode lamps (HCL) are also weak for
these elements, and few laser systems can produce light below 200 nm.
for use at longer wavelengths.
Even inductively coupled plasma mass spectrometry
(ICP-MS), which has become the premier atomic technique, has higher detection limits
for these elements than for most other elements.
This is likely due to a combination of
their high ionization energies and isotopic interferences1
these elements are amenable to a
Using hydride generation, these elements may
be detected at levels below many other elements.
there is a drawback.
Hydride generation is far more complicated than solution nebulization, and more risk
of sample contamination exists because of the necessary addition of several reagents,
sensitivity provided by hydride generation, it is important that the chance of sample
contamination be reduced.
With the electrolytic approach to hydride generation,
reagent used in the largest quantity, the electrolyte, can be purified of hydride forming
elements by electrolysis.
In this dissertation an apparatus and a method for continuous electrochemical
hydride generation (CEHG) will be described and characterized for sample introduction
in atomic spectrometry.
In addition, the use of an ArF laser is examined as a potential
for the detection
laser excited atomic fluorescence
The Scope of This Dissertation
The purpose of this dissertation is to describe original research and the results
perspective on these results.
It is not the intention of the author, nor is it feasible, that
this dissertation contain a complete introduction to the methodology and instrumentation
of atomic spectrometry nor any of the other broad topics that are peripherally touched
upon by the research described here.
Some recommended texts are suggested below.
In this first chapter a general introduction to solution nebulization and hydride
generation for sample introduction in analytical atomic spectrometry is given.
description of the most common sources that have been used for atomic fluorescence
spectrometry (AFS) will follow.
Emphasis will be on sources that have been used for
Finally, the nature of excimer laser radiation will be described.
2 describes three atomic spectrometers constructed during this research,
and compares their performance with commercially available spectrometers.
description of the hydride generator is given in Chapter 3.
In Chapter 4, experiments
designed to compare a variety of electrolytes and cathode materials are described.
results of these experiments are discussed.
focuses on the analytical use of the hydride generator in the continuous
sample introduction mode of operation.
was used to detect arsenic, selenium
Flame atomic absorption spectrometry (FAAS)
and antimony introduced into the flame by the
generation of arsine (AsH3), hydrogen selenide (H2Se) and stibine (SbH3).
coupled plasma atomic emission spectroscopy (ICP-AES) was used to detect arsenic and
In Chapter 6,
the use of an ArF laser for the laser excited atomic fluorescence
Also, suggestions for future research are given.
The order of the chapters presented here does not represent the order of the
performance of the
The idea of CEHG
invention of the actual device by over a year.
Suggested General Texts on Atomic Spectroscopy
As mentioned above, three types of optical atomic spectrometry were used during
the course of the research presented in this dissertation, FAAS, ICP-AES and LEAFS.
A complete description of any of these atomic spectrometric methods is beyond the
scope of this text.
Several general texts are recommended.
Ingle and Crouch have written an excellent text on the subject of spectrochemical
analysis including atomic methods.
They give a bibliography for each major area at
the end of
the classic texts on flame spectroscopy is that of
Mavrodineanu and Boiteux.3
Detailed information about flame methods can be found
classic texts by
spectrometry) can be found in Montaser'
Also, Boumans has edited a text on
Among the general sources of information on atomic fluorescence (AF)
spectroscopy are those of Winefordner and Omenetto.7'
Notes Concerning Atomic Spectrometric Terminology
For example, Mavrodineanu and Boiteux refer to atomization as the process
of breaking a solution into small
droplets,3 while most current sources classify this
process as nebulization and reserve the term atomization for the process of converting
the sample into free atoms.1,4'5'9.10
This dissertation uses the later standard,
device that atomizes the sample is called the atomizer.
source or atomic
ionization source" the term "atomic source" is used to refer to all three devices.
specialized acronyms have been used to describe analytical methods and instrumentation.
Ingle and Crouch list almost 100 acronyms which concern spectrochemical analysis.1
In this dissertation, the use of acronyms will be more limited, but the uninitiated reader
may become confused by such terms as AF, AAS, LIF and LEAFS. Each acronym is
given in parenthesis following its meaning the first time it appears in this text.
author has invented one new acronym, CEHG, which stands for continuous electrolytic
However, he resisted the temptation of inventing more acronyms!
Solution Nebulization for Sample Introduction in Atomic Spectrometry
According to Twyman, the first true examples of spectrochemical analysis were
done by Kirchoff and Bunsen in the middle of the nineteenth century."
salts into flames by placing the salt, or a drop of a salt solution, on a wire and then
supply their flames with sample3, an idea first suggested by Morton.1'2
many new sample introduction methods have been developed.
Browner and Boomrn3
have published a review on sample introduction in atomic spectroscopy; also, several
of the texts suggested above include sections on sample introduction.1-
are several sample introduction methods for use with small liquid or solid samples, this
continuous atomic sources.
For the introduction of solutions in continuous atomic sources, nebulization is the
most popular method.
In FAAS and flame atomic emission spectrometry (FAES), ICP-
AES and inductively coupled plasma mass spectrometry (ICP-MS), the most common
nebulizers are pneumatic.
Ultrasonic nebulizers are also common for use with an ICP.
Pneumatic nebulizers use a jet of compressed gas to aspirate and nebulize the
In cross flow nebulizers, a flow of compressed gas passes over the tip of a
sample capillary, and the resulting Bernoulli effect creates a local pressure drop and
draws the solution through the capillary.
Similarly, in the concentric nebulizer a flow
gas passes over the
tip of a sample capillary
located inside the compressed gas
capillary. In both cases,
the sample solution is broken into tiny droplets forming an
In some applications, the solution is pumped into the nebulizer to control the
solution flow rate.
introduction for solutions.
In the ultrasonic nebulizer, a flow of solution is pumped over
A spray, or cloud, chamber usually surrounds the nebulizer and prevents the
pneumatic nebulizer (the type used for the work described in this dissertation) uses a set
of propeller shaped blades, or spoilers, which help break up large droplets and prevent
those that do not break up from leaving the nebulizer. Other spray chambers use impact
beads for the same purpose.
The Scott type spray chamber, used for the ICP, has a
cylindrical baffle to force the larger droplets to fall from the aerosol before they can
leave the spray chamber.
For some applications, a condenser is employed between the
spray chamber and the atomizer to reduce the amount of solvent vapor that enters the
The efficiency of a sample introduction system can be broken into several steps.
For nebulizers, the solution is first broken into droplets. The percentage of the sample
solution that forms an aerosol is the "nebulizer efficiency." Next the aerosol must be
transported to the atomizer.
"transport efficiency." Of cc
emission or fluorescence can occur.
The total efficiency of the first two steps is called the
)urse, several other processes must occur before absorption,
The aerosol must be dried (desolvation efficiency);
the dry aerosol must be volatilized volatilizationn efficiency); and finally the chemical
bonds (if any) must be broken.
The fraction of the sample (in the atom source) present
as free atoms is termed the free atom fraction (P).
In emission spectrometry, the atoms
must also be excited, and in mass spectrometry, they must be ionized.
While it is useful to think of each of these processes and efficiencies separately,
atomizer, act as a team.
While one nebulizer may have a large transport efficiency, it
may also produce droplets too large to be desolvated.
In other words,
efficiencies (nebulization, transport, desolvation, etc.) are not independent.
while the ( is often the result of a local thermodynamic equilibrium, it is
possible to cool the atomizer with too much sample vapor and thus affect the 3.
also possible to greatly affect 3 in certain atomizers that lack the energy to thermally
For example, when hydride generation is used to introduce samples
into a heated quartz tube atomizer, a large free atom fraction is achieved despite the low
temperature of the atomizer.
The typical pneumatic nebulizer used for FAAS has a transport efficiency of 4-
The typical Meinhard (concentric glass) type pneumatic nebulizer used with
ICP-AES has a transport efficiency of 1-3 %.*,2'13 Transport efficiencies of 20-30 % have
been reported for ultrasonic nebulization.113,15
One hundred percent transport efficiency can be realized in flame spectrometry
using a total consumption burner.2'3
In the total consumption burner (or direct injection
burner), the concentric nebulizer is incorporated into the design of the burner so that
100% of the sample is nebulized directly into a surface mixed flame.
desolvation and vaporization.
Similarly, direct injection nebulizers (DIN) have been
developed for interfacing ICP-AES and ICP-MS with flow injection analyses (FIA) and
flow rates are used with these nebulizers.
Also, the large amount of solvent can "load"
Houk and coworkers'7 have demonstrated a DIN using a microcapillary
(10-25 pm diameter),
which produces very uniform droplets.
However, this type of
nebulizer clogs easily.
In many cases, analytes in solution can be chemically transformed to volatile
spectrometric source as gases.
