Anal. Cham. 1962, 64, 667-672
cientiy large to allow the identification of three natural components of wheat. In a first step, application of stepwise discriminant analysis made it posible to identify the most important wavelengths. Secondly, the images recorded at the chosen wavelengths were linearly combined by canonical discriminant analysis. The pixels were classified in qualitative groups, and a segmented image could be obtained. The segmentation of images was achieved from the spectral information related to chemical composition. Future work includes the characterization of mixtures of raw materials in
667
(9) Newman, P. B. Meat Sd. 1987, 19, 129-137. (10) Newman, P. B. Meat Sd. 1987, 19, 139-150. (11) Aastrup, S.; Gibbons, G. C.; Munk, L. Carlsberg Res. Commun. 1981,
46, 77-88.
(12) Gibbons, G. C. ASBCJ. 1981, 39, 55-59. (13) Heyne, L; Unklesbay, N.; Unklesbay, K.; Keller, J. Can. Inst. Food Sd. Techno!. J. 1985, 18, 188-173. (14) Geladi, P.; Wold, S.; Esbensen, K. Anal. Chlm. Acta 1988, 191,
473-480.
(15) Gulllobez, S. Agronomía Troplcale 1985, 40, 81-88. (16) Munk, L; Fell, C.; Gibbons, Q. Cereals tor Food and Beverages, International Conference on Cereals and Food Beverages, Copenhagen, DK, August 13-17, 1979; Inglett, G. E„ Munk, L, Eds.; Academic Press: London, 1980; pp 27-40. (17) Taylor, S. K.; McClure, W. F. Proc. of the 2nd NIRS Conference, Tsukuba, Japan, May 29-June 2, 1989; Iwamoto, M., Kawano, S„ Eds.; Korin Publishing Co., Ltd. (Japan), 1990; pp 393-404. (18) Williams, P.; Norris, K. H. Near Infrared technology In the agricultural and food Industries·, American Association of Cereal Chemists: St Paul, 1987; p 330. (19) Geladi, P.; Esbensen, K. J. Chemometr. 1989, 3, 419-429. (20) Robert, P.; Devaux, M. F.; Bertrand, D. Sel. des Aliments, In press. (21) Downey, G.; Robert, P.; Bertrand, D.; Kelly, P. M. Appl. Spectrosc. 1990, 44, 150-155. (22) Romeder, J. M. MSthodes et programmes d'analyse discriminante·, Dunod: Paris, 1973. (23) Foucart, T. Analyse factoriete programmatton sur micro ordlnateurs; Masson: Paris, 1982. (24) Law, D. P.; Tkachuk, R. Cereal Chem. 1977, 54, 256-285. (25) Osborne, B. G.; Fearn, T. Near Infrared spectroscopy In food analysis; Longman Scientific & Technical: Harlow Essex, England, 1988; pp
the food, pharmaceutical, and chemical industries. In addition, the method will be tested on tranverse sections of seeds such as wheat grains in order to measure the proportion of different tissue constituents.
Registry No. Starch, 9005-25-8.
REFERENCES (1) Goodman, D. E.; Rao, R. M. J. Food Sd. 1984, 49, 648-649. (2) Neuman, M.; Sapirsteln, H. D.; Shwedyk, E.; Bushuk, W. J. Cereal Sd. 1987, 6, 125-132. (3) Sapirsteln, H. D.; Neuman, M.; Wright, E. H.; Shwedyk, E.; Bushuk, W. J. Cereal Sd. 1987, 6, 3-14. (4) Symons, S. J.; Fulcher, R. G. J. Cereal Sd. 1988, 8, 211-218. (5) Neuman, M. R.; Sapirsteln, H. D.; Shwedyk, E.; Bushuk, W. J. Cereal Sd. 1989, 10, 175-182. (8) Neuman, M. R.; Sapirsteln, H. D.; Shwedyk, E.; Bushuk, W. J. Cereal Sd. 1989, 10, 183-188. (7) Newman, P. B. Meat Sd. 1984, 10, 87-100. (8) Newman, P. B. Meat Sd. 1984, 10, 181-186.
