Anal. Chem. 1988, 60,2031-2035
203 1
Volatilization of Arsenic as the Trichloride for Sample Introduction in Atomic Spectroscopy Solomon Tesfalidet and Knut Irgum*
Department of Analytical Chemistry, Uniuersity of Umei, S-901 87 Umel, Sweden
A method for volatilization of arsenic for facile and interference-free introduction of the element into a flame atomic spectrometer Is presented. The generation Is based on converslon of trlvalent arsenic in solution into low-boiling arsenic trichloride by strong hydrochloric acld. The ASCI, gas Is produced In a flow system and separated from the liquid sample stream by a permeable tube, whose outer side Is purged with hydrogen gas. Subsequent detectlon of As( I I I ) is achleved by using an oxygen-hydrogen flame-in-tube atomizer and atomic absorption spectrometry. The tolerance toward the interferents cobalt, copper, nlckei, and iron is improved conslderably, when compared with hydride generation methods using sodium tetrahydroborate(I1 I ) as reagent.
Many metalloid elements can be successfully determined by atomic absorption spectrometry after volatilization as hydrides, a technique that was introduced by Holak ( I ) . Conversion of these elements into gaseous form allows them to be determined with good sensitivity and selectivity, since they are largely separated from the matrix in the detection stage. For a recent review, see, e.g., ref 2. From reports in the literature, however, hydride generation appears to be extremely sensitive to interference from certain common transition-metal ions (3-6). These interferences have recently been shown to stem from the use of the strong reducing agent sodium tetrahydroborate (5-8), a reagent that was introduced by Schmid and Royer (9)as a highly efficient hydride ion donor. Various attempts have been made to explain and reduce these interferences (%5,10, II), but several of the more common transition-metal ions still interfere severely. This work was from the start aimed toward the development of a method for the volatilization of such elements without the use of tetrahydroborate(II1) as reduction reagent, in order to reduce the interferences from transition-metal ions. Arsenic was chosen as a model substance, since the body of literature on the hydride generation of this element is larger than that of many other hydride-forming elements. In our first experiments we used a flow system where an electrochemical reduction step was substituted for the addition of reducing reagent, and a gas permeable tube (hereafter referred to as a GPT) was used for liberating the gas formed in the generating step. During that work we noticed that i t was not a t all necessary to reduce the arsenic in order to make it traverse the membrane. Volatilization could be accomplished simply by making the hydrochloric acid concentration in the sample sufficiently high. We have followed up this concept and can here present the first data on a method for the sample introduction based on AsC1, formation. EXPERIMENTAL SECTION Apparatus. A flow system with a porous gas permeable membrane tube was used to test the generation of gaseous AsC13. A four-channel peristaltic pump (Gilson Minipuls 3) was used to pump the carrier solution at a flow rate of 1mL/min. Samples were introduced with another peristaltic pump through an Omnifit
six-port PTFE injection valve, which was fitted with injection loops of 80- or 320-pL volume. A gas permeable membrane tube 0.4 m long with 1mm i.d., made from porous PTFE (Gore-Tex, Type TA001, W. L. Gore & Co., Haar, FRG), was used for separating the generated gaseous species from the carrier flow. The tube was assembled with polypropylene tees and jacketed in 4 mm i.d. poly(viny1 chloride) tubing as shown in Figure 1. Thick-wall silicon rubber tubing was used to connect the inlet and outlet of the outer tube to the H2(g)delivery tube and the atomizer, respectively. See Figure 2 for a schematic drawing of the flow manifold used. A Perkin-Elmer Model 372 atomic absorption spectrophotometer (AAS) was used for monitoring arsenic at the 193.7-nm line. An arsenic electrodelessdischarge lamp operated at 8 W was used as light source. The spectral band width of the AAS instrument was set to 2 nm, and the deuterium background corrector was used. The atomizer was an (oxygen-hydrogen) h e - i n - t u b e type, built