Unlike solution nebulization,
where over 90% of the
sample is usually discarded, gas generation techniques often approach 100% transport
which have been
applied to atomic spectrometry are, the generation of molecular halides (12,
19 Br2 and
generation of volatile metal-chelates.24
The two most widely used techniques are the
generation of atomic mercury and hydrides of As, Sb, Bi, Ge, Sn, Pb, Se and Te.
is fortuitous that these elements form volatile hydrides, since As, Sb, Se and Te only
have resonance lines below 220 nm.
This complicates the optical atomic spectrometric
wavelengths, and a high excitation energy is required to produce atomic emission.
a result, solution nebulization detection limits are poorer for these elements than for
The use of hydride generation in analytical chemistry predates its use as a sample
introduction technique for atomic spectrometry.
Arsine generation has long been used
for the separation and preconcentration of arsenic from complex matrixes.Y Hydride
generation is also the basis of the Gutzeit and Marsh tests.
Holak first demonstrated the use of hydride generation for sample introduction
including several recent reviews.27'28,29,30,31
review is primarily concerned
with the use of hydride generation with atomic absorption (AA) detection in flame-in-
tube atomizers, externally heated quart tube atomizers and
A good review of the mutual interelement effects of the hydride forming elements is
emission and absorption detection.
concentrate on the generation of
AsH3 and H2Se.
the review by
and coauthors is a thorough
review of hydride generation for sample
introduction into an ICP with almost 150 references.
Chemistry of Hydride Generation
Holak26 originally used zinc metal as the reducing agent.
borohydride has become the reducing agent of choice.
The general reaction is,
+ 31-J +
+ NaC1 + 8fH
where E is the specific element, m is the formal charge on the element and n is the
number of hydrogens in the product hydride.
Table 1-1 lists several hydrides that may
be formed and their names.
All of these compounds are poisonous.
Less than 0.5 ppm
(by volume) of arsine can cause death.32
Volatile hydrides of main group elements that may be formed in aqueous
FORMULA NAME(S) BOILING pK,
POINT (C ) VALUES
AsH3 arsine, arsenic hydride -61"
SbH3 stibine, antimony hydride, -17"
BiHl3 bismuthane, bismuth hydride 16.8"
H2Se hydrogen selenide, selenium hydride -43.3" 3.89
HTe hydrogen telluride -2" 2.64
GeH4 germane, germanium hydride -90b
SnH4 stanane, tin hydride -52"
PbH4 plumbane, lead tetrahydride -18.4c
" Value from reference 32.
b Value from reference 28.
Values from reference 33.
an acidified solution
of the analyte is treated
(usually in solution).
The rate of hydride production depends on the chemical
form of the analyte, the concentration and type of acid used, the concentration of the
uerd and the de ion nf the opPnratnr 27,28,32
Arsenic(III) reacts almost instantaneously in 1 M hydrochloric acid (HC1),
arsenic(V) reacts only slowly except in 4 M HC1.34
Since the higher oxidation states of
oxidation states (As(III),
Sb(III) and Se(IV)),
The iodide ion (KI,
these species often are prereduced before
Nal) is often used for the prereduction of Sb
while Se is either heated with HC1 or acidified bromide.'29
Methods Used to Generate Hydrides
Two approaches, continuous and batch,
are used in hydride generation methods.
Generally, in the batch method, an aliquot of the solution to be analyzed is acidified and
transferred to a reaction vessel; the NaBH4 (usually a solution) is then added to the
The resulting hydride is swept out of the vessel by hydrogen gas
, usually, an additional flow of argon or nitrogen.
The generator itself can
be an Erlenmeyer flask, a test tube or some specially designed vessel.
In the continuous process,
the reagents are continuously introduced into a mixing
chamber (or in a length of tubing) where the reaction takes place.
Pumps are required
to pump the solutions into the system.
The mixture is
swept into a gas-liquid separator
where the liquid passes to a drain and the gaseous products pass to the atomizer or
Several groups have developed continuous systems in which
sample solution is mixed with the NaBH4 solution as an aerosol.35'36
the advantage of being easily incorporated into a FIA system,37 oa
This approach has
r it may be coupled
increased dependence on the kinetics of the hydride generation reaction.
The efficiency of hydride generation can be defined as the percentage of analyte
leaving the generator as the hydride.
The efficiency of hydride generation depends on
the use of optimized chemical and physical parameters,28 and many published studies do
not include any estimation of the efficiency.
In general, efficiencies of nearly 100% can
efficiency and analysis time may be preferred.
Once formed, the hydride may be swept directly into the atomic source, or it
may be trapped.
cooled U-tube traps have been used to
condense AsH3, SbH3 and H2Se.31'39'40
generator, the trap is warmed, and thi
into the atomic source. Also, the hyd
vapor which are also produced by the generator.
limit because of the preconcentration. However,
Once all of the hydride has been released by the
e hydride is introduced as a preconcentrated plug
ride can be separated from the H2, CO2 and water
There is a decrease in the detection
there is a great increase in the time of
analysis and in the complexity of the apparatus and procedure.
Several companies market commercial hydride generators, including most of the
major atomic spectrometric instrument manufacturers (i.e. Baird Corporation, Hitachi
Instruments, Leeman Laboratories, Perkin-Elmer Corporation, Thermo Jarrell-Ash and
The earliest commercial generators were of the batch type, but the recent
trend is toward fully automated FIA systems.
All of these generators are designed to
Ward and coworkers were able to generate the hydrides
Sn and Te simultaneously using a continuous hydride
Method of Detection for Hydride Generation
ICP-AES, microwave induced plasma atomic emission spectrometry (MIP-
capacitively coupled microwave plasma atomic emission spectrometry (CMP-
AES) and direct current plasma atomic emission spectrometry (DCP-AES).31'42'43
these emission sources, the ICP has been the most often utilized.
For AAS detection, the flame is rarely used, since the typical acetylene/air flame
background absorption of 15%.44 The most common atomizers are the externally heated
quartz tube and the flame-in-tube atomizer.27
In both cases, the hydride is heated in the
presence of hydrogen and decomposes forming free atoms.
These atomizers have also
used.27,47'48 The hydride is purged through a heated graphite tube (300-600 C) where
it is retained. The trapped hydride is subsequently atomized at temperatures higher than
2000 C. Table 1-2 lists some detection limits for comparison.
While hydride generation can eliminate spectral interference, a variety of species
can interfere with the production, or the release of the hydride from the solution.
have been many publications concerning interference effects in hydride generation; for
examples see references 50-54.
The chemical interference can be broken into two
those which act in the solution phase and those which act after the hydride
is released from solution.
Detection limits (ng/mL) by solution nebulization and hydride generation.
ELEMENT SAMPLE INTRODUCTION ICP- AAS Flame- ICP-
METHOD AES" AFS MSd
As Pneumatic nebulization 41 100b 100b 0.4
Continuous hydride generation 0.8 1 0.8c 0.0015
LN2 trap preconcentraion 0.02 0.02b 0. 1
Se Pneumatic nebulization 75 70b 40b 1
Cont. hydride generation 0.8 P1 0.5 0.03
LN2 trap preconcentraion 0.1 0.02b 0.06b
Sb Pneumatic nebulization 32 30b 50b 0.02
Cont. hydride generation 1 0.0003
LN2 trap preconcentraion 0.1 0.5b 0.1b
a Values From reference 28.
b Values From reference 43.
SValues From reference 49.
d Values From reference 31.
Dissolved noble metal
, Pt and Pd) can reduce the hydride generation
efficiencies of As and Se at concentrations less than 1 tg/mL.
as copper, cobalt and nickel,
suppress the production of hydrides at levels that depend
greatly on the concentration of NaBH4 used, the concentration of acid (generally HC1)
such as Fe
Mn and Zn
, have little effect on hydride generation.
mechanisms of transition metal interference are not well
suggested that easily reduced metals may be preferentially reduced.
In turn, the metal
precipitate may coprecipitate the analyte, the metal may absorb the hydride, the metal
decompose the hydride,
or the metal may retard its release
Bye57 has suggested that the formation of highly reactive metal borides may
be responsible for the decomposition of the hydride before they reach the detection
, Aggett and Hayashi58 have studied the effect of transition metal ions on
They concluded that the interference is due to "the formation of a
species between arsenic and the interferant in a lower than normal
oxidation state," reference 58,
The findings of Bax and coauthors59 suggest
that the effect of transition metal ions on hydrogen selenide production is due to the
decomposition of the hydride by metal borides formed by the action of NaBH4 on the
Whatever the case,
the interference of transition metals is less important
when the NaBH4 concentration is kept low and when the hydride is separated quickly
from the solution.
In addition to the transition metals, there are interelement interference between
hydride forming elements that may depend upc
after they have been separated from solution.
a mechanism affecting the hydrides
Interferences between hydride forming
elements are generally more serious in heated quartz tube and flame-in-tube atomizers,
the effect of
and SbH3 can reduce the concentration of hydrogen free radicals (believed to be involved
in the mechanism of hydride decomposition)
and accelerate the decay of free selenium
(Also, it is suggested that modification of the quartz surface is involved).
