Quartz tube atomizers for hydride generation atomic absorption spectrometry_ mechanism for atomization of arsine

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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 307 
Quartz Tube Atomizers for Hydride Generation Atomic Absorption 
Spectrometry: Mechanism for Atomization of Arsine 
Invited Lecture* 
Jiii DQdinat and Bernhard WelzS 
Department of Applied Research, Bodenseewerk Perkin-Elmer, W-7770 Uberlhgen, Germany 
The mechanism for the atomization of arsine was studied in externally heated quartz tube atomizers of various 
designs. A continuous flow of arsine was generated either by reaction with sodium tetrahydroborate or by direct 
arsine sampling from a cylinder. The latter, together with precautions taken to ensure that inadvertent addition of 
oxygen to the system was minimized, made possible full control of the composition of the atmosphere in the 
atomizer. The effect of atomizer design, purge gas type, purge gas flow rate and atomizer temperature on the 
oxygen supply required for optimum sensitivity and on the curvature of the calibration graph was investigated. 
An extremely low supply of oxygen is required for efficient atomization of arsine in heated atomizers with narrow 
inlet arms. At sub-optimum oxygen supply flow rates, calibration graphs are curved significantly, and gradually 
approach a limiting absorbance, which depends on the flow rate of the oxygen if hydrogen is the main 
component in the purge gas. If there is an excess of argon over hydrogen in the purge gas, the calibration 
exhibits a roll-over. At least a slight stoichiometric excess of hydrogen over oxygen is essential for the 
atomization. The results of the experiments gave a deeper insight into the mechanism of radical formation 
involved in atomization of the hydride in quartz tube atomizers. The influence of various experimental 
parameters on the cross-sectional density of hydrogen radicals in a radical cloud, which controls atomization 
efficiency, was established. Possibilities for improvement of analytical performance, as a consequence of the 
results, are discussed. 
Keywords: Hydride atomization mechanism; atomizer design; quartz tube atomizer; calibration graph curvature; 
hydrogen radicals 
The analytical applicability of hydride generation atomic 
absorption spectrometry (HGAAS) was significantly en- 
hanced by the recent introduction of flow injection (FI) 
hydride generation. However, at present the knowledge of 
the process involved in the atomization of the hydride is 
not sufficient to make the systematic optimization of 
atomizer design possible. Diffusion flames, quartz tubes 
and graphite furnaces are devices used to atomize hydrides 
for AAS. Quartz tubes and graphite furnaces are superior to 
diffusion flames in almost all respects. However, the 
potential of these hydride atomizers with regard to sensitiv- 
ity and prevention of atomization interferences has not yet 
been fully realized. In spite of the differences between the 
atomizers, there are remarkable similarities in the mecha- 
nisms of atomization of the hydride in quartz and graphite 
furnaces.' A detailed knowledge of the processes taking 
place in these atomizers is essential for the efficient 
optimization of their design. 
Quartz tubes are the most widely used atomizers for 
hydrides. Traditionally, atomizers of this type are subdi- 
vided into two groups: externally heated cells and flame-in- 
tube atomizem2 It was found that in both groups, atomiza- 
tion of the hydride proceeded via interaction with hydrogen 
radicals.2-6 It has recently been suggested, on the basis of 
experiments with atomization of selenium hydride, that 
these two types of quartz tube atomizer are, in principle, 
identical. The only difference is in the technique of H 
radical cloud creation: either in a self-supporting small 
flame or by external heating of the gases containing a small 
portion of oxygen. Consequently, the only function of 
elevated temperature reached in the heated quartz atom- 
* Presented at the XXVII Colloquium Spectroscopicurn Interna- 
tionale (CSI), Bergen, Norway, June 9- 14, 199 l . 
t Permanent address: Institute of Nuclear Biology and Radioche- 
mistry, Czechoslovak Academy of Sciences, Videiiskd 1083,142 20 
Prague 4, Czechoslovakia. 
$To whom correspondence should be addressed. 
izers is to initiate reactions between hydrogen and oxygen 
which generate hydrogen radical^.^ 
The aim of the present work was to investigate in detail 
the arsine atomization process in both groups of quartz tube 
atomizers in order to make possible the efficient optimiza- 
tion of the design of such atomizers. 
Experimental 
Reagents 
All reagents were of analytical-reagent grade or higher 
purity. A 0.5% m/v solution of sodium tetrahydroborate 
(Riedel de Haen), stabilized with 0.8% m/v sodium hydrox- 
ide, was freshly diluted from a ten times more concentrated 
stock solution. The stock solution had been filtered and 
frozen for storage and was stable for several months. 
Working solutions of As1I1 (2-10 ng ml-I in 1 mol dm-3 
hydrochloric acid) were obtained by dilution of a 
100 ng ml-l solution. This solution was prepared daily 
from 2 ml of a 10 pg ml-l AsV stock solution by adding 20 
ml of a solution containing 5% m/v potassium iodide and 
5% m/v ascorbic acid, diluting the mixture with 5 mol dm-3 
hydrochloric acid to 200 ml and leaving the solution to 
stand for 10 min to allow for completion of the reduction. 