-
28-42.
Received for review July 8,1991. Accepted November 15, 1991.
Prereduction of Arsenic(V) to Arsenic(III), Enhancement of the Signal, and Reduction of Interferences by L-Cysteine in the Determination of Arsenic by Hydride Generation Hengwu Chen Department of Chemistry, Hangzhou University, Hangzhou, Zhejiang, People’s Republic of China 3100028
Ian D. Brindle* and Xiao-chun
Le1
Chemistry Department, Brock University, St. Catharines, Ontario L2S 3A1, Canada
Since Holak1 published his paper on the determination of arsenic by hydride generation-atomic absorption spectrometry, many papers have been published which deal with arsenic
determination by arsine generation combined with atomic absorption spectrometry (AAS),2"6 inductively coupled plasma atomic emission spectrometry (ICPAES),3,4,7 and direct-current plasma atomic emission spectrometry (DCPAES).8-10 Since trivalent and pentavalent arsenic show different behavior and different sensitivities in the arsine generation process, several workers have developed techniques for the selective determination of arsenic(III) in the presence of arsenic(V).11-14 On the other hand, where the oxidation state of the arsenic has been less important, workers have preferred to reduce arsenic(V) to arsemc(III) with various prereducing agents before arsine generation.16,16 The most popular prereductant is potassium iodide. It has been used in combination with ascorbic acid, which can prevent the oxidation of iodide to triiodide by air, or with oxidants such as iron(III) and copper(II).17 However, potassium iodide can only be used in strong acid media. At acid concentrations less than 0.3 M, iodide failed to reduce arsenic(V) to arsenic(III) completely.13,17 Furthermore, some workers reported that, when potassium iodide was used as prereductant, it took as long as 4-5 h to reduce arsenic(V) to arsenic(III) completely at room temperature.17,18 Potassium iodide has also been used in combination with
To whom correspondence should be addressed. Department of Chemistry, University of British Columbia, Vancouver, BC V6T 1Z1, Canada.
sodium sulfite,19 thiourea,20 and tin(II) chloride.21 Anderson et al. reported that mercaptoacetic acid at low concentrations gives almost identical signals for similar concentrations of
L-Cystelne reduce· areenlc(V) to areenlc(III) rapidly on warming. In addition, the algnal from araenlc(III), after reduction by tetrahydroborate(III), I· enhanced by the presence of L-cyetelne In the arsenic solution. Interferences from transition element· and other hydride-forming element· (with the exception of selenium and tellurium) are substantially reduced by the presence of L-cystelne. Because of the different rates of reaction at different pH·, speclation of areenlc(III) and arsenlc(V) Is pósetele. The detection limit, by do plasma atomic emission spectrometry, for arsenlc(III) Is 0.62 ng mL"1 for a 5-mL sample. The relative standard deviation for 10 determinations of a solution containing 50 ng mL"1 arsenle(III) was 1.1%. A major advantage of this technique Is that It requires low acid concentrations and, furthermore, the prereducing agent fulfills several functions and Is relatively Innocuous.