Electrolytic Hydride Generation
It is possible to form arsine, stibine, stanane, germane, hydrogen selenide and
some of the
potentials observed for arsenic using dropping mercury electrode polarography.
Several groups have used electrolytic hydride generation as an alternative to the
chemical reduction of arsine in variations of the Gutzeit or Marsh tests, for example see
Rogers and Heron.61
Rigin62 was the first to use electrolytic hydride generation for
sample introduction in atomic spectrometry.
Rigin62 used a stirred electrolytic cell to produce arsine.
platinum wire was the catho<
NaOH was the electrolyte.
A flat spiral of smooth
A current density of 250 mA/cm2 was used, and
The hydride, and hydrogen,
were directly transferred to a
heated quartz tube atomizer.
to detect the arsenic. Rigin
Hollow cathode lamp excited atomic fluorescence was used
i reported that most of the arsine was generated within 45-
Using the peak area as the analytical signal, he reported a detection limit of
15 pg, or 3 pg/mL in a 5 mL sample, and his relative standard deviation was
The linear dynamic range was 0.2-200 ng (only 40 ng/mL).
Rigin62 tested 20 ions as possible interferants.
The results were impressive.
Standard potentials for several reactions.
HALF CELL REACTION STANDARD FORMAL
POTENTIAL (E'), V
, + 3H-
+ 2e- =
+ 6e- =
- 3e" =
+ 2H20 +
= As+ +
+4 2H20 +
Ge + 4H+
Sn + 4H+
= Sb + HzO
= Sb + 40H-
All values from reference 60 unless noted.
Co(II), Cu(II)and Ni(II) had no effect when 50 mg were added to
the electrolytic cell.
the volume of the cell
was not given,
corresponding concentration cannot be calculated.
If 500 mL is assumed, 50 ug/mL of
Au, Ag, Hg, Sb and Se did not interfere with arsine generation!
Rigin has also reported
electrolytic stanane generation from 10% NaOH using a smooth lead cathode with AA
Reported half wave potential for dropping mercury electrode polarography.
Polarizer SUPPORTING REACTION HALF WAVE
ELECTROLYTE (CHANGE IN POTENTIAL
OXIDATION STATE) (E)A,V
As(IV) 11.5 M HCL 5 0 not measured
0 -- -3 -0.26
As(III) 1 M H2SO4 3 0 -0.455
0 -3 -0.755
As(III) 0.5 M KOH 0 --3 -0.575
All values from reference 60.
a Reported versus the standard hydrogen electrode.
b Half wave potential for As(V) is less than 0 V.
Despite these excellent results by Rigin, no further work has been reported in the
literature concerning electrolytic hydride generation for sample introduction in atomic
Because of the success of NaBH4
the rush to improve the
associated methodologies and the desire to understand matrix effects, work toward other
methndz nf rerdntii n hn hben cere
Light Sources for Atomic Fluorescence Spectrometry
The light sources that have been used for AFS can be categorized as incoherent
and coherent (laser) sources.
Incoherent sources may be further categorized as line and
continuum, in reference to their spectral distributions.
Laser sources can be spectrally
narrow or spectrally broad in comparison to the spectral profile of the atomic transition.
produce less scatter than continuum sources.
The ideal light source for analytical AFS would have a spectral profile smaller
than the spectral profile of the atomic absorption line of interest (under the conditions
existing in the atom reservoir).
In addition, no light would be emitted by the perfect
source beyond the limits of the atomic absorption line's profile, thus maximizing the
selective excitation of the element of interest, and minimizing the laser scatter.
course, the center wavelength of the ideal source spectral profile would be tunable to
match any atomic absorption line wavelength.
The perfect source would have a high
spectral radiance, and there would be no fluctuations in that radiance.
To become even
the ideal source could produce a spectrum of narrow lines matching
several atomic transitions (choice of the analyst),
for multielement analysis.
Incoherent Line Sources
"conventional" sources for AFS. The most widely used sources for AFS are the HCL
and the electrodeless discharge lamp (EDL). These lamps are atomic line sources that
a low pressure direct current discharge or a radio-frequency or microwave excited
In their basic form,
HCLs are direct current glow discharges utilizing a cup
shaped cathode made of, or coated with, the element of interest.
approximately 0.002 nm2'1'9
times narrower than
The typical HCL lamp has a radiance of 103 to
lamps have a radiance of approximately 5 x 10i5 W/cm2sr.
x 106 W/cm2 sr, and
66 Since fluorescence
intensity is proportional to excitation intensity, HCL lamps are often operated at higher
currents for AFS than those used for AAS.
However, if the current is increased too far,
broadened and self-reversed.
HCL operating lifetimes are also greatly reduced at high
Several methods have been developed to increase the excitation efficiency within
HCLs without increasing the rate of sputtering from the cathode.
The simplest method
involves using short (15-40 /s), high current (200
pulses at frequencies of
Walsh68 and, later, Lowe69 developed "boosted" HCLs through the use
of an additional electrode.
In these lamps, a low current is used to provide atomization,
a higher current
Lamps operating on this principle are available from Photron Pty,
Ltd., and produce atomic lines with up to ten times the intensity of ordinary lamps.
Several other methods have been developed to increase the intensity of HCLs.
based on the coupling of the discharge with a microwave or radio frequency field.64
Electrodeless discharge lamps are usually more intense than common HCLs.
an EDL, a sealed tube containing the element, or a compound of the element, is placed
in an intense radio frequency or microwave field.
For certain elements (As, Se, Sb, Te
and Hg), the improved performance of EDLs make them preferable to HCLs.
can be over an order of magnitude more intense than hollow cathode lamps."
Since both EDLs and HCLs are line sources, they can be used for AFS with or
without a dispersive light collection system (i. e., a monochromator).
A simple filter
can be used to select the fluorescence wavelength, or a solar blind photomultiplier tube
(PMT) can be used.64"1
The excellent light throughput makes this type of detection
in atomizers that do not produce much scatter and have
Atmospheric plasmas such as the DCP and ICP have also been used as line
sources for AFS.
However, these sources have been rarely used and have never been
used for the detection of arsenic.70
1-5 lists some AFS limits of detection for
arsenic and, for comparison, a more typical element, cadmium.
Dye Laser Sources
For many elements, dye lasers approach the ideal source for atomic fluorescence.
They can have an extremely high radiance and are spectrally narrow.
Pulsed dye lasers
can produce peak powers of megawatts or more and have spectral linewidths that can
system does not extend below 200 nm.
Therefore, there have been almost no reports
of LEAFS for arsenic detection.
Detection limits for As and Cd using several light sources with several
Pneumatic solution nebulization was used.
Data from reference 60.
Only Leong, D'Silva and Fassel71
arsenic using a dye laser system. In their
have reported an analytical detection limit for
system, the output of a frequency doubled dye
laser was used to generate anti-Stokes stimulated Raman scatter in a liquid nitrogen
cooled Raman cell.
Anti-Stokes stimulated Raman scatter is a nonlinear effect used to
shift coherent light to shorter wavelengths.
They measured arsenic and several other
using an ICP atomizer with
The limit of
(LOD) for arsenic was 20 ng/mL,
which is not significantly better then ICP-AES.
improvement of only two times was seen when the pneumatic nebulizer was replaced
by an ultrasonic nebulizer.
However, the same authors observed an improvement of
over 50 times for mercury and phosphorus by LEAFS when ultrasonic nebulization was
and undesolvated droplets were the limiting noise for mercury and phosphorus but that
some other source of noise was limiting for arsenic.
Unfortunately, the authors did not
discuss limiting noise sources.
All dye laser systems are complex and difficult to operate and maintain;
system described by Fassel and coworkers would be more so.
for LEAF detection of arsenic is the ArF excimer laser. Selw
Another possible source
'yn7 has demonstrated the
use of a simple untuned Lumonics ArF
fluorescence in a low pressure RF sputl
excimer laser (al
t 193 nm) to excite arsenic
The ArF laser pumped the
193.7 nm line of arsenic, and fluorescence was observed at 245.6 nm.
performance of the ArF laser was never evaluated because of "the extremely low ArF
power available at the RF sputtering cell."n
The Nature of ArF Laser Radiation
In an excimer laser, a mixture of a rare gas and a halogen are contained between
two parallel conduction plates.
For an ArF laser, these gases are argon and fluorine.
A high power discharge is formed between the plates in fast pulses.
excimers) are created within the discharge.
Excited dimers (or
A population inversion is automatic, since
these excimers are not stable in
state of an
excimer is bound, but the ground state is dissociative (or very weakly bound).
common excimer lasers types are ArF, KrF, XeC1, XeF and KrC1.