Hydrogen (99.999%), oxygen (99.999%), pure argon 
(99.9996%) and argon containing both 3% and 0.1% 
oxygen, were obtained from Linde. Hydrogen and pure 
argon were passed through an Oxisorb low-pressure car- 
tridge (Messer Griesheim) to reduce the concentration of 
oxygen to less than 1 x Standard-purity argon 
(99.99%) was obtained from Sauerstoffwerk Friedrich- 
shafen. Arsine (diluted in helium) was obtained from 
Messer Griesheim. Comparison of the AAS signal for the 
diluted arsine with the signal obtained using the standard 
arsine generation procedure yielded an estimated arsenic 
concentration of 73 ng ml-', which was significantly lower 
than the 0.005% v/v arsine concentration stated by the 
manufacturer. 
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308 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 
Arsenic working solutions and sodium tetrahydroborate 
solutions were flushed for at least 15 min with the standard- 
purity argon to remove dissolved oxygen. Flushing for 10 
min reduced the oxygen content in water from the original 
value of about 0.6% v/v to 0.0 12% v/v. The oxygen content 
in water was measured using a Dissolved Oxygen Test Kit 
(Hanna Instruments). 
Spectrometer 
A Perkin-Elmer Model 1 1 OOB atomic absorption spectro- 
meter equipped with a Perkin-Elmer arsenic electrodeless 
discharge lamp, which was operated at 6.5 W, a power 
slightly above the acceptable minimum, was used at a 
wavelength of 193.7 nm, with a 1 nm spectral bandpass. 
Background correction was not employed. The instrument 
was operated in the absorbance mode; hard copies of the 
resulting data were obtained using an Epson FX-85 printer. 
Atomizers 
Quartz tube atomizers of three different designs were 
employed. The commercially available heating device, 
which has a temperature control facility, produced by 
Perkin-Elmer for the MHS-20 and FIAS 200 systems was 
used for heating all of the atomizers to temperatures of 
between 700 and 1000 "C. The heating device heated an 1 1 
cm long central section of the horizontal bar and about 10 
mm of the inlet arm near the atomizer T-tube junction. All 
atomizers were periodically cleaned in 40% hydrofluoric 
acid. 
A standard quartz tube atomizer supplied by Perkin- 
Elmer for the FIAS 200 system8 (FIAS atomizer) is shownin Fig. 1. It is a quartz T-tube with a horizontal bar (total 
length 166 mm) closed at both ends by two removable 
quartz windows, which are attached to the T-tube body by 
silicone sleeves. The hydride, carried by a flow of purge gas, 
is introduced through the inlet arm (90 mm long, 8 mm 
o.d., 1 mm i.d,) in the centre of the horizontal bar and 
removed through plastic tubing connected to nipples at 
each end of the bar close to the windows. The central 
section of the bar (122 mm long, 14 mm o.d., 7 mm i.d.) is 
heated by the heating device, the outer sections of the bar 
(1 7 mm i.d.) are unheated. 
The second quartz tube atomizer used in this work was 
that supplied by Perkin-Elmer for the MHS-20 system 
(MHS atomizer). Whereas the thickness of the walls for 
both the inlet arm and the central section of the bar of the 
FIAS atomizers is 3.5 mm, that for the MHS atomizers was 
only 1 mm. The only other differences between the MHS 
and FIAS atomizers were the internal diameters of the inlet 
arm and of the central section of the bar. For the MHS 
atomizers these diameters were 6 and 12 mm, respectively. 
The flame-in-tube atomizer is similar to the FIAS 
atomizer, differing only in the design of the inlet arm. The 
horizontal bar is identical with the bar of the FIAS 
atomizer. The only differences between the inlet arm and 
Fig. 1 Quartz tube atomizer for the FIAS 200 system 
the arm of the FIAS atomizer are: (i) the diameter (3.5 mm 
id. , 5.5 mm 0.d.) and (ii) it contains a concentric capillary 
which extends to within about 0.5 mm of the T-tube 
junction (this capillary delivers the oxygen). A further 
flame-in-tube atomizer, differing only in the inside dia- 
meter of the central section of the bar (1 5 mm), was used for 
a single experiment. 
Delivery of Gases to Atomizer 
Two methods of arsine delivery were used to produce 
siteady-state signals: (i) standard hydride generation by 
reduction of Asi1' using sodium tetrahydroborate; and (ii) 
direct delivery of arsine from a gas cylinder. 