*
f Present address:
0003-2700/92/0364-0667$03.00/0
©
1992 American Chemical Society
668
·
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
arsenic(III) and arsenic(V) in the hydride generation reaction and, in addition, for methylarsonic acid and for dimethylarsinic acid, although the maximum responses for some species were lower than others when different systems were used.13 This was confirmed by Takahashi et al.,22 who reported that, when mercaptoacetic acid was used as a prereducing agent, the sensitivity in the determination of inorganic arsenic by atomic absorption spectrometry was lower than when potassium iodide was used. For applications other than hydride generation, other prereducing agents have been used. Sodium thiosulfate by itself23 and in combination with sodium hydrogen sulfite,24 and titanium(III) chloride25 have been used to reduce arsenic(V) to arsenic(III) in solvent extraction techniques that have been used in speciation studies. Sodium sulfite has been used in stripping voltammetry.26 In our previous work,10·27 we found that, when hydride generation was allowed to proceed for 20-30 s in 5 M HC1 before arsine was stripped, peak heights may be used to determine the total (arsenic(III) plus arsenic(V)) arsenic concentration. A delay column was necessary, for determination by DCP, to control the surge of hydrogen which would otherwise destabilize the plasma. Although the hydride generation technique is relatively simple and has high sensitivity, it suffers from interferences that arise from certain metal ions in the sample solution.15,28"32 We have found that L-cystine and L-cysteine are excellent reagents to reduce interferences in the hydride generation process for the determination of arsenic,27 tin,33 and germanium.34,35 In addition, we have shown that L-cysteine can speed up the rate of reduction by sodium tetrahydroborate(III), thereby increasing the sensitivity in the determination of germanium.34 Anderson et al.13 suggested that the “sulphur containing ligand” was important in the reaction. We have confirmed this by observing that thioglycerol also produces a similar effect. We have investigated this reaction in some depth35 and have shown that there is a compound formed between the tetrahydroborate(III) and the sulfhydryl group of L-cysteine which may be the reagent which is a more potent reducing agent than tetrahydroborate(III) alone. It is possible, in fact, that this species may be formed in the case of the reaction with L-cystine,27 since it may be generated by the reduction of the disulfide by the following reaction: R-S-S-R + BH4" — R-SH + R-S-BH3-
R
-CH2CH(NH2)C02H In this paper, we report our investigation of the characteristics of L-cysteine as a prereducing reagent for arsenic (V). We also report a technique to determine total inorganic arsenic at low acid concentration with a flow-through batch hydride generator in which L-cysteine acts not only as a prereducing agent but also also as a reagent which reduces interferences and increases sensitivity. The particular advantages of Lcysteine are that it is almost odorless and produces no odorous byproducts during reaction, a decided advantage over mercaptoacetic acid; also, it reacts most effectively at low acid concentrations, thus minimizing the danger to personnel and corrosion of the instrument. =
EXPERIMENTAL SECTION Apparatus. The DCP spectrometer used is
a
Beckman
Spectraspan V dc plasma atomic emission spectrometer equipped with a background correction system, a Dataspan manipulation and storage system. The hydride generator was described in previous work.10,33 Signals were recorded on a Fisher Recordall Series 5000 recorder. Equipment parameters are shown in Table
I.
Reagents. Arsenic(III) standard solution (1000 Mg mL"1) was prepared from As203 (Thorn Smith, Beulah, Michigan). Arsenic(V) standard solution was prepared from Na2HAs04-7H20 (BDH
Table I. Operating Parameters of the DC Plasma Atomic Emission Spectrometer nebulizer pressure sleeve pressure slit size
entrance
exit wavelength carrier gas flow sample volume
26 psig 50 psig 50 X 300 Mm (w X h) 100 X 300 mbs (µ X h) 193.696 nm 1.6 L min"1 5
mL