A typical single
laser may be used as an ArF, KrF, XeCl or XeF laser by changing the fill gas mixture.
However, some time is required to
"passivate" the laser to each halogen.
spectral wavelength peak is approximately
exact wavelength depends
slightly on the composition of the gas fill.73
= 1 nm.74'75
The spectral profile typically shows a complex fine
Since the typical excimer laser chamber is about 1 m long,
6-7 ns to make one round trip between the rear mirror and the output coupler.
laser pulses last only 10-20 ns, so it is not possible for the laser radiation to make more
than one full pass between the output coupler and the rear mirror.
coupler is not a reflector, but a MgCI2 or quartz plate.
In fact, the output
Therefore, the resonance cavity,
constructive interference do occur, and some fine spectral structure is superimposed on
a broad peak.
Since the ArF laser is so broad, and only the wing of the band reaches out to
very little of its power is available to excite arsenic fluorescence.
the unabsorbed energy passes through the atomizer with some being scattered by small
molecules and particles.
molecules present in air, flames and plasmas.
Among these are O,, NO, CO and H2.
Laufer and coworkers" have studied the radiative processes in air excited by
They found several strong oxygen fluorescence lines between 200 and 260 nm.
247.8 nm. Presumably, CO2 is photodecomposed to CO, which is further decomposed
to C.74'7 Scatter (fluorescence and Raman) from such small molecules is an important
disadvantage in a LEAFS source,
since this scatter will be a source of noise.
important to remember that far
is available in
output to be
scattered by small molecules than is available to excite arsenic.
available, but they are far more expensive and complex than the untuned excimer laser.
Like most tunable excimer lasers,
the Lambda Physik uses two discharge devices.74
the first discharge,
the tuned laser radiation is created,
The ArF laser used in these studies,
the Questec Model 2100,
had a spectral
0.47 + 0.02 nm full width at half maximum (FWHM) and a peak center of
The pulse duration was 10-12 ns FWHM.
(See Chapters 2 and 6 for the
maximum pulse energy is 150 mJ/pulse;
although, only 80 mJ/pulse was sustainable at
high repetition rates;
the maximum repetition rate is 50 Hz;
the beam dimensions are typically 6-14 mm by
the pulse to pulse stability
20-23 mm (a rectangular
beam); and the beam divergence is 1.0 mr in the vertical direction and 3.0 mr in the
INSTRUMENTATION USED FOR FAAS, ICP-AES AND AFS
dissertation. Minor modifications made for particular measurements will be described
in later chapters. The goals of the experiments concerning the electrolytic hydride
generator were to develop and characterize this new method of sample introduction.
Because the analytical figures of merit measured depended on both the generator and the
spectrometers, it is important to understand the limitations of the spectrometers used.
Three instruments were constructed in the laboratory chiefly of commercially
available components: a flame atomic absorption spectrometer (FAA), an inductively
coupled plasma atomic emission (ICP-AE) spectrometer,
and a laser excited atomic
The emission instrument and the fluorescence instrument
resided on the same experimental setup and shared much of the same equipment.
the sake of clarity, they will be discussed below as if they were completely separate
The atomic sources (the flames or the ICP) and optical systems will be described
will be described
in a separate section
be compared to commercially available instruments.
Because the spectrometers
were used at wavelengths near 200 nm, this comparison will focus on the use of these
instruments at short wavelengths.
Since there has never been a successful commercial
LEAF spectrometer, no comparison can be made between the laboratory constructed
LEAF spectrometer and a commercial counterpart.
Atomizers. Emission Sources and Optics
The FAA Spectrometer
The FAA spectrometer was a simple single beam spectrometer.
Figure 2-1 is
a schematic diagram of the instrument, and Table 2-1 lists some of the equipment used.
All the lenses were made of fused quartz (silicon glass). The light from a HCL was
focused, modulated by a mechanical chopper, and then collimated. The collimated light
was focused by a 15 cm focal length lens into the center of the flame and then refocused
by another 15 cm focal length lens.
An iris was used to block the outside portion of the
beam to match the f-number of the monochromator.
Next, the beam passed through a
filter (when used) and fell onto the entrance slit of the monochromator.
image of the lamp emission was formed on the entrance slit.
The entire optical train
was carefully aligned to ensure that the system had a maximum throughput.
The burner was a premix type burner with a 5 cm slot burner head.
line was attached to the fuel inlet.
The auxiliary air inlet of the mixing chamber was
introduced through the nebulizer-air port.
The nebulizer was a common pneumatic type,
and the sample uptake tube of the nebulizer was blocked when the nebulizer was not in
hydrogen and air
regulated by two stage regulators and
metered by needle valves in the base of two separate rotameters.
Equipment used for FAAS measurements.
EQUIPMENT DESCRIPTION / MANUFACTURER AND MODEL
Atomic line source Boosted hollow cathode lamps, Photron Pty. Ltd.,
Chopper and control Stanford Research Systems, model SR540
Premix burner Perkin Elmer, part no. 0040-0146
Nebulizer Perkin Elmer, part no. 0303-0352
For As and Se Acton Research Corp., 193-N-1D (195.0 peak
measurements wavelength) 20 nm bandwidth
For Sb measurements none
Monochromator Thermo Jarrell Ash (0.5 M focal length, 1200 groves
mm' grating, F-number 8.6), model 82020
High voltage supply Bertran Associates Inc., Model 301 (-1000 V)
Photomultiplier Hamamatsu, model R 1547 (1.3 x 106 gain, 1 nA dark
The ICP-AE SDectrometer
spectrometer, and Table
lists some of the equipment used.
The emitted light
from the 40 MHz argon ICP was collimated by a fused quartz planoconvex lens,
I I H
O a- e0
:3 ***** .***.OleO********U**Se
of the monochromator.
A second planoconvex lens focused the light onto the entrance
of the monochromator where it was focused into a one-to-one image of the ICP.
Optionally, one or more filters could be placed in front of the entrance slit to filter the
The plasma was viewed through the observation window in the front
access door of the ICP housing rather than
the window in
the side of the housing
normally used for this purpose.
(This geometry allowed the laser beam to enter at a 90
angle to the collection optics during the fluorescence experiments.)
The lenses were
mounted on to a kinematic base so that they could be easily removed and replaced when
the plasma access door was opened.
Equipment used for ICP-AES measurements.
EQUIPMENT DESCRIPTION / MANUFACTURER AND MODEL
Torch Precision Glass Blowing of Colorado (standard)
Nebulizer Precision Glass Blowing of Colorado (concentric)
Monochromator Spex Industries Inc. (0.5 M focal length, 1200
grooves mma' grating, F-number 6.9), model 1870
Photomultiplier Hamamatsu, R 166
High voltage supply Bertran Associates Inc., Model 301 (-1000 V)
Radio frequency Baird Corp., 300-D
Generator (and ICP
The argon gas sources,
coolant and plasma/nebulizer,
were regulated by two
were supplied by
rp nI t rn- ft-. a ,,-an-,1nnl ,-,n 1an TC'TD f^.-^l 'fo'.. ^,.n,,ar-fA 4f a l,,, a ,,- C tM70
generator, depending on the experiment being performed.
The ICP torch and nebulizer
were manufactured by Precision Glass Blowing of Colorado, and both were of the type
, which supports an annular plasma.
The nebulizer was a concentric glass
nebulizer similar to the type developed by Meinhard.so
Solutions were not pumped into
the nebulizer, but were drawn in by the vacuum created by the Bernoulli effect in the
The ICP-LEAF Soectrometer
used for atomic
ICP and the
flames, is shown in Figure
and some of the equipment used is listed in Table 2-3.
For the flame measurements, a flame was in the position occupied by the ICP in Figure
Monochromator #1 was used for the early experiments and was later replaced by,
the higher resolution, monochromator #2.
was placed on its side,
so that the entrance slit was horizontal, parallel to the ground.
This allowed a larger
area of the fluorescence "stripe," formed as the laser passed through the atomizer, to
be imaged along the monochromator entrance slit.
In contrast, monochromator #2 was
used in the conventional vertical entrance slit attitude.
A 10 mm2 area of the edge of a quartz microscope slide was placed in the path
of the edge of the beam was thus reflected toward the photodiode
The laser beam was then focused through a 50 cm focal length, fused
********* "*** "* ** -***, .. *
... - -v ""**1" ".- .,
Equipment used for LEAFS measurements.
EQUIPMENT DESCRIPTION / MANUFACTURER AND
Excimer laser Questec (50 Hz. maximum pulse rate, 150 mJ
maximum pulse energy in ArF mode) model
Monochromator #1 Spex Industries Inc. (0.33M focal length, 1200
grooves mm-1 grating, F-number 5.9), model
Monochromator #2 Spex Industries Inc. (0.5 M focal length, 1200
grooves mm-' grating, F-number 6.9), model
Photodiode trigger EG&G Photon Devices (<1 ns risetime) FND-
Arsenic electrodeless Imaging and Sensing Technology model WL-
discharge lamp 40011
Photomultiplier Hamamatsu, R 166
High voltage supply Bertran Associates Inc., Model 301 (-1000 V)
ICP torch Precision Glass Blowing of Colorado (standard)
ICP nebulizer Precision Glass Blowing of Colorado (concentric)
Radio frequency generator Baird Corp., 3000-D
(and ICP housing etc.)