The experimental set-up for hydride generation is de- 
picted schematically in Fig. 2. Components from the 
Perkin-Elmer FIAS 200 system were used wherever pos- 
si~ble. All the connecting pieces were made from polypropyl- 
ene; the tubing was made from Tygon or silicone. A two- 
channel peristaltic pump (I) was used to deliver the sample 
and tetrahydroborate solutions to a 1 mm i.d. mixing T- 
piece. Typical flow rates of the sample and tetrahydroborate 
solutions were 7 and 4 ml min-l, respectively. The sample 
channel was switched manually between the working 
solution and 1 mol dm-3 hydrochloric acid, which was used 
as a blank to set the baseline. The reaction coil was 120 cm 
long with a 1.6 mm i.d. A standard gas-liquid separator 
(made of glass) from the Perkin-Elmer FIAS 200 system was 
weds8 The internal volume was about 2.2 ml. A peristaltic 
pump (11) was employed to draw the reacted solution from 
the separator to waste. The pump rate was chosen to be 
sliightly higher than chat necessary to remove all the liquid 
fi-om the separator. This prevented liquid from penetrating 
into the atomizer; the fraction of the hydride drawn to 
waste was considered to be negligible. Hydride was carried 
by the flow of argon, together with hydrogen arising 
from decomposition of the tetrahydroborate (about 50 
nil min-l), through a connecting tube (70 cm long, 1 mm 
id. , made of Tygon) to the atomizer. Flows of purge gas and 
oxygen were controlled by rotameters with high precision 
needle valves (ROTA Apparate Dr. Hennig). The flow of 
oxygen to the capillary of the flame-in-tube atomizer (a 
separate channel) was controlled in the same way. The 
minimum controllable gas flow was 2 ml min-l. For the 
introduction of lower oxygen flows, air or oxygen diluted in 
argon (see above) was used. All gas flows were calibrated 
using soap-bubble flow meters. To prevent the diffusion of 
oxygen into the system, components shown within the 
Oxygen 
I 
' Atomizer 
Waste 
B U I I 
Purge 
gas 
Fig. 2 Schematic diagram of the set-up used for hydride genera- 
tion: I and 11, peristaltic pumps; 0x1, Oxisorb cartridge; GL, 
gas-liquid separator; and ROT, rotameter 
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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 309 
broken line in Fig. 2 were placed in a plastic box, which was 
covered by a lid, and flushed continuously with a flow of 
standard-purity argon. For the same reason, the connecting 
tubing between the gas-liquid separator and the atomizer 
was placed in an outer sleeve tubing, which was also flushed 
with argon. 
For the direct delivery of arsine, the purge gas (hydrogen 
or a mixture of hydrogen with pure argon) was intro- 
duced, after passing through the Oxisorb cartridge, directly 
to the connecting tubing (70 cm long, 1 mm id., made 
of Tygon) to the atomizer. A peristaltic pump was used 
to introduce hydrogen flows lower than 2 ml min-'. All 
connections were polypropylene T-pieces of 1 mm i.d. If 
any of the gas lines was not in use, the inlet arm of the 
corresponding T-piece was blocked. The flow of the arsine- 
helium mixture was controlled using a high-precision 
membrane valve on the cylinder (output pressure up to 
2 x los Pa) and a peristaltic pump with a digital speed 
setting (ISMATEC) of 1-100 rev min-l. The lowest 
selectable flow was 0.1 ml min-I. As the flow was not 
exactly stable, even for the same settings of all control 
elements, it was calibrated regularly using soap-bubble flow 
meters. In this work the rate of supply of the analyte is 
expressed as that of the arsine-helium mixture, as the 
arsine concentration in the mixture is not known precisely. 
The valve outlet was connected to the pump with 30 cm of 
Tygon tubing (1 mm i.d.). The outlet from the pump was 
interfaced directly to the connecting tubing leading to the 
atomizer (see above). A polypropylene T-piece was inserted 
just upstream of the pump in order to vent the arsine-hel- 
ium mixture at about 70 ml min-l. Without the venting, the 
atomic absorption signal was lower and the rising edge of 
the signal (after opening the valve) was extremely slow. All 
the other gas flows were controlled as described for hydride 
generation. The peristaltic pump, the Oxisorb cartridge and 
T-pieces connecting the purge gas, oxygen and arsine were 
placed in the box flushed with argon. 
Results and Discussion 
Temperature Distribution Within the FIAS Atomizers 
The temperature distribution, measured using a thermo- 
couple, in the bar and the inlet arm, with the control unit of 
the heating device set to 900 "C, are shown in Figs. 3 and 4. 
It can be seen in Fig. 3 that an increase in the purge gas flow 
rate cooled the junction and transported the hot gas further 
downstream but only gas flows higher than 1 1 min-l 
significantly changed the temperature distribution in the 
bar. The cooling effect of the gas flow was more pronounced 
in the inlet arm. For example, the temperature 4 mm 
upstream of the T-tube junction that was 820 "C in the 
absence of gas flow (Fig. 4) decreased to 790, 700 and 
550 "C for hydrogen flow rates of 100, 200 and 550 
ml min-I, respectively, 
Mechanism of Atomization of Arsine 
It has recently been shown7 that the mechanisms involved 
in selenium hydride atomization and in the reaction of free 
atoms are identical in flame-in-tube and in externally 
heated 'flameless' quartz tubes: H radicals are formed either 
in the flame or by reactions between oxygen and hydrogen 
that take place at the entrance to the hot region of the tube. 
