Anal. Chem. 1980, 52, 1028-1031
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(17) Kolthoff, I . M.; Sandell, E. B.; Meehan, E. J.; Bruckenstein, S. "Quantitative Chemical Analysis", 4th ed.; The Macmillan Go.. New York, 1969; p 849.
RECEIVED for review December 10, 1979. Accepted March 3,
1980. This work was supported by the Air Force Office of Scientific ~~~~~~h under AFOSR ~~~~t78-3621. presented in part at the Air Force Office of Scientific Research Workshop on the Status of Chemical Instrumentation and Analytical Program in the Air Force, Dayton, Ohio, J u n e 15, 1979.
Separation and Determination of Volatile Hydrides by Gas Chromatography with a Gold Gas-Porous Electrode Detector P. R. Gifford and Stanley Bruckenstein" Chemistry Department, State University of New York at Buffalo, Buffalo. New York 14214
The gaseous hydrides of As(III), Sn(II), and Sb(II1) were generated by sodium borohydride reduction. The hydrides were swept from solution onto a Porapak Q column where they were separated and detected at a gold gas-porous electrode by measurement of the respective electrooxidation currents. Detection limits for 5-mL samples were: As(II1) (0.2 ppb); Sn(I1) (0.8 ppb); Sb(II1) (0.2 ppb).
T h e determination of trace levels of arsenic and antimony in environmental samples has received widespread attention due to their high toxicities. The U.S. Public Health Service recommends t h a t arsenic concentrations in drinking water should not exceed 0.01 mg L-' ( I ) , illustrating the need for sensitive methods of determination. For those elements forming volatile hydrides, the use of hydride generation for analysis has gained wide usage due to the improved sensitivities achieved over direct measurement of t h e dissolved species. T h e development and application of hydride generation methods for atomic spectroscopic analysis has been recently reviewed ( 2 ) ,and numerous applications of hydride methods have appeared in review articles ( 3 , 4 ) . The application of hydride generation for determination of a number of elements was summarized in a recent article
(5). T h e hydride method frequently uses a sodium borohydride reduction of the sample to generate the volatile hydride or hydrides. The hydride may then be collected using a balloon trap or a freeze trap. T h e trapped hydride is then vaporized and determined by atomic absorption spectrometry. Detection limits for this method range from near 1 ng to 10 ng analyte for As, Sb, and Se (6, 7). Direct measurement of the evolved hydrides has been used for AsH3 and SbH, using a dc discharge detector. Detection limits of 0.5 ppb and 1 ppb for S b a n d As, respectively, are reported for 1-3 m L sample volumes (8). Methods based on the direct determination of hydrides are subject to a number of interferents, including the formation of other volatile species, e.g., ASH,, SbH3, SnH,, SeH2,and Hg (9, 10). Moreover, direct determination of these hydrides does not provide for simultaneous determination of two or more hydrides formed concurrently. These problems have led to the development of methods for the simultaneous determination of volatile hydrides. Talmi a n d Norvell (11) determined As and S b in environmental samples by gas chromatography using a microwave spectro0003-2700/80/0352-1028$01 OO/O
metric detector. They report relative sensitivities of 50 and 123 ng L-l for As and Sb. This sensitivity is achieved by lengthy 100-fold preconcentration steps. Kadeg and Christian (12) used gas chromatographic separation on a Porapak Q column with mass spectrometric detection to determine Ge, Sn, As, and S b as their volatile hydrides. Using a freeze trap method, detection limits for 5 mL samples were: Ge (1ppb), S n (10 ppb), As ( 2 ppb), and S b (20 ppb). In the previous paper (13) we outlined a method for trace level determinations at a gold gas-porous electrode (Au GPE) using gas-evolving reactions. In this study, the Au GPE is used as a gas chromatographic detector to simultaneously determine As(III), Sn(II), and Sb(II1) as their volatile hydrides. The generated hydrides are separated on a Porapak Q column and are detected by measurement of the respective electrooxidation currents at the Au GPE. Detection limits for 5-mL samples were: As(II1) (0.2 ppb), Sn(I1) (0.8 ppb), Sb(II1) (0.2 PPb). EXPERIMENTAL Instrumental and Apparatus. A Princeton Applied Research Model 173 potentiostat equipped with a Model 179 digital coulometer and a Model 178 electrometer probe (Princeton Applied Research Corp., Princeton, N.J.) was employed in this study. To reduce high frequency noise, a 0.024-pF capacitor was connected between the auxiliary and reference electrodes with a 50-kR resistor in series with the auxiliary electrode. Current data were recorded on a Bristol millivolt strip-chart recorder Model 1PH 560-51-T46-T82 (The Bristol Co., Waterbury, Conn.) which was modified to accommodate voltages from 1 mV t o 10 V full scale, or a Heath Model SR-204 strip-chart recorder (Heath Co., Benton Harbor, Mich.). The input signal to the recorder was conditioned through an RC filter of R = 1 MR and C = 0.22 pF to 1.0 pF. The electrochemical cell and reaction vessel are described in (19). All potentials are reported vs. the saturated calomel electrode (SCE). Au GPE. The Au GPE was constructed using the method developed by previous workers in our laboratory ( 1 4 ) and is shown in Figure 1 of (13). For hydride determinations, the Au GPE was potentiostated at +1.35 V until a stable residual current of 1-2 pA was obtained. The electrode was then conditioned by electrooxidation of Hg vapor generated by N&H4 reduction of Hg(I1) and swept across the gas phase side of the Au GPE. Treatment of the Au GPE with Hg vapor was found to significantly enhance electrode sensitivity for the electrooxidation of the hydrides. Reagents and Solutions. All reagents were of A.R. grade and used without further purification. Solutions were prepared with reagent grade water obtained from a Milli-Q Reagent Grade Water Systeni (Millipore Corp., Bedford, Mass.). C 1980 American Chemical Society
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980 Teflon C o m e tors
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SCE
N2 Poropak column
Figure 1.
