2048
Anal. Chem. 1983, 55,2043-2047
LITERATURE CITED (1) Ahrens, L. H.; Taylor, S. R. "Spectrochemlcal Analysis", 2nd ed.; Addison-Wesley: Heading, MA, 1961. (2) Barnes, R. M. I n "Progress in Analytical Chemistry"; Simmons, I. L., Ewing, G. S,, Eds.; Plenum: New York, 1973; Vol. 6. (3) Walters, J. P. Science 1977, 198, 787-797. (4) . . Coleman. D. M.; Walters, J. P. Spectrochlm. Acta, Part 8 1976,318, 547-587. (5) Klueppel, R. J.; Coleman, 11. M.; Eaton, W. S.; Goldstein, S. A.; Sacks, R. D.: Acta. Part 8 1978. 338, 1-30. . .. ., Walters. . .-. .-. -, J. P. Saectrochim. -r-(6) Scott, R. H. Spectrochlm. Acta, PartB 1978,338, 123-125. (7) Salin, E. D.; Horlick, G. Anal. Chem. 1979,51, 2284-2286. (8) O'Reilly, J. E.; Hicks, D. G. Anal. Chem. 1979, 51, 1905-1915. (9) Mohamed, N.; Brown, R. M., Jr.; Fry, R. C. Appl. Spectrosc. lg81, 35, 153-164. (IO) Price, W. J.; Dymott, T. C.; Whiteside, P. J. Spechochlm. Acta, Part8 1980, 358,3-'10. (11) Chakrabarti, C. I_.; Wan, C . C.; LI, W. C. @ectrochlm. Acta, Part B 1980, 358 93-105. (12) Chakrabarti, C. L.; Wan, C. C.; Hamed, H. A.; Bertels, P. C. Anal. Chem. 1981,53,444-4510. (13) Chakrabartl, C. l.,; Wan, C. C.; Teskey, R. J.; Chang, S. 6.; Hamed, H. A,: Bertels. P. C . SDeCtfOChlm. Acta. Part 8 1981, 368 427-438. (14) Gdldberg, J.; Sacks,'R. Aiial. Chem. 1982, 54,2179-2185. 115) . , Clark, E. M.; Sacks, R. C). Spectrochlm. Acta, Part 8 198% 358, 47 1-488. (16) Sacks, R . D.; Ling, C. S. .4ppl. Spectrosc. 1979,33, 258-268. (17) Duchane, D. V. Ph.D. Dissertation, University of Michigan, 1978. ~~
(18) Suh, S. Y.; Collins, FI. J.; Sacks, R . D. Appl. Spectrosc. 1981, 35, 42-52. (19) Araki, T.; Uchida, 'T.; Mlnami, S . Appl. Spectrosc. 1977, 31, 150-155. (20) Suh, S. Y. Ph.D. Dissertation, University of Michlgan, 1980. (21) Goldberg, J. M. Ph.D. Dissertation, University of Michigan, 1982. (22) Duchane, D. V.; Sacks, R. D. Anal. Chem. 1978, 5 0 , 1752-1757. (23) Falk, H.: Tham, T. K. Spectrochim. Acta, Part 8 1980,358 465-469. (24j Collins, R. J. Ph.D. Dlssertation, Unlversity of Michlgan, 1982. (25) Flowers, J. W. Phys. Rev. 1943, 64 (7), 225-235. f?R\ .I. .hA... "Theorv Excitation": Ple,-- , -Roumnna. --. . ._, P. . . W. ... -. . . ... . of - Snectrochemical - r - num: New York, 1906. (27) Dawson, J. 6.; Ellis, E). J. Spectrochlm. Acta, Part8 1967,238, 565. (28) Cordos, E.; Malmstaclt, H. V. Anal. Chem. 1973, 45,27-32. (29) Manabe, R. M.; Peipnneler, E. H. Anal. Chem. 1979,51,2066-2070. (30) Ariki, T.; Walters, ,I. P. Spectrochim. Acta, Part 8 1979, 3413 371-383. (31) Mossotti, V. G.; Laquci, K.; Hagenah, W. D. Spectrochim.Acta, Part 13 1967,238, 197-2068. (32) Bennett, F. D. I n "Progress in High Temperature Physics and Chemistry"; Rouse, C . A,, Ed.; Pergamon: Oxford, 1968; Vol. 2. (33) Karioris, F. 0.; Fish, B. R.; Royster, G. W., Jr. I n "Exploding Wires"; Chace, W. G., Moore, H. K., Eds.; Plenum: New York, 1962, Vol. 2 . I
~~
~
RECEIVED for review h r i l 2 1 , 1983. Accepted July 21, 1983. This work was supported by the National Science Foundation through Grant No. CHE 78-25542.