The concentration of radicals is several orders of magnitude 
higher than their equilibrium concentration. The recombi- 
nation of H radicals is too slow to establish equilibrium but 
it is fast enough to keep H radicalsin the form of a cloud 
confined to a small volume. Atomization of hydrides takes 
place in this cloud.' During the course of this work it was 
900 
800 
0 
f 700 
5 600 
f 500 
8 400 
3 
a 9 
300 
1 1 1 1 I 
0 10 20 30 40 50 
Distance from T-tube junction/mm 
200' ' 
Fig. 3 Temperature distribution inside the central section of the 
bar of the FIAS atomizer, heated nominally to 900 "C, for various 
hydrogen flow rates: A, without gas flow; B, 0.3; C, 1; D, 2; and 
E, 6 1 min-' 
8001 700 \ 
600 Y g 500 
g 300 
c 
$ 400 
n 
t- 
200 
100 
L I I I I I I 
0 10 20 30 40 50 60 
Distance from T-tube junction/mm 
Fig. 4 Temperature distribution inside the inlet arm of the FIAS 
atomizer heated nominally to 900 "C, in the absence of a gas flow 
possible to observe, under certain conditions, a type of 
'flame' within the inlet arm of the FIAS atomizer. For 
example, at flows of 50 ml min-l hydrogen, 50 ml min-I of 
argon and 8 ml min-l of oxygen, the flame oscillated in a 
region about 30 mm upstream of the T-tube junction. A 
decrease in the oxygen flow or an increase in the argon flow 
shifted the flame downstream. Increasing the oxygen flow 
moved the flame as far as 50 mm upstream of the junction. 
The temperature measured at this position without any gas 
flow was 70 "C (Fig. 4). Generally, the observation and 
position of the flame depended on the flow rate and 
composition of the gas mixture flowing to the atomizer and 
on the, temperature profile within the atomizer. Thus it was 
possible for the cloud of H radicals to be situated in the inlet 
arm of the atomizer a long way upstream of the T-tube 
junction. 
The hydride atomization proceeds via reactions with H 
radicals.s Atomization of arsine consists of three consecu- 
tive step^:^?^ 
AsH3+H-AsH2+H2 A H = - 196 kJ mol-l (1) 
AsH2+H+AsH+H2 AH=-163 kJ mol-I (2) 
ASH + H-AS + H2 (3) 
It has been reported previously that hydrides cannot be 
atomized in the heated quartz tube atomizer in the absence 
of hydrogen .3* lo- l 
The direct arsine delivery mode was used to investigate 
the influence of the amount of hydrogen in the atomizer on 
the efficiency of atomization of arsine. The investigations 
were made under an argon flow of 50 ml min-l at 800 "C 
and for oxygen flows of between 0.1 and 9 ml min-I. No 
AH= - 163 kJ mol-I 
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3 10 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 
signal was obtained at hydrogen flows of less than twice the 
oxygen flow. When this ratio was just exceeded the signal 
appeared and reached a maximum for slightly higher 
hydrogen flows. For example, no signal was obtained for an 
oxygen flow of 0.95 ml mine' when the hydrogen flow was 
less than 1.8 ml min-l. The signal started to appear then 
and reached a maximum for hydrogen flows of 2.1 and 4 ml 
min-l, respectively. This is consistent with similar observa- 
tions for selenium hydridel* and for arsine15 and suggests 
that the 0 or OH radicals, which should be prevalent for 
hydrogen to oxygen ratios of less than two, do not atomize 
hydrides. 
The data on oxygen demand and calibrations discussed 
below give a deeper insight into the mechanism involved in 
hydride atomization. 
Oxygen demand 
The relationship between oxygen flow rate and arsenic 
absorbance under various conditions (purge gas flow, 
temperature and atomizer type) was studied to determine 
the oxygen demand, i.e., the minimum oxygen supply 
needed to maximize absorbance. 
The standard procedure to determine oxygen demand 
under given experimental conditions began with the mea- 
surement of a 'reference' signal for a sufficient oxygen 
supply. Part of the procedure was verification that the 
'reference' signal fell in the straight portion of the calibra- 
tion graph. However, as shown below, the curvature of the 
calibration graph is pronounced under oxygen deficient 
conditions even for the lowest feasible arsine supply. 
Consequently, in some instances the values determined by 
the present method may be biased towards a higher oxygen 
demand. 
The influence of oxygen flow on absorbance in'the FIAS 
atomizers at various temperatures and purge gas flows is 
illustrated in Figs. 5 and 6. It is shown in Fig. 5 that there 
was a strong dependence of oxygen demand on purge gas 
flow at 700 "C. At purge g a s flows of about 100 ml min-l, 
the oxygen demand was less than 0.3 ml min-'. At higher 
temperatures (Fig. 6), the oxygen demand was largely 
independent of the purge gas flow rate in the range 
investigated, 50-250 ml min-l. 
An extremely low oxygen flow is required for efficient 
atomization in the FIAS atomizers (Fig. 6). This adequately 
explains why optimum sensitivity has often been reported5 
even without the addition of oxygen to such heated quartz 
tube atomizers. 
ot-y , I I I 1 1 
0 1 2 3 4 5 6 7 8 9 10 
Oxygen fiow/mi min-' 
Fig. 5 Influence of oxygen supply at 700 "C on the relative arsenic 
absorbance signal for various purge argon flow rates: A, 95; B, 235; 
and C, 235 ml min-' (contaminated atomizer). FIAS atomizer, 
hydride generation and hydrogen flow rate of 50 ml min-I 
- +-- -*====-I t r .,/x 100 A 
S I 
/ I 
20cH D I 
0 0.05 0.10 0.15 0.20 
Oxygen fiow/mI m i d 
Fig. 6 Influence of oxygen supply at: A, 1000; B, 900; C, 800; and 
D 900 "C (contaminated atomizer) on the relative arsenic absor- 
bance signal for a purge argon flow rate of 50 ml min-'. FIAS 
atomizer, hydride generation and hydrogen flow rate 50 ml min-* 
Also shown in Figs. 5 and 6 are curves for an atomizer of 
the same type in which the inner surface had been 
contaminated by liquid carried from the gas-liquid separa- 
tor into the quartz tube. The oxygen demand was markedly 
increased for this atomizer. In general, this effect was 
irreproducible, and the original sensitivity was restored by 
treatment with hydrofluoric acid. Radical recombination 
takes place primarily on the atomizer surface; the increased 
reactivity of a contaminated atomizer surface leads, there- 
fore, to an increased radical recombination rate, and hence 
to a large increase in the oxygen demand. 