by Dedina (12). The oxygen and hydrogen gas flow rates were 15 mL/min and 2 L/min, respectively. Reagents and Solutions. All chemicals used were of reagent grade. Water was purified with Milli-Q (Millipore, Bedford, MA) equipment. Stock solutions, 1.ooO g/L in As(II1) or As(V), were prepared by dissolving Ultrapure As203and As205(Ventron-Alfa, Karlsruhe, FRG), respectively, in 20 mL of 1 M NaOH and diluting to 1L with 2 M HCl. A 2% (w/v) solution of NaBH, was prepared by dissolving sodium tetrahydroborate(II1) (Merck, Darmstadt, FRG) in 0.1 M KOH. As(II1) solutions containing the potential interferents Cu(II), Fe(III), Co(II), and Ni(I1) were prepared by adding their chloride salts to 20 mL of 2 pg/mL As(II1) in 9 M HC1. Procedures. The continuous sample flow experiments were carried out by pumping an acidic solution containing arsenic(II1) at the appropriate level directly into the GPT with the pump normally used for the carrier solution. Pumping continued until the absorption reached a plateau; thereafter the carrier stream was again switched to the carrier solution. In the series of flow injection experiments, the samples (80 pL of a solution of 10 pg/mL As(II1))were injected into the flow line of the carrier solution through the injection loop. The hydrochloric acid concentrations of the samples were adjusted to match the varying HCl concentrations of the carrier solutions. The stopped flow experiments were carried out by pumping the carrier solution until the entire injected sample was within the GPT. The pump and the chart recorder were then switched off and started, respectively, at the same time. The flow remained stopped until the signal peak had returned to the base line. Interference Studies. Metal ion interference studies were carried out in the flow injection mode with 9 M HCl as carrier solution. Samples containing 2 pg/mL As(II1) and varying concentrations of the interfering metal ions (one at a time) were injected into the flow line with the 80-fiL loop. Influence of Temperature on the Signal Delay. The temperature of the membrane reaction tube was increased from the normal temperature of 22 "C to 70 " C by the means of a temperature-controlled heat gun. This was done while the sample was situated in the GFT by using the stopped flow mode described above. A solution of 5 pg/mL As(II1) in 2 M HCl was used as sample. The carrier solution was 2 M HCl. Electrochemical Verification of the Formation of AsCl,(g). The membrane tube was operated in continuous mode with 20 mL of 5 fig/mL As(II1) pumped through the GPT at a rate of 1 mL/min. Hydrogen gas was used to purge the outer jacket of the reactor. An impinger containing 10 mL of 2 M HCl was used to trap the AsC13(g)from the gas emerging from the
0003-2700/88/0360-2031$01.50/00 1988 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
HzAsOzCl
tA
+ HCI
F=
HAsOC1,
+ HzO
K = 3.4
X
R
(2)
HAsOC1,
Flgure 1. Gas permeation tube assembly: 1, Gas permeable tube; 2, PVC outer tube; 3, polypropylene tee; SI, sample inlet; A, to atomizer; GI, gas inlet; W, to waste.
-- 1
Pump
3
2
Sample Waste
% H L lc
02
1 -q I---
Figure 2. Schematic diagram of the flow manifold: 1, flow system; 2, AsCI,(g) gas generation and separation; 3, atomizer cell.
membrane tube. The impinger solution was then directly analyzed for As(II1) by differential pulse polarography, using a PAR 174A electrochemical analyzer with a Model 303 static mercury drop electrode (Princeton Applied Research, Princeton, NJ). A similar experiment was also performed with NaBH4 as reductant. This time the reaction took place in a 50-mL round-bottom flask, into which 20 mL of 5 gg/mL As(II1) and 8.8 mL of a 2% (w/v) solution of NaEiH, were pumped at a rate of 1.0 and 0.44 mL/min, respectively. Reduction of As(V) to As(II1). One milliliter of a 0.6 M solution of potassium iodide in water was added to 10 mL of the sample solution, which was in 9 M HC1. The reaction was allowed to proceed for at least 15 min before the sample was subjected to analysis using the GPT in flow injection mode. Role of the Carrier Gas. In this experiment, helium was used at a rate of 1.5 L/min in lieu of hydrogen gas for purging the outer jacket of the GPT. Before the helium purge gas entered the atomizer, H,(g) was added at a rate of 200 mL/min to provide fuel for the flame in the atomizer tube.