AnalaR, Toronto, Ontario) and standardized against arsenic(III) standard solution as previously described.10 Sodium tetrahydroborate(III) (Aldrich, Milwaukee, Wisconsin) solutions (0.5-100 mg mL"1) were prepared in 8 mg mL"1 sodium hydroxide solution. L-cysteine was obtained from Sigma Chemical Co. All other reagents were of analytical-reagent grade or better. Samples of metallic standard reference materials (SRM) 55E and 363 were obtained from the National Institute of Standards and Technology (NIST), Gaithersburg, MD. The Czechoslovakian SRM Cast Iron CKD 229 was donated by General Motors of Canada, St. Catharines, Ontario. Hydride Generation Conditions. Analyte solutions should contain 5 mg mL"1 L-cysteine and should be approximately 0.01 M in either nitric or hydrochloride acid. A 5-mL aliquot of sample is placed in the reactor, followed by 1 mL of 20 mg mL"1 tetrahydroborate(HI) solution containing 8 mg mL"1 sodium hydroxide. The signal is recorded on the chart recorder, as described previously.33
Sample Preparation. Metallic Samples (NIST Open Hearth Iron 55E, NIST Low Alloy Steel 363, and Czechoslovakian SRM Cast Iron CKD 229). A sample of the iron (approximately 1 g) or the steel (approximately 0.7 g) was weighed accurately and transferred to a beaker. Nitric acid (15 mL of 1:1 v/v) was added. The sample was reacted, with gentle warming, for 1 h. The solution was then transferred into a 500-mL volumetric flask and diluted to the mark with distilled water. A 10-mL aliquot was placed in a 100-mL volumetric flask, 0.5 g of L-cysteine was added, and the solution was diluted to approximately 90 mL. Adjustment of the pH to 2.10-2.15 was made with 1 M nitric acid or 1 M sodium hydroxide solution before the solution was made up to the mark. The 100-mL volumetric flask was placed in a boiling water bath for 10 min, and then the solution was cooled to room temperature. Water Samples. Water samples were obtained from the Ontario Ministry of the Environment (MOE) Laboratory Services Division, (P.O. Box 213, Rexdale, Ontario M9W 5L1, Canada), Inorganic Trace Contaminants section, which had been previously analyzed either by inductively coupled plasma mass spectrometry (ICPMS), or by the hydride method routinely used at the laboratory. An appropriate volume of sample was placed in a 150-mL beaker and diluted to approximately 90 mL, after which it was treated in a similar manner to the iron samples above. Speciation. The best conditions for the determination of As(III) in the presence of As(V) are as follows: to a sample (5 mL with an acid concentration of 0.01 M) in the flow-through batch system vessel is added 1 mL of 20 mg mL"1 L-cysteine; this is immediately followed by the addition of 1 mL of 5 mg mL"1 sodium tetrahydroborate(III) solution.
RESULTS AND DISCUSSION L-Cysteine as Signal Enhancer. We have shown that acid concentration is critical in the determination of other elements by hydride generation.33,34 Thus we have investigated the effect of acid concentrations on the generation of arsine in the presence and absence of L-cysteine. Results from these investigations are summarized in Figure 1. As we can see, in the absence of L-cysteine, the production of arsine, meapeak height, increases steadily as the acid concena plateau when the acid concentration reaches 0.08-0.10 M. In the presence of L-cysteine, on the other hand, arsine production reaches a maximum over a range of acid concentrations from 0.01 to 0.03 M. The advantage of L-cysteine is also clearly shown in Figure 1. Over sured
as
tration increases, and reaches
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Sodium tetrahydroborate(III) concentration 25 50 75 0
·
669
(mg mL·1) 10 0
Figure 2. Effect of L-cystelne concentration (+) (1 mL of 20 mg mL"1 sodium tetrahydroborate(III) solution used for reduction) and effect of sodium tetrahydroborate(III) concentration (O) (L-cysteine concentration in sample, 5 mg mL"1) on arsenlc(III) signal. sence of L-cysteine. Measurement of the pH showed that, after 5 mL of the test solution reacted with 1 mL of 20 mg mL"1 sodium tetrahydroborate(III) solution (stabilized with 8 mg mL"1 sodium hydroxide), the final pH value was greater than 8. It was not surprising, therefore, that no arsine was released
Figure 1. Effect of acid concentration on arsenlc(III) signal: (O) nitric acid with 5 mg mL"1 L-cystelne, 20 mg mL"1 NaBH4; (·) nitric acid, 20 mg mL-1 NaBH4; (X) hydrochloric acid with 5 mg mL"1 L-cystelne, 20 mg mL"1 NaBH4; (V) hydrochloric acid with 5 mg mL"1 L-cystelne, 5 mg mL"1 NaBH4; (+) hydrochloric acid, 20 mg mL'1 NaBH4.