The oxygen demand for purge gas flows of higher than 
2!50 ml min-l was studied using direct arsine delivery and 
hydrogen as the purge gas. In FIAS atomizers heated to 
950 "C the demand increased from 0.08 ml min-l at a 
hydrogen flow rate of 100 ml min-l to 0.4 ml min-l at a 
hydrogen flow rate of 1 1 min-l and to about 30 ml min-l at 
a1 hydrogen flow rate of 3 1 min-'. Oxygen flow rates higher 
than optimum did not change the sensitivity further. For 
example, for a hydrogen flow rate of 1 1 min-l, the 
sensitivity was the same for oxygen flow rates of between 
0.4 and 40 ml min-I. The oxygen demand was considerably 
higher for MHS atomizers in which the i.d.s of the bar and 
of the inlet arm are 1.7 and 7 times greater, respectively, 
than in FIAS atomizers. For a hydrogen flow rate of 
1 1 min-l, at 700 and 950 "C about 10 and 
2 ml min-' of oxygen, respectively, were required to reach 
maximum absorbance. When oxygen was introduced into 
the flame-in-tube atomizer in the same way as to the FIAS 
and MHS atomizers, i. e., with the centred capillary blocked, 
the oxygen demand was similar to that for MHS atomizers. 
These measurements suggest that the cross-sectional 
density of hydrogen radicals is the factor determining the 
efficiency of hydride atomization, in analogy with observa- 
tions made for flame-in-tube at0mizers.*9~ The higher 
oxygen demand in cells with a larger inlet arm immediately 
reflects the fact that the same oxygen supply and, conse- 
quently, the same production of H radicals results in a 
lower cross-sectional density of radicals in the larger tube. 
Similarly, the higher oxygen demand in the flame-in-tube, 
compared with the 'flameless'arrangement, for a hydrogen 
flow rate of 940 ml min-l (Fig. 7) appears to be due to a 
difference in the location of the H radical cloud: in the 
'flameless' mode, the cloud is formed in the inlet arm, 
which is hot enough to initiate the reaction (see Fig. 4) but 
in the flame-in-tube mode the cloud is located in the 
broader T-tube junction. This explanation is supported by 
the fact that for a flame-in-tube atomizer with the same 
inlet arm but an optical tube with a wider i.d. of 15 mm, the 
oxygen demand in the 'flameless' mode was the same as in 
the cell with an optical tube diameter of 7 mm, whereas in 
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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 31 1 
the flame-in-tube mode it was about twice as high. The 
dependence of the oxygen demand on gas flow rates in FIAS 
atomizers indicates that the cloud is transported down- 
stream to the wide sections of the atomizer by the increased 
gas flow. Consequently, more oxygen is required to generate 
the same cross-sectional density of radicals. The dramatic 
increase in the oxygen demand when the gas flow rate is 
increased from 1 to 3 1 min-l is obviously due to 
overcoming the 'threshold' of the cooling of the T-tube 
junction (Fig. 3). The much lower temperature in the 
junction then shifts the radical cloud into the wide diameter 
optical tube. In the flame-in-tube atomizer, there was no 
significant difference in the oxygen demand from the results 
shown in Fig. 7 even for purge gas flow rates of up to 
6 1 min-l. This could be attributed to the fact that in the 
flame-in-tube atomizer in contrast to the 'flameless' atomiz- 
ers the radical cloud remains in a stable position at the tip 
of the capillary, regardless of purge gas flow. 
The 10-fold higher oxygen demand in FIAS atomizers at 
700 "C for a purge gas flow rate of 235 ml min-l compared 
with 95 ml min-l (Fig. 5 ) is accounted for by a similar 
mechanism, reflecting a temperature dependent difference 
in the position of the radical cloud in the inlet arm of the 
atomizer: at 700 "C, the cloud is closer to the T-tube 
junction than at higher temperatures. When the purge gas 
flow is increased the cloud moves even further downstream 
to a point where the inlet arm becomes wider. 
The decrease in oxygen demand with increasing tempera- 
ture (Figs. 5 and 6) and analogous observations for 
selenium hydride' suggest an increased efficiency of H 
radical formation and/or a decreased rate of H radical 
recombination at higher temperatures. However, as shown 
above, the decrease in the demand might be partially due to 
the effect of temperature on the position of the radical 
cloud in the atomizer. 
Generally, there was no significant difference in the 
absorbance obtained for hydride generation and for direct 
arsine introduction. The only exception was the absorbance 
when a very low oxygen supply was used. It is shown in Fig. 