Q
Sampling train for generation and determination of hydrides at a Au GPE
f 100 n A
c
Q)
L
3
0
0
0.5
1.0
1.5
2.0
2.5
3.0
3.5
4.c
Time (minutes) Figure 2.
Chromatographic response for As(II1). Curves A-E correspond to 0, 0.8, 1.5, 3.1, and 4.6 ng As(III), respectively
The reductant solution used was 4% w / v NaBH, in 0.1 hl NaOH and was prepared fresh daily. Sodium borohydride powder was obtained from Fisher Scientific Co. Nitrogen carrier gas was obtained as boil-off from liquid nitrogen (Union Carbide, Linde Division). The flow rate was controlled through a flowmeter (Union Carbide, Linde Division). Glassware was periodically cleaned in a 50-50 volume mixture of concentrated nitric and sulfuric acids followed by extensive rinsing with reagent grade water. The electrochemical cell supporting electrolyte was 1.0 M H2SOd. Procedure. The sampling train used for hydride generation. separation, and detection at a Au GPE is shown in Figure 1. The separation column was a 20-inch, 6-mm o.d. glass column packed with Porapak Q, 50-80 mesh (Waters Associates, Milford, Mass.). For sample determinations, 5 mL of saturated potassium bitartrate (KHTar) buffer were placed in the reaction vessel. Portions of analyte stock solution were added with a 10-pL syringe (Hamilton Co., Reno, Nev.). Kitrogen carrier gas was bubbled through the sample at a flow rate of 100 mL rnin~'and 25 pL of 4% NaBH, were injected into the sample through the rubber septum fitted on the reaction vessel injection port. The current response of the Au GPE, potentiostated at 1.35 V vs. the saturated calomel electrode, was recorded vs. time and the current maxima (ipeak) determined for each hydride. Between samples, the reaction vessel was rinsed with 1.0 hl H2S04followed by KHTar buffer before adding fresh buffer and sample. It was necessary to precondition the apparatus by running three samples of about 100 ng each of As(II1) and Sb(II1). This pre-
conditioning yielded good reproducibility and prevented low results for the first few samples run a t the start of a day. At low analyte levels (510 ppb), the buffer was pretreated using NaBH, under the same conditions used for an analysis. The procedure reduced blank readings due to trace levels of impurities present in the KHTar buffer. All experiments were performed a t room temperature.
RESULTS A N D DISCUSSION Determination of As(II1). T h e determination of As(II1) as ASH, a t a Au G P E in the absence of other hydride generating species was reported previously without employing a column separation (13). Therefore, this system was chosen as the model to characterize the gas chromatographic procedure. T h e arsenic determination procedure is based on the reduction of arsenic species with sodium horohydride to produce volatile arsine:
NaBH,
As(II1)
Hf
AsH3 (+ H2 + other products)
(1)
T h e arsine is swept from solution by the carrier gas through the column to the Au GPE where it is electrooxidized to As(V) to give the measured current response. As shown in Equation 1, H2 is produced due to sodium borohydride hydrolysis in acid media, both in the absence and presence of arsenic and concurrent with ASH, generation. A
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ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
Determination of Sb(II1). Since the chromatographic Au G P E system gave excellent sensitivity for As(II1) determinations, other hydride systems were studied under identical experimental conditions. Antimony was determined by NaBH, reduction of Sb(II1) and electrooxidation of SbH3. The chromatogram shows a H2 peak a t 0 s followed by a second peak a t 345 s for SbH3 electrooxidation (Figure 4). A plot of peak current vs. amount of Sb(II1) was linear and had a slope 20.7 f 0.5 nA/ng Sb(II1) and intercept 4.5 2.3 nA in the range 0 to 10 ng Sb(II1) (Figure 3). A detection limit of 1.0 ng Sb(II1) (0.2 ppb) was obtained. Determination of Sn(I1). Sn(1I) was determined by the electrooxidation current for the generated hydride SnH,. The retention time for SnH, is 160 s. Because of the buffer impurity previously mentioned, a blank response of 100 nA was obtained for 0 ng Sn(I1). This blank response limited the detection of Sn(I1) to 