RESULTS AND DISCUSSION In the early stages of this work, when we saw that trivalent arsenic could be volatilized without adding a reducing agent to the solution, our initial assumption was that the hydrogen gas used M purge the outer sleeve of the GPT was permeating into the liquid flow and reducing the arsenic to arsine. In order to check if this was the reason for the volatilization of As(III), we substituted helium for hydrogen gas. The results obtained with helium were practically identical with those obtained with hydrogen gas, the only difference being a slightly higher noise level, which can be attributed to the change in flame stoichiometry and the dilution of the flame gases by helium. This proves that the hydrogen gas does not contribute chemically to the reactions that cause arsenic volatilization in the GPT. Arsine, or any other arsenic species requiring reduction of As(II1) to lower oxidation stages, could thus not be the species responsible for the evolution of the gaseous arsenic in this case. These findings directed our interest toward investigation of the mechanism behind the volatilization of arsenic without reduction, and we consequently checked what arsenic species could be present under the conditions prevailing in the solution where volatilization took place. The formation constants of various chloro complexes of As(II1) known to exist in hydrochloric acid at various concentrations (13)are shown in eq 1-3. H3As03
+ HC1*
HzAsOzCl+ HzO
K = 8.5 X lo-' (1)
+ HC1+
ASCI,
+ HzO
K = 6.25 X
(3)
According to Myron (13),AsC1, is the only arsenic species present in significant quantity in 12 M HCl. AsC13 and As(OH)C12are the predominant species in 9 M HC1, while in 5.9 M HCl, AsC13, As(OH)C12, As(OH),Cl, As(OH)Z+,and H ~ A s O ~ all exist in significant quantities. Nelson and Kraus (14) reported, on the other hand, that As(II1) is adsorbed by an anion exchange resin at hydrochloric acid concentrations higher than 10 M. This is indicative of formation of an anionic species, probably ASCI,. Optimum hydrochloric acid concentration for AsC13 formation should thus be in the vicinity of 9 M. Liquid AsC13 has a boiling point of 130 OC a t atmospheric pressure. Even though this may seem high, its heat of vaporization is low compared with, e.g., water. It will also evaporate quite rapidly from aqueous solutions, even at room temperature, since it does not form hydrogen bonds with water. We thus conclude that the only possible gaseous As(II1) species that can be evolved under these circumstances is AsCl,(g). Influence of Hydrochloric Acid Concentration on AsC13 Volatilization. The rate at which arsenic is volatilized is strongly dependent on the HC1 concentration used, and thus on the type of hydrochloric acid-As(II1) complex present in the sample solution. The signals obtained for 320 p L of 10 pg/mL As(II1) in various hydrochloric acid concentrations by using the stopped flow mode are shown superimposed in Figure 3. Some data extracted from this figure are also summarized in Table I. It is apparent that the time it takes for the signal to appear depends strongly on the hydrochloric acid concentration used. The signal appears immediately as a sharp peak at elevated hydrochloric acid concentrations, but when the HC1 concentration is decreased, the signal does not appear until after a delay. The shape of this delayed signal is also largely independent of the HC1 concentration within the range tested. When an induction period is seen, the hydrochloric acid concentration is too low to promote immediate vaporization of AsC13, so the arsenic dwells in the tube until the evaporation of water produces a sufficiently high HCl concentration inside the tube to effect formation of AsC13 in the solution. The gradual increase in HC1 concentration as a result of successive loss of water through evaporation favors the formation of AsC13(g),and can be written as HC1
As(OH)2Cl -~,br As(0H)ClZ
HC1
-HtO
AsC13
(4)
We are certain that a significant evaporation of water from the G P T actually takes place when the flow is stopped, because bubbles appear at the membrane outlet when the carrier flow is turned on again after the prolonged stops necessary for the signal to appear a t low hydrochloric acid concentrations. When the flow injection system was used (Figure 4) no detectable arsenic signal could be observed with 2 and 4 M hydrochloric acid. This is due to the small fraction of the arsenic prevent as AsC13 at these comparatively low acidities. In these experiments the carrier stream was pumped at a rate of 1mE/min, which gives a residence time of the sample in the GPT of less than 20 s. The volatilization of AsC13(g) effected by evaporative concentration takes 15 and 5 min in 2 and 4 M HC1, respectively, so AsCl, formation can therefore not take place by this mechanism either. Further evidence for the formation of AsC13 gas is provided by the experiment done to study the effect of temperature
ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
I
0
2033
I
I
I
20
40
60
T i m e/m in Flgure 5. Signals obtained from 80 pL of 10 pg/mL As(1II) in 2 M HCI by use of the stopped flow mode with the gas permeabletube kept at two different temperatures: A, 22 OC; B, 70 OC.
Table 11. Differential Pulse Polarographic Diffusion Currents (Zwak) Obtained in the Electrochemical Analysis of Trapped As(II1) from Different Test Mixtures
[HCll, M
I
I
0
20 Timelmin
I
I
40
60
Figure 3. Signals obtained from injections of 320 pL of 10 pg/mL As(II1) In various HCI concentrations using the stopped flow mode: A, 9 M; B, 7 M; C, 6 M; D, 4 M; E, 2 M HCI.