6 that 70, 90 and 95% of the maximum absorbance was 
measured at 800,900 and 1000 "C, respectively, for hydride 
generation when no oxygen was added to the flow of 100 
ml min-l of a 1 + 1 mixture of hydrogen and argon. All of 
the observations are fairly compatible with an oxygen flow 
rate to the atomizer of about 0.04 ml min'l. Under the 
same conditions, except that the direct introduction of 
0 0.5 1.0 1.5 2.0 , 2.5 3.0 3.5 I/ 32.0 32.5 
Oxygen fiow/ml min-l 
Fig. 7 Dependence of arsenic absorbance on oxygen flow intro- 
duced to either: A, the oxygen delivery capillary of the heated 
flame-in-tube atomizer or B, mixed with the other gases upstream 
of the atomizer ('flameless' mode). Temperature 950 "C, direct 
arsine introduction, arsine-helium mixture supply rate of 2.0 
ml min-l and hydrogen flow rate of 940 ml rnin-l 
arsine was used, much smaller absorbances were observed. 
For example, only 8% of the maximum absorbance was 
measured at 800 "C. As estimated from the flow rate of 
oxygen required to obtain 50% of the maximum absorbance 
(0.15 ml min-l), this is compatible with an oxygen flow rate 
of 0.005-0.01 ml min-l. Such an oxygen supply cannot 
arise from either purge gases or from reagent solutions (see 
Experimental); However, these oxygen flow rates are com- 
patible with oxygen diffusion into the apparatus from the 
argon atmosphere, which might contain more than 5% air, 
through plastic tubing, which is fairly permeable to oxygen 
diffusion.16 The higher flow rates estimated for the hydride 
generation assembly might be explained by the greater 
length of tubing that is employed in comparison with the 
set-up for direct arsine introduction, which results in a 
higher rate of diffusion into the system. 
Cali brat ion graphs 
The shape of the calibration graphs depends markedly on 
atomizer type, oxygen supply and purge gas flow and 
composition. 
The calibration graphs shown in Fig. 8 are for a medium 
flow of pure hydrogen of about 100 ml min-l as the purge 
gas with an oxygen supply too low to reach full sensitivity 
and also with an oxygen supply sufficient to give maximum 
sensitivity. The curves for oxygen flows of 2.4 and 
0.4 mi min-l did not differ significantly. A calibration 
graph without added oxygen is also shown. The calibration 
graph for an adequate oxygen supply is reasonably linear 
but it is curved when there is an oxygen deficiency. Fig. 9 
shows that the calibration graphs are markedly influenced 
by the hydrogen flow: at low hydrogen flow, but with all 
other parameters being the same as for Fig. 8, the 
calibration graph is curved even for an oxygen flow of 1.2 
ml min-l. This did not change significantly for oxygen flow 
rates of 0.03 (Fig. 9) and 2.4 ml min-l. The oxygen supply 
could not be increased much further at low hydrogen flows 
as this resulted in a pronounced shift in the baseline and an 
increase in noise. 
The calibration graphs shown in the figures exhibit 
curvature in two distinct situations: (i) under oxygen 
deficient conditions (oxygen deficiency curvature) and (ii) 
when there is an adequate supply of oxygen but curvature 
becomes more pronounced at low hydrogen flow rates (low- 
flow curvature). 
Calibration graphs apporoaching a low limiting absor- 
1.4 
1.2 
al 
f 1.0 
n $ 0.8 
a 0.6 
0.4 
n 
0 1 - 
B 
0.21 J/ C 
I TLx*-- " I 
of$ , 
c 
b I I 1 1 I I 
0 1 2 3 4 5 6 7 8 9 
Analyte supply/ml min-' 
Fig. 8 Calibration graphs with a hydrogen flow rate of 
93 ml min-' and for: A, adequate oxygen flow rate (2.4 ml min-l); 
B, oxygen deficiency (0.03 ml min-l); and C, no oxygen flow. 
Temperature 950 "C, FIAS atomizer and direct arsine introduction 
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312 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 
I I I I I 1 
0 1 2 3 4 5 6 7 8 9 
Analyte supply rate/rnl rnin-' 
Fig. 9 Calibration graphs with a hydrogen flow rate of 
46 ml min-I for oxygen flow rates of: A, 1.2 ml min-l; B, 0.03 
ml min-l; and C, no oxygen flow. Temperature 950 "C, FIAS 
atomizer and direct arsine introduction 
bance are typical of oxygen deficiency curvature. The 
curvature is apparently due to an insufficient population of 
H radicals in the cloud: at a very low rate of supply of arsine 
the sensitivity approaches that obtained for an adequate 
oxygen supply as even the low H radical population is 
sufficient for full atomization, but the atomization effici- 
ency decreases with increasing analyte supply as the 
radicals are depleted by reaction with arsine. The oxygen 
deficiency curvature thus results from a type of radical 
population interference59l7 in which the hydride is the 
interferent and analyte at the same time. Onlya limited 
number of hydride molecules are atomized according to 
reactions (1)-(3). The fact that the calibration graphs 
approach the limiting absorbance suggests that reactions (2) 
and (3) are much faster than reaction (l), so that ASH, and 
ASH molecules compete successfully with AsHJ molecules 
for the insufficient number of H radicals. For any oxygen 
flow there is a maximum supply of arsine that can be 
atomized, i.e., even the lowest oxygen supply can fully 
atomize a certain flow rate of arsine. The number of 
molecules atomized, corresponding to the limiting absor- 
bance, is determined by the oxygen supply. For example, it 
can be derived fairly straightforwardly from Fig. 8 that the 
maximum arsine flow that can be atomized for an added 
oxygen flow rate of 0.03 ml min-l (1.3 pmol min-l) is 0.5 
nmol min-l, based on an arsenic concentration in the 
arsine-helium mixture of 73 ng ml-l. In summary, a three 
orders of magnitude excess of oxygen over arsine is 
required for complete atomization under the conditions as 
given in Fig. 8. A lower oxygen supply or any reduction of H 
radical lifetime inevitably induces a lower atomization 
efficiency. Consequently, when a compound which reacts 
with H radicals penetrates the atomizer, pronounced 
interferences in the atomization should be expected under 
conditions of low oxygen supply. 