Determination of Trace Vanadium in Water by a Modified Cata Iyt ic-P hotometric Method Qiang Weiguo The Industrial Health Research Institute of Sichuan Province, Chengdu, The Peoples' Republic of China
Several conditions for th@currently accepted catalytlc determlnatlon of vanadium lin water, which was worked aut by Flshman and Skougstad, were researched In detail. SIX Improvements are suggested: (1) prepare (NH4),S,08-H,P0, reagent slmply at room temperature; (2) age (NH4),S,08H,PO, reagent for 24 h or more In 30 O C water bath; (3) us8 1% concentration of gallllc acld reagent; (4) equlllbrate the temperature of analysls solutions for about 15 mln In 90 O C bath; (5) use a common oallbratlon curve If dates are wlthln 10 days and changes of room temperatures are less than 5 O C ; (6) measure the absorbance of each orlglnal water sample and subtract It from llhe absorbance of the resulting solutlon. These Improvements result In greatly increased reproducibility, shortened anlalysls time, and Improved preclslon (RSD 1.1-5.4%) and acciuracy (recovery 95.8-103%). The, llmlt of detectlon is 2 ng. The determinable lowest concentratlon Is 0.2 pg/L The limproved method has been applled to determlnatlon of V In various types of water samples.
The catalytic-photometric determination of trace vanadium in freshwater is a very sensitive, simple, and easy method. Its sensitivity is much higher than typical photometric methods, Jarabin and Szarvas (I) described a qualitative test for trace V based upon its catalytic action on the oxidation-coloration reaction of gallic acid by (",),S2Os in acid medium. Its sensitivity was very high, in that 25 ng could be detected. In 1964, Fishman and Skougstad (2) adapted this test into EL sensitive, rapid, and accurate spectrophotometric method for
the determination of as low as 0.2 pg/L V in water. Later, many other authors investigated different kinds of oxidizing agents and oxidation-coloration agents (Table I). However, the sensitivities of these modified methods were not superior to the method that uses ("4)zSz08-gallic acid, and some of these modified methods are more tedious. In fact, the (~H,),Sz08-gallic acid method of Fishman and Skougstad (2) has been adopted in the field of water analysis. In "Standard Methods for the Examination of Water and Wastewater", 13th ed., the method was on probation as the only method for the determination of V, arid in the 14th ed., published in 1976 (9), it was further affirmed to be the only standard method for chemical analysis of 'V. Although the Fishman and Skougstad method (2) has generally been acceptled and applied, there are some obvious defects in it, and it seems desirable to modify and perfect it further. The most outstanding defect is that the reproducibility is comparatively poor. This is an actual problem being encountered by every worker who uses this method. That is;, even if under the same conditions of the reagents and apparatus the same standards or samples are determined by the same persons, there will still be great differences among the acquired results if the determinations are performed on different dates, or even at different times on the same day. As a remedy, therefore, their method requires the preparation of a calibration curve at the time of the analysis of each set of samples. In the present work the author has examined the principal factors affecting reproducibility, such as the concentrationsi, preparations, and stability of the key reagents, tested the preservation of V standard solutions and the possibility of
0003-2700/83/0355-2043$0 1.50/0 0 1983 American Chemical Society
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ANALYTICAL CHEMISTRY, VOL. 55, NO.