Premix burner Perkin Elmer, part no. 0040-016
Nebulizer Perkin Elmer, part no. 0303-0352
experiments to achieve various laser beam cross sectional areas in the atomizer.
fluorescence was collected at a right angle to the laser beam by a pair of 15 cm focal
spectrometer, and the collected fluorescence was imaged on the spectrometer entrance
slit to form a one-to-one image.
Optionally, one or more filters could be placed in front
of the monochromator entrance slits.
The arsenic electrodeless discharge lamp was used to calibrate the wavelength
of the spectrometer; it was left off during most of the experiments.
splitter was used only when the EDL was used.
The 90/10 beam
It allowed 90 % of the fluorescence, or
to enter the spectrometer at the same time as
10% of the EDL optical
The ICP used for the fluorescence experiments was the same as that used for the
The burner and nebulizer used for the LEAF spectrometer were
of an identical type to those used in the FAA spectrometer, except that a 3 cm slot
burner head was used instead of a 5 cm slot burner head.
The burner head was placed
so that the laser beam passed through the long axis of the flame.
Electronics for FAAS
The electronic system used for the FAA measurements is shown in Figure 2-4,
and Table 2-4 lists some of the components used.
The HCL was modulated to reduce
The electronic system used for the atomic absorption measurements.
rectification by a lock-in amplifier.
The optical modulator,
reference signal to the lock-in.
The current-to-voltage amplifier was generally operated
2 kHz electronic band pass, the chopper modulated the HCL beam at 200 Hz,
and the time constant of the lock-in amplifier's low pass filter was set to 10% of total
time used for each measurement.
The output of the lock-in was sent to an analog-to-
digital converter (ADC) and recorded by a computer and a strip chart recorder.
Table 2-4: Electronic equipment used for FAA measurements.
EQUIPMENT DESCRIPTION / MANUFACTURER AND MODEL
Current amplifier Keithley, model 427
Lock in amplifier EG&G Princeton Applied Research, model 5101
Analog to digital Stanford Research Systems, model SR245
Computer Northgate 286
When a single beam spectrometer is used to measure absorption, the ratio of the
HCL atomic line intensity to the intensity of the line transmitted through the flame
cannot be directly measured.
intensity of the HCL. Eaci
Therefore, it is necessary to decrease the variation in the
i lamp used in these experiments was always allowed to
warm up for 30-60 min, or until the average intensity of lamp emission changed by less
than 1% in 10 minutes.
The lamps were powered by a current regulated high voltage
Electronics for ICP-AES
All of the equipment listed in Table 2-4 was also used for the ICP-AE
spectrometer, except for the lock-in amplifier.
The current from the PMT (a solar blind
model) was amplified by a current-to-voltage amplifier with a built in, low pass, filter.
A laboratory constructed voltage amplifier was used to adjust the output range of the
current amplifier (-1 to 1 V) so that it more closely matched the input range of the ADC
(-12 to 12 V).
The output of the current-to-voltage amplifier could be monitored either
calibrated at 193.7 nm and 228.8 nm, were used to extend the region of linear response
of the instrument.
The time constant of the detection electronics was adjusted to approximately 10%
of the measurement time used for recording steady state signals.
For transient signal
measurement, a time constant of no more than 1/7t of the peak width of the transient,
or pulse, is required to accurately reproduce the pulse."8 Therefc
of the electronics was set for 1/10" of the fastest transient signal'
)re, the time constant
Electronics for LEAFS
Figure 2-6 is a schematic of
's electronic measurement
However, a current signal from the PMT was converted to a voltage by a laboratory
designed transimpedance amplifier, rather than the Keithly current amplifier.
The electronic system used for the atomic emission measurements.
voltage amp. gain = 10
The electronic system used for the atomic fluorescence measurements.
corner inset is a circuit diagram for the transimpedance amplifier.
Optional voltage Amp (5 or 10X)
X 10 Transimpedance Amp.
Low Pass Filter ->
S70 V Battery
I<-- Trigger Photodiode
OP-07 (50 MHz)
diagram of this amplifier is shown in the inset in figure
average (Stanford Research Systems,
In addition, a boxcar
model 255) was used.
A fast voltage preamplifier was sometimes used to further amplify the signal
before its measurement by the boxcar average.
The fast voltage preamplifier was a
Comlinear Corporation (model CLC100) DC-500 MHz amplifier with a gain of 20dB
However, since the output resistance of the voltage amplifier and the input
resistance of the boxcar were each 50 (0
, a voltage divider was created,
and the effective
gain was only
Whether the voltage preamplifier was used or not, a 50 (f series resistor was
to match the input impedance of the boxcar average to the impedance of the
coaxial cable that connected the various components.
provide by a photodiode.
The boxcar's trigger signal was
The signal from the boxcar was recorded on both a strip chart
recorder and a computer via an ADC.
calibrated neutral density filters were
used to extend the linear response range of the instrument.
to record a
spectrum simultaneously with a laser specular scatter or fluorescence spectrum.
atomic emission lines from the EDL were used.
This was particularly necessary during
experiments designed to measure the laser spectral profile (see Chapter 6.)
emission spectrum was recorded simultaneously with the laser scatter by the computer
and strip chart recorder.
Since the EDL was modulated at a frequency of 500-1000 Hz,
the high pass filter (10 kHz cutoff frequency) built into the boxcar average prevented
emission was measured by a lock-in amplifier connected to the transimpedance amplifier
via a low pass filter (1 kHz
cutoff frequency) which prevented the laser induced signal
from reaching the lock-in device.
The excimer laser not only produced ultraviolet light but also radio frequency
(RF) pulses and X-rays with each laser pulse.
the lead lining of the laser housing. Howeve
The X-rays are effectively contained by
r, the pulses of radio frequency (MHz to
GHz) electromagnetic radiation presented a considerable difficulty in the measurement
of small fluorescence signals.
picked up by the PMT.
shows a typical radio frequency noise pulse
was in a shielded housing and biased with -1000 V
, 50 0,
Packard model 54503A).
internal 50 0 input resistance served as a
(The band width of the oscilloscope prevented the measurement of higher
Figure 2-8 shows the average discrete Fourier transform of four, excimer laser
generated, RF pulses as detected at the PMT.
The frequency content of the RF pulses
generally increased with increasing frequency from
100 kHz to
Unfortunately the atomic fluorescence pulses had a FWHM of approximately 12-15 ns,
and the RF pulses began before the fluorescence signal started and ended long after the
fluorescence signal had decayed.
Therefore, it was difficult to effectively eliminate this
(sl!un AieJiqJe) epn)
filter (such as a boxcar).
Since the exact phase and intensity of each RF pulse was
random, the boxcar output showed random fluctuations (noise) that interfered with the
measurement of small fluorescence signals.
The laser was encased with a copper mesh.
PMT housings were compared, and a model PR1402RF (Products for Research, Inc.)
for the experiments described in
housing was electrically isolated from the monochromator's mounting flange by a rubber
amplifier was built in a shielded box and was connected directly (no cable) to the PMT
As a result, the system'
response to the RF noise was reduced to a level below
that produced by a single photoelectron.
Although the fluorescence signal consisted of pulses of light lasting not much
longer than the laser pulses (12-15 ns FWHM), the transimpedance amplifier stretched
these pulses to a FWHM of approximately 250 ns.
Figure 2-9 shows the shape of the
average of eight fluorescence signal pulses recorded with a simple 50 0 load resistor and
with the transimpedance amplifier.
The speed of the transimpedance amplifier had to
be compromised to prevent ringing due to
Gain peaking can occur
when a fast transimpedance amplifier is connected to a current source that has a high
An unstable resonator results if the product of output capacitance
output resistance of the current source
(ALw) apnl!ldwue leu6!s
A feedback capacitance of 15 nF was experimentally determined to be the
minimum capacitance that prevented ringing.
In theory, it is always better not to stretch
a transient signal in the presence of white noise, since the signal intensity will decrease.
Also, if the boxcar gate width is increased to capture the same area of each peak, the
larger gate width will include more noise.
However, it was empirically confirmed that
the larger gate width did not degrade the signal to noise ratio, likely due to the special
nature of the RF noise.
The larger gate width reduced the bandwidth of the boxcar,
effectively filtering the higher frequencies contained in the noise spectrum as much as,
or more than, the signal.
Performance and Comoarison to Commercial Instrumentation
characteristic concentrations for a variety of elements.