In FIAS atomizers heated to 950 "C, calibration graphs 
were linear over the entire absorbance range tested 
(up to 0.7-0.9) at hydrogen flow rates of between 200 and 
1000 ml min-l and with an oxygen supply sufficient for 
maximum sensitivity. Under oxygen deficiency conditions 
and at a hydrogen flow rate of 1 1 min-l, the calibration 
graph was linear up to an absorbance of about 0.2, which 
corresponded to analyte being supplied at a rate of more 
than 2 ml min-l. The sensitivity was 50% of that obtained 
with an adequate oxygen supply. Significant curvature was 
observed at higher absorbances but the limiting absorbance 
was not reached even for the maximum analyte supply rate 
tested (10 ml min-l). Apparently, in contrast to low 
hydrogen flow rates, there is a minimum oxygen supply 
required for full atomization at even the lowest arsine flow 
rate. This indicates that atomization reactions (1)-(3) are 
not fast enough compared with the 'residence time' of the 
arsenic species within the H radical cloud when the flow 
rate of the purge gas is high. 
It has been shown above that a hydrogen flow rate of 
more than double that of the oxygen is required in order to 
observe an analytical signal. The data given in Fig. 10 
indicate that the fraction of hydrogen in the purge gas also 
an'ects the shape and slope of the calibration graph. If the 
amount of argon present is in excess of the hydrogen in the 
purge gas, the calibration graph under oxygen deficient 
conditions is not only curved, as shown in Figs. 8 and 9 for 
comparable total gas flow rates, but there is a marked roll- 
over. This becomes more pronouned at 700 "C. With an 
adequate supply of oxygen the calibration graph is linear, 
and the curvature is only about 5% at an absorbance of 1.2 
for both 950 "C (Fig. 10) and 700 "C. If the hydrogen flow 
rate was increased to 50 ml min-l using the same argon flow 
rate, no roll-over was observed and similar data as for pure 
hydrogen (Fig. 8) were obtained. This can be interpreted in 
terms of the influence of argon on the relative rate of 
reactions (1)-(3): reaction (1) or (2) becomes much faster 
than reaction (3). Thus an increasing proportion of the 
reacted analyte is transported out of the radical cloud in an 
intermediate, non-atomic form. The calibration graph roll- 
owler observed for very high selenium hydride supply rates 
has been described previously by Agterdenbos et a1.18 The 
roll-over manifested itself as a 'double peak': the leading 
and falling edges of the hydride supply function appeared as 
two peaks separated by zero or a lower absorbance for the 
high values of the hydride supply function. 'Double peaks' 
were observed by Welz and Guoi9 under conditions of 
oxygen deficiency for FI generation of arsine and stibine. 
These observations were made with a purge gas containing 
law proportions of hydrogen in nitrogen18 and argon, l9 
respectively. 
The low-flow curvature for MHS atomizers (Fig. 11) was 
much more pronounced than for FIAS atomizers. However, 
the curvature was less pronounced for higher gas flow rates. 
For flow rates above 400 ml min-l, and for adequate 
oxygen supply, the calibration graphs were linear at least up 
t s an absorbance value of 0.7. Low-flow curvature was also 
observed, in the flame-in-tube atomizer operated in the 
'flameless' mode. This is illustrated in Fig. 12. The curve is 
representative for oxygen flow rates of between 0.1 and 
4.6 ml min-l. Compared with the FIAS atomizers, the 
curvature is more pronounced and occurs at higher hydro- 
gen flow rates (compare Figs. 8 and 9); it is similar to the 
MHS atomizers (compare with Fig. 11). The situation 
changes dramatically in the flame-in-tube mode: with an 
adequate oxygen supply, calibration graphs are linear up to 
an absorbance of 0.9 (Fig. 12). The slope is similar to that of 
Analyte supply rate/ml min-' 
Fig. 10 Calibration graph for: A, adequate oxygen supply 
(0.4 ml min-I at 950 "C) and B and C, oxygen deficiency (0.008 ml 
min-l at 950 "C and 0.06 rnl min-l at 700 "C, respectively) with 
purge flow rate of 50 ml min-' for argon and 8 mi min-I for 
hydrogen. FIAS atomizer and direct arsine introduction 
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JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 313 
0 1 2 3 
Analyte supply rate/mI min-' 
Fig. 11 Calibration graphs with low hydrogen flow rate (110 
ml min-l obtained with two different MHS atomizers for: A, 
MHS-1 and B, MHS-2 with an adequate oxygen supply (2.4 
ml min-I); and C, MHS-1 with oxygen deficiency (0.03 ml min-l). 