13, NOVEMBER 1983
1 1 1
Table I. Methods for the Determination of Vanadium author Fishman et al. Costache Fuller et al. Kreingol'd e t al. Il'icheva e t al. Lisetskaya e t al. Motoharu et al.
oxidizing agent (",)*SZO,
KBrO, KBrO KBrO, HZ02
H*O, KClO,
detectable limit concn, d Lv
oxidation-coloration agent gallic acid gallic acid Bordeaux N-phenyl-N-hydroxy-N'-methylurea 0-phenylenediamine KI + starcha t @-naphthylamineb
0.2 18.2 5 3
ref 2 3
4 5 6
5 20 10
I 8
H,O, oxidizes KI into Iz, first, and then the I, acts on starch to show blue color. KC10, oxidizes phenylhydrazine-psulfonic acid first and then the oxidation product couples with a-naphthylamine to produce color.
shortening analysis time, and determined the standard deviations, the recoveries, and also the V concentrations of a number of water samples. These studies have resulted in an improvement of the Fishman and Skougstad method (2). (It will hereafter be called "the original method".)
EXPERIMENTAL SECTION Apparatus. A thermostatic water bath made in China with a contact-conductivethermometer, a relay for heating, and a stirrer was used. The temperature stability should be within 10.2 "C. A spectrophotometer made in China with 50-mm absorption cells and several 50-mL and 25-mL colorimetric tubes was also used. Reagents. Standard Solutions of V. ( a ) Stock standard solution, 1 mL = 0.1 mg of V. Weigh out 22.97 mg of NH4V03, reagent grade or superior; dissolve "4v03 in 1:lOO HN03 solution and transfer the mixture into a 100-mL volumetric flask and dilute to volume with 1:lOO HN03. ( b )Middle standard solution, 1 mL = 1 pg of V. Dilute stock standard 1 to 100 (1 + 99) with demineralized water. (c) Working standard solution, 1 mL = 10 ng of V. Dilute middle standard 1 to 100 (1 + 99) with demineralized water. 5% (NH4)zSz08-H3P04 Reagent. Prepare fresh for each day's use, Weigh out 2.50 g of (NH4)zSz0sof reagent grade or superior into n SO-mL colorimetric tube, dissolve in 25.0 mL of water, add 25.0 mL of concentrated H3PO4, mix, and place in a 30 "C thermostated water bath to age for 24 h. 1 % Gallic Acid. Prepare fresh daily. Dissolve 0.500 g of C6H2(OH)3COOH.H20 of reagent grade or superior in 50 mL of warm water, heat to just below boiling, filter the hot solution through a layer of close-filterpaper into a 50-mL colorimetric tube, wash with a little water being careful not t o overrun the 50-mL scale; after cooling replenish the volume with water and place it into the 30 "C bath for at least a half hour. Concentrated and 0.035% Hg(N03)zare used as received. Procedure. Preparation of Calibration Graph. Pipet appropriate volume of working standard solution to yield the following V concentrations: 0,0.5,1,2,4,6,8 pg of V/L into 25-mL colorimetric tubes. Bring up volume of standard solution to 10 mL with demineralized water. Add 1 mL of 0.035% Hg(NOJ2, mix, and place in a 30 1 0.2 "C thermostated water bath to equilibrate for 15 min. Add 1 mL of 5% (NH4)2S208-H3P04 reagent that has been aged for 24 h or more at 30 "C, mix thoroughly, and return to the bath. Exactly at intervals of 1 or 2 min, into each tube add 1 mL of 1%gallic acid that has been equilibrated at 30 O C , thoroughly mix, and immediately return to the bath. Thermostat each tube exactly for 40 min (count the time from adding gallic acid). After the tube is removed from the bath, transfer immediately into a 50-mm absorption cell and measure its absorbance at 415 nm with a demineralized water reference. The absorbance of every tube should be quickly read out in the same short time. Subtract the value of the reagent blank (Le., the "0 ng" tube) from the absorbance of each tube, construct a calibration curve, and in the meantime note the room temperature. Samples are determined in the same way by pipetting exact aliquots of water samples (510 mL), the volumes of which are defined such that V contents of them should be less than 80 ng.
,
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H E A T I N Q TIME, MIN. Figure 1. The effect of heating time and aging temperature on va-
nadium absorbance measurements.