The characteristic concentration
is the concentration of the element that gives an absorbance of 0.0044.
can be calculated as 0.0044 divided by the characteristic concentration.
direct measure of the instrument'
the sensitivity times the detection
Perhaps a more
capability is the minimum detectable absorbance, or
limit. This criterion is still a function of the flame
type, the HCL lamp type, and the wavelength, but it is less affected by the nebulizer
efficiency and atomization efficiency.
Another important performance criterion for FAA
spectrometers is the minimum transmittance for linear response, or the minimum optical
transmittance that produces a linear response from the instrument.
From this value and
a linear region
estimated, if the spectrometer response is the limiting factor.
The minimum transmittance for linear response was estimated by using calibrated
neutral density filters.
An arsenic HCL was used, and the arsenic
193.7 nm line was
isolated by the monochromator (spectral band pass 0.1 nm).
The flame was not used.
The instrument responded linearly down to a transmittance of 0.78% (or an absorbance
Stray radiation was the most likely cause of the nonlinear response at high
transmittance down to 1% (an absorbance of 2).2
The minimum detectable absorbance depends greatly on the relative strength of
several noise sources that may be dominant at low absorbance values.
- 220 nm, flame absorption flicker noise is usually the limiting noise source.
The instrument used in this work had minimum measurable absorbance of approximately
0.003 for both the 193.7 nm arsenic line and the 196.8 nm selenium line.
were measured using pneumatic nebulization of an arsenic or a selenium solution into
concentration that gives an absorbance of 0.0044) and detection limits for FAAS.
these values, a minimum detectable absorbance of approximately 0.001 can be calculated
, minimum detectable absorbance of 0.0025 (for arsenic at 193.7 nm) and
0.002 (for selenium at 196.8 nm) can be calculated.
At longer wavelengths, both the
The simple single beam instrument described here lacked two important feature
generally employ some type of background correction technique.
reduces the effect of background absorption and allows the measurement of a lower
absorbance when the background absorption is high.8485
Flame background absorption
is significant at wavelengths shorter than 210-220 nm, even in a H2/air flame.
important feature of most commercial FAA spectrometers is the incorporation of a
reference light beam.
In such an instrument (a spectrophotometer), the ratio of the two
not affect the
magnitude of the signal.
The absence of a reference beam to account for changes in the
lamp intensity decreased the precision of the measurements and necessitated constant
recalibration of the 100% T level.
is more difficult than absorption
spectrometers since the only common figures of merit given for commercial instruments
are the limits of detection for various elements.
A major difference between the ICP-
spectrometer, such as the Perkin-Elmer Plasma 200086 or the Varian Liberty
What is more important is that these commercial instruments take a number of
steps to stabilize the plasma.
the spray chamber of the Plasma 2000 is
, the gas flow rates are more accurately controlled by mass flow meters, and a
solid state RF supply with automatic impedance matching is used.
selenium) at lower concentrations than were possible using the laboratory constructed
Using pneumatic solution nebulization and the laboratory constructed
the arsenic detection limit was approximately
integration time and three standard deviations (3 a) were used to calculate this LOD.
The Plasma 200's arsenic detection limit is 20 ng/mL.
, and the Liberty 110'
detection limit is 12 ng/mL,
based on 10
s integration time and the same, 3 a, definition
DESIGN OF THE CONTINUOUS ELECTROLYTIC HYDRIDE GENERATOR
A canon of all design is that the optimal compromise between performance,
convenience and cost defines the best design.
industry in the 1980's).
(An obvious exception is the defense
In this case, the budget, mechanical abilities of the author, time
and initial ignorance were the limiting factors.
much was learned.
This design is a prototype, from which
That there could be many improvements is obvious, and several will
be suggested in the seventh and final chapter of this dissertation.
The electrolytic hydride generator was designed and constructed at the University
The design was intended for ease of construction.
Much of the actual
fabrication was done in the "student machine shop," and the more difficult machining
was carried out by the UF Chemistry Department'
machine shop staff.
contains shop drawings for the various parts that must be machined.
plastic (polymethylmethacrylate) was chosen for several parts, primarily for its inertness
in strong acids and alkali, excellent machineability and transparency.
polytetrafluoroethylenee) plastic was used for all of the other parts that came in contact
with the solutions.
Chemical resistance was the principal reason for choosing PTFE.
electrical conduction between the cathode in the flow cell and the anode in the anode
The solution to be analyzed was pumped through the cathode cell where
it contacted a coil of wire functioning as the cathode, and the analyte (As, Se or Sb) was
converted to the hydride. A large potential difference (over 20 V) was maintained
between the electrodes of the cell. Obviously, the principal reaction was the electrolytic
decomposition of water into oxygen and hydrogen gas.
The solution and the gases
(hydrogen and hydrides) produced at the cathode flowed up through the cathode cell and
into a "sparge" (phase separator) chamber where they were separated from the aqueous
phase with the aiding flow of a "carrier gas" (argon).
the solution stream below the sparge chamber. The
The carrier gas was mixed with
vaste liquid was drained out from
the side of the sparge chamber, and the gases were directed through a length of tubing
to either the burner or torch.
The oxygen gas generated at the anode was regulated to
maintain a higher pressure within the anode chamber than that within the cathode cell.
This created a net flow of solution through the glass frit from the anode compartment
and into the cathode cell, greatly reducing the possibility of the anode solution becoming
contaminated with the contents of the cathode cell.
A Detailed Walk Through and Around the Generator
Referring to Figure 3-1, the cathode flow cell was a cylindrical channel cut into
a solid block of extruded PTFE plastic.
A slot was cut into one side of the channel (A).
The chamber was shaped with a round flange (B) which was used to attach it to the
linking flange described below.
A cylindrical extension (C) above the flange had six
The cathode flow cell (not to scale).
Side cross section
The solution to be analyzed was pumped into the bottom of the flow cell through a
fitting (E) that allowed the chamber to be connected to 1/8" PTFE tubing via a 1/4" to
1/8" PTFE compression union.
(All the tubing fittings and tubing unions were made
of PTFE unless otherwise note
(SwageLok Co., Solon, Ohio).
brand compression fittings were used
The entire chamber was -
10.5 cm long, while the slot
cut in the channel wall was t 4.3 cm long and 0.4 cm wide.
The channel was
cm in diameter and was lined with a coil of wire forming the cathode.
The cathode coil
(F) extended above and below the slot in the channel wall and was = 6 cm long.
the wire continued up and out of the cathode flow cell and into the sparge
chamber which is described below.
An indentation was cut into the cathode flow cell to accept a porous fitted glass
This plate was cut from a fitted glass disc (supplied by Kontes,
nominal pore size 10-15 gm).
A thin layer of silicon sealant was spread along the
edge of the plate which pressed against the cathode flow cell to form a gasket.
plate was held in
place with a
TFE clamping plate (H)
was attached to the
cathode flow cell by four nylon screws (not shown).
The platinum wire mesh anode (I)
was held in place between the clamping plate and the porous plate.
The linking flange is illustrated in Figure 3-2.
This flange provided a means of
forming gas tight seals between the other parts of the reactor.
which sat in
groove A, provided a gas tight seal between the linking flange and the flange on the
cathode flow cell.
The flow cell was held in place by six PTFE screws.
The PTFE linking flange (not to scale).
a gas tight seal.
Two sets of six threaded holes (C and D) accepted the twelve nylon
screws which held the cathode flow cell and sparge chamber tightly in place.
linking flange was attached to the anode chamber by six additional screws.
in groove E,
provided a gas tight seal between
the anode chamber and the
Sparge gas entered through a compression connection in port F in the
linking flange and was forced into the small holes in the cylindrical extension of the
cathode flow cell.
Figure 3-3 illustrates the assembled generator.
sparge chamber (B) were made of acrylic. The solution
The anode chamber (A) and
1 to be analyzed flowed through
the wall of the anode chamber in 1/8" PTFE plastic tubing (C) which was connected to
the bottom of the cathode flow cell (D) by a reducing compression union (E).
anode mesh (F) was electrically connected to a platinum wire that passed through a
compression fitting (G) in the anode chamber wall.
The oxygen produced at the anode
left the chamber through a port (H) near the top of the anode chamber.
electrolyte solution could be pumped into the anode chamber through a connector (I).
The solution in the anode chamber was stirred during operation by the magnetic stirring
The anode chamber was 17.8 cm long, the outside diameter was 8.9 cm, and
the inside diameter was 6.35 cm.
The sparge chamber was 8.9 cm long and 3.6 cm in outside diameter.
inside diameter was 2.3 cm.
The compression connection that forms the drain of the
sparge chamber (K) was about 1 cm from the base of the sparge chamber.
A length of
The assembled generator (not to scale).
ma a a St,0
,..*.; *S*.eS. 0a
*a. ............................ ~!
OS. 0*05.... *.* abates
a.' ~ S.