the best range of acid concentrations the peak heights, where L-cysteine was used, are some 75% greater than peak heights obtained in 0.1 M acid in the absence of L-cysteine. No significant difference in behavior was observed when nitric acid or hydrochloric acid was used as the acid medium. A 20 mg mL'1 sodium tetrahydroborate(III) solution, stabilized with 8 mg mL"1 sodium hydroxide gave the best results over the widest range of acid concentrations. If the concentration is reduced, the range of acid concentrations, over which the maximum signals is obtained is narrowed, as can be seen in Figure 1 for a 5 mg mL'1 solution of sodium tetrahydroborate(III). The maximum signal occurs at the same acid concentration, however. The low acid concentrations required for this technique lend themselves to the gas-flowing batch system, since relatively small amounts of hydrogen are produced, compared to the amount generated at high acid concentrations. Thus the distortion of the plasma, caused by large volumes of hydrogen produced by high acid systems, is minimized and the need for a “delay column”10 is eliminated. This technique should lend itself to users of ICP systems, since the ICP is particularly susceptible to changes in flow and in gas composition. Figure 2 shows the peak height as a function of L-cysteine concentration and also as a function of sodium tetrahydroborate(m) concentration. As the sodium tetrahydroborate(m) concentration increases, peak heights slowly diminish as the concentration exceeds 40 mg mL"1. When the concentration of L-cysteine exceeds 4 mg mL"1, signals remain constant over the range studied. Prereduction of Arsenic(V) by L-Cysteine. When the effect of L-cysteine on arsine generation from arsenic(V) was studied, it was found that, over the range of acid concentrations from 0.01 to 0.02 M, no arsine was detected in the ab-
from the arsenic(V) solution, since arsenic(V) has been reported to be reduced to arsine only in a strongly acidic medium.1112 When L-cysteine solution was added to the arsenic(V) solution just before the NaBH4 solution was added, almost, no arsine was measured. If the solution was allowed to stand for 5 min before the sodium tetrahydroborate(III) solution was injected, a small arsine signal was detected. The signal gradually increased with increased standing time. This phenomenon suggested that L-cysteine probably plays a role as a prereducing agent which reduces arsenic(V) to arsenic (HI) and is itself oxidized to L-cystine. A series of investigations was made on the prereduction of arsenic(V) with L-cysteine as well as other reducíante. The reducíante tested were hydroxylamine hydrochloride, sodium sulfite, ascorbic acid, and potassium iodide. The first three reagents did not produce measurable arsenic signals in 0.01 M acid, even after heating in a boiling-water bath for 30 min. With respect to 5 mg mL'1 KI, only 4% of the arsenic(V) was reduced to arsenic(III) under the above conditions. Using L-cysteine as prereductant, however, the situation was much different, as shown in Figure 3. At room temperature and in 0.02 M acid medium, the reduction of arsenic(V) to arsenic(III) was complete within 35, 60, and 135 min when L-cysteine (1,0.5, and 0.25 g, respectively) was added to 100 mL of 50 ng mL'1 arsenic (V) solution. When the solution in the 0.02 M acid medium was heated in boiling water for 5 min, arsenic(V) was completely reduced to arsenic(III). Heating for as long as 45 min resulted in no arsenic loss. In addition, when prereduction was carried out in 1 M acid with 0.5 g of L-cysteine, arsenic(V) was completely reduced to As(IH) within 5 min. Compared with the other systems tested, L-cysteine is an excellent reagent for the prereduction of arsenic(V) to