Temperature 950 O C and direct arsine introduction 
0 0.5 1 .o 
. I , " " ' 
2 3 4 5 6 7 8 9 
Analyte supply rate/ml min-' 
Fig. 12 Calibration graphs obtained with the flame-in-tube 
atomizer heated to 950 "C for: A, an adequate oxygen supply 
(2.4 ml min-I) introduced through the capillary; and B and C, 
insufficient oxygen supply (0.6 ml min-I) introduced through the 
capillary (B) or (C) mixed with the gas upstream of the atomizer. 
Hydrogen flow rate 93 ml min-l and direct arsine introduction 
the calibration graph for the 'flamless' mode extrapolated to 
zero analyte supply (Fig. 12). Only oxygen deficiency 
curvature appears in the flame-in-tube mode. 
In summary, the extent of the low-flow curvature for a 
given hydrogen flow rate depends on the inner diameter and 
the quality of the surface of the atomizer: higher hydrogen 
flow rates are required to eliminate the curvature in wider 
and/or contaminated atomizers. It has been shown previ- 
ously that the decay of analyte free atoms is enhanced for 
lower purge gas flows and for wider atomizer diameters; the 
decay is controlled by the surface quality of the atomizer 
and it is significantly increased in the presence of other 
volatile hydride forming elements in the ample.^^*^ This 
might suggest that the low-flow curvature is due to the decay 
of free atoms and is enhanced with higher analyte supply. 
The curvature of the calibration graph with a low-flow of 
purge gas, which could not be straightened by an increase in 
oxygen supply, has already been observed for the atomiza- 
tion of selenium hydride by Agterdenbos et a l l4 These 
workers ascribed the phenomenon to analyte dimerization 
in the atomizer, which does not contradict the above 
suggestion. However, a radially inhomogeneous distribu- 
tion of free atoms in the observation volume of an 
atomizer, must also result in curved calibrationgraphs.** 
The inhomogenous distribution is also more likely to occur 
under low gas flow rates and in wider as well as in 
contaminated atomizers. Nevertheless, neither of the above 
two explanations of the low-flow curvature is entirely 
consistent with the fact that the curvature of the calibration 
graph obtained with oxygen mixed with the gas upstream of 
the atomizer is eliminated when oxygen is introduced in the 
flame-in-tube mode (Fig. 12). 
Conclusions 
Considering results presented in this work together with 
recent investigations6v7 the following mechanism for hy- 
dride atomization in quartz tube atomizers emerges. A 
cloud of H radicals is formed by reactions between oxygen 
and hydrogen either in a flame burning at the end of an 
oxygen delivery capillary or at the beginning of the hot zone 
of externally heated atomizers. The cloud fills only a small 
portion of the volume of the atomizer. The exact position of 
the cloud, if not fixed by the capillary, is controlled by the 
temperature profile within the atomizer, by the purge gas 
flow and composition, and by the atomizer design. The 
hydride is atomized within the radical cloud. The number 
of H radicals is determined mainly by the oxygen supply to 
the atomizer, however, the amount of radicals is not a 
decisive factor in ensuring that an efficient atomization is 
achieved but their cross-sectional density in the cloud is. 
Consequently, the oxygen demand is controlled more by the 
i.d. of the atomizer section where the H radical cloud is 
situated than by the purge gas flow rate. 
How to reduce the interference effect of volatile com- 
pounds penetrating into the atomizer and hence depleting 
the H radical concentration (radical population interfer- 
ence5vI7) follows conclusively from the above consideration. 
Obviously, the radical population interference, as well as 
the oxygen deficiency curvature, are controlled by the cross- 
sectional density of H radicals in the cloud. The density is 
controlled by the oxygen delivery rate to the atomizer and 
by the i.d. of the part of the atomizer where the radical 
cloud is located. The extent to which the oxygen delivery 
rate can be increased is limited. Consequently, it is highly 
recommended that the part of the atomizer, where the 
radical cloud is located, is kept as narrow as possible. 
The authors are indebted to B. Radziuk for valuable 
comments and assistance in preparation of the manuscript. 
J. D. gratefully acknowledges a research fellowship granted 
him by Bodenseewerk Perkin-Elmer GmbH and the sup- 
port by the internal grant of CSAS no. 41 147. 
References 
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Anal. At . Spectrom., 1989, 4, 143. 
2 DEdina, J., and RubeSka, I., Spectrochim. Acta, Part B, 1980, 
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3 Welz, B., and Melcher, M., Analyst, 1983, 108, 213. 
4 Welz, B., and Schubert-Jacobs, M., Fresenius' 2. Anal. Chem., 
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5 DEdina, J., Prog. Anal. At. Spectrosc., 1988, 11, 251. 
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314 JOURNAL OF ANALYTICAL ATOMIC SPECTROMETRY, MARCH 1992, VOL. 7 
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Paper I /02944D 
Received June 17, 1991 
Accepted November 15, 1991 
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