* A .
*I S a
*l Sl S
gl 11j ga.*tg L 4pOb
.~ .00*000 .... .asSeeet0eeaSasaa.~~ -
*~ .................. 0 S ...S~ Ste...
-.***** "****'*" **.,
1/8" plastic tubing (L) formed a pressure equalizing arm between the sparge chamber
and the drain tubing (M).
the sparge chamber. Th
A stainless steel compression T-union (N) was attached to
e hydrogen and hydride gases passed out of the generator
through 1/4" TFE plastic tubing (0).
The cathode coil lead passed out of the sparge
chamber through a compression connector (P), and the sparge gas entered through a
compression connection (Q) in the linking flange (R).
additional devices and parts that comprised the complete apparatus.
The oxygen formed
at the anode was forced through a length of plastic tubing (B) and under several inches
of liquid in a cylinder (C).
The pressure in
the anode chamber was regulated by
adjusting the depth of the tubing in this cylinder (the vent of this cylinder was attached
to the laboratory exhaust system).
(D) was connected to the
electrode leads by two wires (E and F).
The solution being analyzed, or blank electrolyte solution,
the generator by a peristaltic tubing pump (G),
was pumped through
while the second pump (H) was used to
keep the anode chamber filled with clean electrolyte solution.
(I) was used to impel the stirring bar in the anode chamber.
A magnetic stirring motor
A loop of tubing (J) on the
liquid drain line prevented the gases from escaping out the drain port.
The drain bottle
(not shown) was carefully vented to the laboratory exhaust system to prevent the release
of toxic hydride gases into the laboratory.
The carter gas flow rate was controlled by
a needle valve in the flow meter (K).
power supply used was a Hewlett Packard Model 6268A, operated in
the voltage regulated mode.
Since this electrolytic cell contained no reference electrode,
only cell potential could be regulated and not the actual cathode potential.
mode of the power supply was used at these high
regulation circuit fluctuated greatly.
It is assumed that the hydrogen generated within
the confined cathode flow cell erratically blanketed part of the cathode surface from the
This may be aggravated by the use of PTFE for the cathode flow cell since
it has a hydrophobic surface and gas bubbles tend to stick to the walls of the cell.
, the actual working surface area of the cathode changed rapidly, and the current
regulation circuit could not respond. In any
production (as observed by FAAS and ICP-
potential is regulated rather than the current.
' event, the rate of hydride and hydrogen
AES) varied less with time when the cell
The total cell potential could be monitored
at one of the ADC channels simultaneously with the atomic spectrometric data.
current could also be monitored.
The Cell Voltage and Overpotential
Contributions to the Cell Potential
There are several types of voltage drops across an electrolytic cell.
The total cell
potential for water electrolysis may be given as,
the conditions used (temperature, acid or base concentration, etc.) used, wr and fla are
the overpotentials of the cathode and anode respectively,
I is the current,
R, is the
electrical resistance of the solution, Rb is a correction factor (with units of Ohms) which
accounts for the effective loss of electrode surface area because of gas bubbles, and the
increase in solution resistance when small gas bubbles are dispersed into the solution.
The resistance of the metal parts of the circuit is denoted by Ra.
The value of
can be estimated using the Nernst equation for the hydrogen half cell reaction, or maybe
found in tables,88 and is about 1.15 V.
The IRm1e term can be ignored in most cases.
The resistance of the solution, R,,
can be estimated as,
where K is the conductivity of the solution, and C is the cell constant, or the distance
between the anode and
the cathode divided by the average surface area of the two
Solution conductivities can be found in tables for most common electrolytes.
it is unlikely
approximation of the true R,.
(Because the electrodes are in very close proximity, but
are separated by a porous glass frit,
must travel further than the apparent
distance and must migrate through narrow channels in the barrier.)
The bubble effect
factor, Rb ,is difficult to estimate theoretically without specific experiments to measure
significance of these resistance terms is that they create heat within the cell (power
that they do not
become too large.
To reduce the
resistance of the cell, the
electrodes should be small
. electrode areas should be large,
the distance between the
, the barrier should be very porous and thin,
the void fraction
(fraction of space occupied by gas bubbles) within the cell should be small, and the
electrolytes to increase conductivity because hydroxide and hydronium ions have very
large ion mobilities.
The overpotential of a half cell is the difference between the equilibrium half cell
given by the Nernst equation, and the actual cell potential necessary to pass
a Faradaic current.
In this cell, ;h,
like the resistive losses
, is largely a nuisance,
the potential loss at the anode requires the use of a larger cell voltage.
On the other
, a large ar is the goal.
If a reaction
, such as the hydride generation reactions,
which has an
more negative than
the hydrogen half
cell reaction for hydrogen
is to be forced to occur in aqueous solution,
a large cathode overpotential is essential.
There are two primary causes of overpotential (or polarization): mass transfer
Concentration polarization is important when the rate limiting
step is the diffusion of reactants toward the electrode.
This is usually important when
the reactants are dilute or when very high currents are used.
The activation polarization
is important when an electron exchange step is the rate limiting factor.
A large concentration polarization should be avoided since this leads to a voltage
drop across a thin solution boundary layer (or double layer) adjacent to the electrode and
does not increase the reducing power of the electrode;
activation energy to the hydride generation reaction. (T
this potential cannot provide
he rate of diffusion across this
double layer is the basis for the electroanalytical method of polarography.)
to reduce concentration polarization is to provide a high concentration of reactants.
Another method is to reduce the effective depth of the double layer.
In practice this is
accomplished by stirring the solution or, in a tubular reactor, but using forced solution
The activation polarization for a reaction that is controlled by a single electron
transfer can be approximated by the famous Butler-Volmer equation,90
The symbol, f, denotes F/RT or the ratio of the Faraday constant to the product of gas
constant and the temperature.
The symbol, a, is the transfer coefficient (about 0.5).90
The term, i, denotes the current density (current per unit area of the electrode).
overpotential is present.
(A zero net current does not imply a zero io).
If a reaction has
a large io, it'
rate limiting step has a low activation energy; if io is low, the activation
energy is high.
At high negative overpotentials,
the second bracketed term is negligible,
and the current is proportion to the first term.
Solving for ',
= n (In (i),
= a + blog(i)
.3 RT log(io)
This relation was derived from empirical data by
Tafel in 1905.90
For well studied reactions
in tables for a large variety of
, such as water decomposition,
io and b can be found
One of the largest factors affecting the
exchange current is the nature of the electrode surface.
Some materials are active,or
to the hydrogen evolution reaction.
Other materials are less active
, such as Hg and
Active electrodes include Pt and Ag.
Pb. Table 3-1 lists some kinetic data
for hydrogen evolution on various materials.
Using the Tafel equation and the data in tables,
such as those in Table 3-1, it is
the cell current varied rapidly (as is noted above) and so only
anaverage current can be measured.
Figure 3-5 shows the current versus cell potential
for the CEHG
exposed surface area of the cathode was roughly 50 cm
. The two curves were acquired
on two separate days,
the glass frit was replaced between experiments.
averaged for 1 mm, and the error bars represent a 90
The current was
data in Table 3-1 and the Tafel expression,
is roughly -0.55 V
the activation overpotential at 110 mA/cm2
. If the same current density were used with a lead cathode,
activation overpotential is about 1.6 V
. One thing should be stressed:
just because an
produce a large overpotential
that it will
produce hydrides quickly.
A surface which is not catalytic for hydrogen generation may
not necessarily be catalytic for hydride generation,
and it may interact with the analyte
in some way that is unfavorable for hydride generation.
Some kinetic data for hydrogen evolution.
ELECTRODE SOLUTION TAFEL SLOPE -LOG i,
b (mV) (A/cm2)
Aga 1MHC1 130 3.7
1 M HSO4 110 5.7
1 M NaOH 120 6.5
Cd" 0.5 M HzSO4 135 10.77
Fe" 0.5 M H2SO4 118 5.18
Hgb 1.0 M H2S04 124 11.8
Pbb 1 M H2SO4 120 12.7
0.5 M NaOH 129 6.5
Pd" 2.0 M H2SO4 120 3.2
Ptb (smooth)0 1 M H2SO4 120 3.6
0.5 M NaOH 117 4.1
a Values from reference 90.
b Values from reference 91.
c Platinum roughened by platinization (fast platinum electrodeposition), has a
larger exchange current.
o o O0 0!
pewnsse eeJe eoe ns epo jeo
. I a S'S II\ rl lnnllHA %SE *fI ~ HA
CHOICE OF ELECTRODE MATERIALS AND ELECTROLYTES
The choice of materials for the anode is limited to conductive materials that will
not corrode or contaminate the anode solution
platinum mesh was chosen.
under a large positive potential.
This mesh has an apparent surface area of 36 cm2 while the
platinum lead wire had a surface area of 31.7 cm2.
the cathode must be made of a conductive material
that will not
corrode or contaminate the solution under a large negative potential.
present electrolytic hydride generator design, the cathode material must be easily formed
into a coil.