arsenic(III). Speciation. Since L-cysteine reduces As(V) to As(III) slowly at room temperature and at low acid concentration, it was felt that it might be possible to determine As(III) in the presence of As(V). Thus, a solution containing As(III) and As(V) was injected into the generator; L-cysteine solution was then added, followed immediately by the NaBH* solution. In this way, only As(III) reacts to form arsine. A separate sample can be reduced with L-cysteine prior to the hydride generation step to give the total As (III) and As(V) concentration. Acid and NaBH4 concentrations are critical factors
670
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ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Table II. Interference from Transition Elements and the Effect of L-Cysteine % recovery
ion
Au(III)
concn, Mg mL"1
no
L-cysteine0 95 78 8.0 100 55 2.0 76 ndc
0.2 2
20
Pt(IV)
0.2
Pd(II)
20 0.2
Ag(I)
20 200 0.2
2
2
-
-
-
87 42
2
Ni(II)
20 200 0.2
-
76 11.5
2
20
Co(II)
-
0.2
82
2
25.8
20
Cu(II)
-
200 0.2
-
82 67 12.4
2
Fe(III) Figure 3. Prereduction of arsenlc(V) (50 ng mL-1) to arsenic(III). (·) 2.5 mg mL"1 L-cystelne In 0.02 M HN03, room temperature; (X) 5 mg mL"1 L-cystelne In 0.02 M HN03, room temperature; (A) 10 mg mL"1 L-cystelne In 0.02 M HN03, room temperature; ( ) 5 mg mL"1 L-cystelne In 0.02 M HN03, heated In boiling water; ( ) 5 µg of As(V) and 0.5 g of L-cystelne In 2 mL of 1 M HN03, room temperature (subsequently diluted to 100 mL before the determination step).
for reducing As(V) interference in the determination of As(III). Under these conditions, 2.5 µ% of As(V) did not interfere in the generation of arsine from 250 ng of As(III). In an attempt to control interference from larger amounts of As(V), an attempt was made to control the pH with a citric acid-citrate buffer system. During the arsine generation step, the buffer caused a foaming problem, which created instability in the
Fe(II)
Hg(II)
Interference Studies. When interference was studied, mL of the interfering ion, at different concentrations, was directly added to the generator containing 5 mL of arsenic ( ). The signals obtained in this way were compared with those where no interfering ion had been added. Table II shows the interference of some transition elements and the interference-reducing effect of L-cysteine. Without L-cysteine, Pd, Pt, Au, Ag, Ni, Co, Cu, and Fe interfere with the hydride generation process and the interferences were more severe than in 5 M HC1 medium,27 which agrees with the findings of Welz and Melcher.32 Moreover, the interference resulted in a memory effect. Interfering ions, introduced to the generator in a determination, would depress the arsenic signal of the next determination even in the absence of interfering ions. This could be due to a residue of finely divided metal particles, precipitated on the generator, which was not completely removed during the cleaning. Interference in the generation of the hydride by finely divided metal has previously been reported by Welz and Melcher.32 Molybdenum was exceptional in that, in the absence of Lcysteine, enhanced recovery of arsenic was observed. The interference-free level of the above elements that could be tolerated was increased by 1-3 orders of magnitude, due to the presence of L-cysteine, and there was no observable
2
20 200
Mn(II) Zn(II) Cr(IV) Cd(II)
Mo(VI)
20
200 20 200 2
20 200 20 200 2
20 200
plasma. 1
20 200 20 200 20 200 2000
-
93 -
36 -
-
-
95 -
91 -
65 -
-
101 87 96 -
102 189
with 5 mg mL"1 L-cysteine1 102 91 77 100 96 92 92 94 85 54 105 101 95 87 -
98 66 -
-
98 64 -
-
101 96 104 99 99 99 91
99 103 105 99 100 99 96 99 102 94 102 96 105 98 96
“The concentration of As(III) was 50 ng mL-1. 6The concentration of As(III) was 125 ng mL"1. cnd, not detected; -, not determined.