0.4
1
1
1
1
1
1
1
5
Time/min Figure 4. Influence of hydrochloric acid concentration on the signal when 80 pL of 10 pg/mL As(II1) is injected by using the flow injection mode: A, 11 M; 8, 9 M; C, 7 M; D, 6 M; E, 4 M; F, 2 M HCI.
Table I. Dependence of Peak Height, Peak Area, and Signal Delay on HCl Concentration for 320 p L of 19 pg/mL Arsenic(II1) in the Stopped Flow Mode
[HCl], M
peak height, AU
peak area, AUSmin
2
0.215 0.255 0.260 0.275 0.875
3.32
9
pg/mL
IF&, pA
2.0
0.2
2.0 2.0 2.0
0 0.3 0 0.7 0
9.0 9.0 9.0 9.0
20 2.5
generation methodb GPT RBF GPT RBF GPT RBF GPT RBF
"Indicates whether or not 8.8 mL of 2 % NaBH, was added to the HC1-arsenic mixture. GPT, gas permeable tube; RBF, round bottom flask.
0
4 6 7
[As(III)], [NaBH4]"
4.40
4.31 4.26 4.10
signal delay: 9
1850 1080 610
215 20
"Time elapsed from when the gas permeable tube was entirely filled with sample to peak maximum. on the rate of AsC1, gas evolution; see Figure 5. By elevation of the temperature of the G P T from 22 to 70 OC, the AsC1, gas evolution rate, i.e., the time required for the signal to appear, could be decreased from 15 to 5 min. The evaporation of water from the G P T increases, resulting in a faster evaporative concentration of HCl. The HC1 concentration required for AsC13 formation is therefore reached more rapidly.
The final test performed to ensure that the volatilized arsenic species really was AsC1, was to purge the gas swept from the membrane into a gas washing bottle containing 10 mL of 2 M HC1. The acid solution was thereafter subjected to polarographic analysis. The diffusion currents for the reduction of As(II1) to As(0) are tabulated along with the various hydrochloric acid concentrations used in Table 11. As can be seen from the table, the diffusion current obtained when As is reduced to ASH, with NaBH, and 2 M HCl is insignificant in comparison to that obtained when As is volatilized to AsCl, gas by using the GPT and 9 M HCl. When NaBH4 is used for hydride generation in 9 M HCl, the diffusion current from the reduction of As(II1) to As(0) shows that a significant fraction of the As(II1) is in fact volatilized as AsCl,(g), rather than as AsH,(g). Interference Studies. Table I11 collates the results from our studies of the interferences from the metal ions Ni(II), Co(II), Cu(II), and Fe(II/III). These ions are all potent interferents in the hydride generation technique, as can be seen from the literature data also present in Table 111. Our results, which are based on peak height signal evaluation and averages of four replicates, show that the range of interference-free determination of arsenic can be considerably extended by volatilizing the arsenic as the trichloride instead of as arsine, Le., without the use of sodium tetrahydroborate. Studies on the mechanisms of transition-metal interferences in hydride generation performed by Welz and co-workers (8,10,15) show the severe interferences caused by the presence of the interfering metals above are due to the reaction of the hydride with metal ions that have been reduced and precipitated in a colloidal form by the tetrahydroborate. In the most recent of the cited publications (8),which was focused on the influence of the acid and tetrahydroborate concentrations on the interferences in arsenic determination, Welz and Schubert-Jacobs showed that the range of interference-free determination of As(II1) could in fact be improved by increasing the hydrochloric acid concentration and decreasing the sodium tetrahydroborate concentration, conditions that will favor the
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ANALYTICAL CHEMISTRY, VOL. 60, NO. 19, OCTOBER 1, 1988
Table 111. Observed Interference from Iron, Copper, Cobalt, and Nickel in the Determination of As(II1) by the Present Method and Comparison with Values Previously Reported for Hydride Generation Methods
inter-
ferent
Fe
cu
co
Ni
[interferent], wg/mL
amt of NaBH4
33.3 100 1000 1000 12000
1mL, 1% 3% w/v 0.3 g 2 mL, 5% none
33.3 100 200 1000 1000 12000
1mL, 1% 3% w/v 2 mL, 2% 0.3 g 2 mL, 5%
33.3 1000 1000 10000
1 mL, 1% 0.3 g 2 mL, 5%
0.1 33.3 60 1000 1000 6000
none
none 3% w/v 1 mL, 1% 2 mL, 2% 0.3 g 2 mL, 5%
none
signal change, % -3
-5