A large hydrogen overpotential might also be advantageous.
The electrolyte must be highly conductive. Any strong, or nearly strong, acid or
a base meets this criterion.
It must also not be reduced at the highly negative cathode.
In addition, if the electrolyte forms an oxidation product at the highly positive anode,
the product must not be very soluble in water. An oxidation product may be drawn
through the glass frit and into the cathode flow cell. Once in the cathode flow cell, the
Thus, the cathode activation overpotential would be reduced.
acid and HBr were not used as electrolytes to avoid chlorine or bromine generation at
JiLt ..-- -A
To choose the electrolyte and cathode material, several comparative tests were
CEHG was used
with four different cathodes and three electrolytes to
generate AsH3, SbH3 and H2Se.
The four cathodes were made of platinum, silver, lead
The three electrolytes were 1 M sodium hydroxide, 1 M sulfuric acid,
and 1 M phosphoric acid.
Silver was chosen on
the basis of the availability of pure silver wire in the
It was also included in order to test a relatively inexpensive and harmless
Lead was chosen for its high hydrogen overpotential.
Lead is very toxic, and
pure lead wire is soft.
Care should be taken not to touch the wire with bare hands.
Palladium and platinum were chosen for their chemical resistance and their resistance
platinum can be cleaned in concentrated nitric acid.
particularly convenient during the concomitant tests to be described in Chapter 5.
The output of the hydride generator was directed into the burner of the FAA
standard solutions of As(III), Sb(III) or Se(IV),
was monitored as a function of cell
Most of the possible combinations of the three analytes, four cathodes and
three electrolytes were tried.
Sb or Se
For comparison, a solution having an equal concentration
was also nebulized at the conclusion of each hydride generation
The ratio of the analytical signal from hydride generation to the analytical
signal from solution nebulization was thus calculated for each test.
The four cathodes were 1) a coil of silver wire, 50 cm long (1.0 mm diameter
99.9% pure), 2) a coil of platinum wire, 85 cm long (0.5 mm diameter
3) a coil of lead wire, 50 cm long (1.0 mm diameter
palladium wire, 50 cm long (1.0 mm diameter 99.
- 99.95% pure),
- 99.9985% pure) and 4) a coil of
Reagents and Stock Solutions
The stock arsenic, selenium and antimony solutions used throughout this study
are listed in Table 4-1.
The electrolyte being tested was used to make dilute standard
solutions from these stock solutions daily.
SOLUTION SOLUTE CONCENTRATION SOLVENT
As(III) As2O3 2000 xg/mL 0.05 M NaOH
Sb(III) Sb2O3 1000 pg/mL 0.1 M HCL
Se(IV) Na2SeO3 1000 pg/mL 0.1 M HC1
Preparation of the FAA SDectrometer
The FAA spectrometer was fitted with the appropriate HCL and aligned using
solution nebulization to produce an AA signal for alignment.
As stated in Chapter
each HCL lamp was "warmed up" for at least 30 minutes.
Preparation of the Generator
To prepare the generator for operation,
the electrode to be tested was placed
inside the cathode flow cell, and the generator was assembled. The anode compartment
was rinsed with distilled water and then clean electrolyte solution. The generator could
be quickly filled through the oxygen exhaust port using a large tubing pump, while the
the anode solution
chamber was filled with electrolytic solution to a point just under the oxygen exhaust
The generator was then attached to all the equipment shown in Figure 3-4 and was
connected to the burner chamber by teflon tubing as described in Chapter
of blank electrolyte through the cathode cell was started, and the DC power supply was
turned on to begin electrolysis at 15-20 V
before any samples were introduced to
. The electrolysis was continued for 2-3 min
allow the anode compartment to pressurize.
During this time, the generator was carefully inspected for leaks to prevent the release
of hydride gases into the laboratory.
The anode solution level was always maintained
to within 1 cm of the oxygen vent port by adjusting the flow rate of the pump used to
Next, the flame was ignited, and a solution containing 0.1-1 pg/mL of the
analyte being tested was introduced at
- 6 mL/min for 2 or 3 minutes to pretreat the
electrode. (The production of AsH3, H2Se and SbH3 generally increased slowly when a
clean electrode was used.
After the initial few minutes of generation using a As(III),
Se(IV) or Sb(III), the electrodes did not need to be retreated for at least four hours of
the atomic absorption signal reached a steady state,
the blank electrolyte was again
In these cases, a more concentrated analyte solution, up to 200 pg/mL,
Determination of Optimal Carrier Gas Flow Rate
Arsine was generated using a solution of 10 pg/mL As(III) in 1 M H2SO4 and
a silver electrode.
The solution flow rate was 6.0 mL/min.
The arsenic absorption
signal was recorded as a function of argon carrier, or sparge, gas flow rate.
increased rapidly with increasing argon
the argon flow rate reached 200
The arsenic absorbance reached a plateau after 400-500 mL/min and did not
significantly increase even when 1000 mL/min of argon was used.
An argon sparge gas
rate of 500-700
for use with
confirmed that the carrier gas flow rate had a similar affect on the Sb and Se signals.
Procedure for Cathode and Electrolyte Tests
The optimal flame gas flow rates and flame height,
which gave the maximum
absorbance signal, were determined before each test using solution nebulization.
confirmed that the optimal flame parameters were not different for sample introduction
using CEHG for arsenic and selenium.
However, the optimal air flow rate for antimony
measurement, using CEHG, was so low that the nebulizer could not be used.
purpose of these texts was to compare the hydride generation sensitivity to the solution
nebulization sensitivity, the optimal solution nebulization conditions were used.
While a blank was being introduced to the hydride generator, the power supply
was set to a high voltage setting (around 25 V).
The blank response, or 0%
Next, several standard solutions of various concentration were introduced.
A solution that produced an easily measured absorbance,
linear response range,
but was clearly within the
was chosen for use in the remainder of the test.
Finally, the FAAS response to a blank solution and to the analyte solution was
recorded at a number of cell potentials.
The signal was averaged for 10-30 seconds.
After each test the absorbance due to a standard solution of equal concentration was
measured using solution nebulization.
Table 4-2 lists the conditions used.
All of the
solutions used were in a 2-50 pg/mL range.
Results and Discussion
In Figure 4-1
the atomic absorption signal shows the recorded relative arsenic
absorbance signal as a function of cell potential using 1 M H2SO4 for each cathode.
absorbance of arsenic introduced by hydride generation to the absorbance recorded for
a solution of identical arsenic concentration introduced using the nebulizer.
bars in the figure represent one standard deviation from 4-5 measurements,
represent a best fit,
third order polynomial equation.
Figure 4-2 shows similar data recorded for H2Se generation in 1 M sulfuric acid,
obtained for AsH3 generation using 1 M H3PO4 and 1 M NaOH, respectively. Finally,
Table 4-3 lists some results for a variety of less successful combinations of the various
cathode materials and electrolytes.
Experimental conditions for FAAS measurements.
Spectral bandpass (interference filter used) 0.32 nm
As 193.7 nm
Sb 217.6 nm
Se 196.8 nm
Hydrogen (flame gas) flow rate
As and Se 6.5 L/min
Sb 7.5 L/min
Air (flame gas) flow rate 4.2 L/min
Cathodic (sample) solution flow rate 5 mL/min
Nebulizer sample flow rate (when used) 5 mL/min
Carrier gas (argon) flow rate 600 mL/min
HCL current, boost current
As 25 mA, 60 mA
Sb 18 mA, 40 mA
Se 18 mA, 40 mA
In general, sulfuric acid was the best electrolyte for the generation of all three
There was no significant difference between the effectiveness of Pt, Ag and
Pd cathodes for AsH3 generation. Palladium gave the best results for H2Se generation,
be easily cleaned.
The lead cathode gave the largest signal for SbH3, but it corroded
quickly in sulfuric acid at cell potentials larger than 18 V.
Hydride generation results with several cathode materials and electrolytes.
ELEMENT ELECTROLYTE CATHODE MAX. SIGNAL3 (CELL
Se HzS04 Ag 2.25 0.22 (18 V)
Se NaOH Pt no signal0
Se NaOH Pb no signal
Se NaOH Ag no signal
Se H3PO4 Ag no signal
Sb H2SO4 Ag 0.43 .08 (25 V)
Sb NaOH Pt 0.57 0.09 (20 V)
Sb NaOH Pb no signal
Sb H3PO4 Pt 1.59 + 0.15 (18 V)
Sb H3PO4 Pb 1.64 0..17 (18 V)
a The ratio of the atomic absorbance signal for hydride generation to the atomic
absorbance signal for solution nebulization.
b The potential which gave the maximum signal, or the maximum potential used.
c No signal was observed for solutions of up to 200 ug/mL.