memory effect. Compared to L-cystine in 5 M HC1,27 L-cysteine, even in 0.02 M HC1, reduced interferences more effectively from gold and palladium, though it was less effective in reducing interferences from cobalt and nickel. In addition, at the low acid concentrations used, L-cysteine readily dissolves in solution and thus simplifies the technique. Table III shows that, except for tellurium and selenium, other hydride-forming elements, at concentrations up to 20 Mg mL"1, have no influence on arsine generation in the system. It is also interesting to note that tellurium and selenium interfere more severely when L-cysteine is present than when it is absent. Although selenium, and by extension, possibly tellurium, form selenotrisulfides (and tellurotrisulfides) by reaction with L-cysteine, it is not clear at this point why these elements interfere so strongly. Detection Limit and Precision. The limit of the detection for As(III) (3 times the standard deviation of the blank) was 0.62 ng mL"1 for a 5-mL sample. The relative standard deviation of 10 determinations of 50 ng mL"1 As(III) was 1.1%.
ANALYTICAL CHEMISTRY, VOL. 64, NO. 6, MARCH 15, 1992
Table III. Interference of Hydride-Forming Elements and the Effect of L •Cysteine
ion
Bi(III)
2
20
Ge(IV)
2
20
Sb(III)
2
“
L-cysteine1
96 75 94 95 -C
100
20
Sn(II)
2
20 200
Pb(II)
2
Se(IV)
20 0.2
Te(IV)
20 200 0.2
2
2
200
with
no
found, ng mL'1
-
101 -
76 56 -
106 100 63 96 100 67
5
mg mL"1 L-cysteine6 102 101 101 102 101 101 98 99 100 97 100 97 94 78 7.7 100 53
10.4
“The concentration of As(III) was 50 ng mL"1. 6The concentration of As(HI) was 125 ng mL ^ “-, not determined. Table IV. Determination of Arsenic in Standard Reference Materials sample Open Hearth Iron 55E Low Alloy Steel 363 Cast Iron CKD 229
found, Mg/g ± sd (n)
mean
71.5 » 2.4 (3) 95.7 ± 1.2 (4) 130,140 (2)
certified value, #tg/g 70 100 1.5 X 102
Determination of Arsenic in Standard Reference Materials and in Waters. To test whether or not a systematic error existed in the newly developed technique, a calibration curve was compared with a standard addition curve, using the National Institute of Standards and Technology (NIST) Open Hearth Iron 55E and Low Alloy Steel 363 as samples. The slopes of the standard addition curves (in units of mm (ng mL"1)'1), 3.08 ± 0.13 and 3.04 ± 0.17, respectively, were almost the same as that of calibration curve (3.17 ± 0.21). The results shown in Table IV obtained with the calibration technique, show good agreement with certificate values. In addition, since nickel is a strongly interfering species in the determination of several hydride-forming elements, a Czechoslovakian standard reference cast iron, containing 22.5 mg g"1 nickel, was included to determine whether the interference from nickel was significant. Table IV shows that there is good agreement with the certified value for arsenic.
The determination of arsenic in water samples provided a useful illustration of the technique. A total of 25000 water samples are submitted to MOE for arsenic determination per year. They are assigned to different sections, depending on the type of sample. Typically, drinking waters are analyzed by ICPMS. Waters, whose composition would be expected to fall outside the range of normal drinking waters, are analyzed by other methods; arsenic is determined by hydride generation AAS with iodide prereduction. Table V shows the results of arsenic determined by the technique described in this paper, compared with results obtained by the methods in the MOE laboratories. Reasonable agreement is found in most cases between values determined in MOE laboratories and in this study. Exceptions are found in the case of samples identified as M43-277, M44-211, and IW46-0026. The sample M44-211 was found
671
Table V. Determination of Arsenic in Waters
% recovery
concn, Mg mL"1
·
this technique sample
mean
± sd
(
=
3)
MOE initial redeter· determination mination
M43-295 M43-288 M43-202
15.2 ± 0.3 7.5 ± 0.3 31.4 ± 0.5 30.9 ± 0.3 (dup)
15.5“ 8.3“ 28“
17.7“
M43-277 M44-211