4 ng (0.8 ppb). A linear plot having a slope 26.2 f 0.8 nA/ng and intercept 87.9 f 8.3 nA was obtained for 0 to 20 ng Sn(I1) (Figure 3). S i m u l t a n e o u s D e t e r m i n a t i o n of A s ( I I I ) , S n ( I I ) , a n d Sb(II1). A typical chromatogram for a sample containing As(III), Sn(II), and Sb(II1) in the presence of Hg(I1) is shown in Figure 5 . T h e order of elution is Ha, ASH,, SnH,, SbH,, and Hg, i.e., the order of increasing molecular weight. This is in agreement with the results of Kadeg and Christian (12) who postulate a molecular sieve type separation for hydrides on Porapak Q. The peaks for ASH, and SnH, are poorly resolved and even at low As(II1) levels (1--2 ppb), a positive error occurs in the Sn(I1) determination. Thus, while Sn(I1) can be detected in the presence of As(III), its quantitation would be difficult a t low concentrations. The As(II1) determination is not affected by Sn(I1) up to a tenfold excess of Sn(I1). At higher Sn(II)/As(III) ratios, the SnH, current peak masks the As(II1) response. The chromatographic responses of As(II1) and Sb(II1) were found to be independent of one another, even when one analq-te was present in large excess with respect to the other. The Sb(I1I) response was also independent of Sn(I1) concentration. Chromatograms for the simultaneous determination of 0 to 10 ng As(II1) and Sb(II1) are shown in Figure 4. A peak is obtained for Hg due to the NaBH, reduction of Hg(I1) to Hg. However, the chromatographic procedure used here is not sufficiently sensitive to be of analytical interest
*
Nanograms Analyte Figure 3. Plot of peak current vs. nanograms of analyte. (0)As(III), (0)Sn(II), (A)Sb(II1)
small fraction of this H 2 is electrooxidized a t the Au GPE giving rise to a current response. Thus the chromatogram for As(II1) determination shows two peaks (Figure 2) due to H 2 and ASH, electrooxidation. The retention time for ASH, is 100 s. By use of a column separation, the detection limit for As(II1) is significantly improved over that obtained previously (13) by eliminating the H 2 electrooxidation current interference. A linear plot of peak current vs. weight of As(II1) was obtained (Figure 3). T h e slope is 65.2 f 1.9 nA/ng As(II1) and intercept 47.5 f 6.9 nA for 0 to 10 ng As(II1). A plot of As(II1) peak current response is linear up to 400 ng (80 ppb) and then shows slight negative curvature. A detection limit of 1 ng As (111) (0.2 ppb) was obtained. A blank experiment yielded a small peak. Pretreating the buffer solution as described in the Experimental reduced the magnitude of this peak but did not completely eliminate it. This impurity did not interfere with the determination of As(II1) since it eluted from the column after ASH,.
100 nA L
c
2 L
3
0
0
1
2
3
4
5
6
7
8
Time (minutes) Figure 4. Chromatographic response for determination of As(II1) and Sb(II1) Curve A: 0 ng As, 0 ng Sb; curve B: 1.8 ng As, 2.5 3.0 ng As, 4.1 ng Sb; curve D: 4.6 ng As, 6.2 ng Sb; curve E: 6.2 ng As, 8.2 ng Sb
c:
ng Sb; curve
ANALYTICAL CHEMISTRY, VOL. 52, NO. 7, JUNE 1980
I
I
1"\
1031
Table I. Interference Studies for As(II1) and Sb(II1) Determinations Hg(I1) Cr(V1)
Cu(I1)
As(II1) 0.5 ppm 1 ppm 0.4 ppm
Se(I1)
NO,-
none up i o none up to
20ppm 20ppm Sb(II1) 0.5 ppm 1 ppm 0.03 ppm none up t o none up to 20ppm 20ppm Table 11. Comparison of Detection Limits for Hydride Methods, ppb AASI 1n-Cc-j 0
2
,
4
, 5
1
8
solution nebuliza- AAS/ tion' hydrideu
w 1c
'2
'4
T i m e 'minutes)
Figure 5. Chromatogram for sample of As(III), Sn(II), Sb(III), and Hg(I1). (B) 15 ng As(III), (C) 25 ng Sn(II), (D) 20 ng Sb(II1). (E) 500
(A) H, ng "1)
in the detection of very low levels of Hg. This is due in part to the long retention time (11 min) and severe band broadening for Hg. Also, the Au G P E oxidation reaction involves only a two-electron process for Hg as compared to eight electrons for AsH3. Hg was studied as a potential interferent and is discussed further below. Reproducibility. Studies of the reproducibility of the chromatographic method were performed for each analyte and for simultaneous determinations of As(II1) and Sb(II1). After the high level analyte preconditioning step, relative standard deviations of