RESULTS AND DISCUSSION Fishman and Skougstad ( 2 ) explained the mechanism of catalyzed oxidization of ("4)2Sz08 on gallic acid in the presence of V, i.e., that what really controls the oxidation is the first-step hydrolysis product of (NH4)zSz0sin an acid solution-peroxymonosulfuric acid (HzS05). Obviously, the HzSO5 concentration will increase first and decrease later along with proceeding of hydrolysis. Therefore, one must allow the hydrolysis t o proceed t o a certain extent, i.e., allow (NH4)zS2O8-H3Po4reagent to age for appropriate time, so that a better catalyzed oxidization efficiency could be obtained. In fact, the following experiments will prove that if the conditions of preparation and aging are different, the catalyzed oxidization efficiency of (NH4)2S20s-H3P04reagent will suffer substantial effects. The effects of heating times of (NH4)&08 water solutions in a boiling bath, and aging temperatures of (NH4)2SzOsH3P04reagents are shown in Figure 1. In the upper curve, (NH,)2SzOs-water solutions were heated for different times (C-30 min) in a boiling water bath, and then H3P04was added and the reagent samples were aged for 24 h a t thermostatic 30 "C. The lower curve was obtained by the same way but aged a t room temperature 15 "C instead. One can find the following: (a) As the heating time increased from 0 rnin (Le., not heated) to 30 min, the net absorbance given by 20 ng of V decreased steeply from 0.139 to 0.032 (the 30 O C curve). (b) All the absorbances on the curve aged a t 30 "C were higher than ones a t 15 "C, and along with extending the heating time they should all tend to the same low value. So it is obvious that the heating process at a temperature approaching boiling will exert a marked effect on catalyzed oxidization efficiency of the reagent. Since the demand of the original method, Le.,
ANALYTICAL CHEMISTRY, VOL.
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55,NO. 13, NOVEMBER 1983
2045
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GALLIC ACID CONCENTRATION,%
Figure 2. The comparison of vanadium absorbance measurements made under different aging temperatures.
/
A m in o n I u rn P e r 8 u I f a t e C o n c e n t r a t lo n, %
Flgure 3. The relationship of vanadium absorbance measurements and (NH,),S,OB concentration.
to bring the reagent "just to boil" on a direct heater, will result in intense heating as wlell as the diversity of heating times among different sets of reagents, the author discovers that this is one of the principal reasons causing the poor reproducibility of the originail method. Therefore the (NH4)ZSz08-H3P04 reagent should not be heated during preparation but should instead be prepared simply at room temperature. Figure 2 further s h o w the effects of aging temperatures of (NH4)2S208-H3P04 reagents. After the reagent samples were prepared at room temperature they were aged at respective different room temperatures (- 15, 19.5, and -25 "C, read off on the starting morning) and thermostatfc 30 "C for 24 h. It is obvious that the effect of the aging temperature on catalyzed oxidization efficiency of the reagent was also very marked, so the author is sure that this is another pirincipal reason for the poor reproducibility of the original method. Therefore the (NH4)2S208-H3P04 reagent should be aged in a 30 "C thermostated bath and not a t room temperatures. The effect of the aging time of (NH4)2S208-H3P04 reagent in a 30 "C bath has been found. That is, the catalyzed oxidization efficiency of the reagent gradually reached a maximum within 2 to 3 days and gradually decreased thereafter. Fishman and Skougstaid ( 2 ) discovered that the reagent reached the optimal efficiency within 24 to 48 h, but their reagent was prepared by boiling. According to the result here, it should have been better to age the reagent for 2 to 3 days (48-72 h), nevertheless there would be certain difficulties in practical work. Therefore, the author still suggests the reagent be aged for 24 In and discarded after each day's use.
-
Flgure 4. The relationship of vanadium absorbance measuremeints and gallic acid concentration.
The effect of the concentration of (NH4)2S208 on analyiais result is seen in Figuire 3. In order to simplify the preparation of the reagent the concentration unit used here is in terms of weight/volume. That is, the sum of the volumes of water and concentrated &PO4 used for preparing (NH4)2SzC)8H3P04 reagent is approximately regarded as the total volume of the solution. The result shows that, although in the presence of V the catalyzed oxidizing capacity of the (NH,)zS208-H3P04reagent would increase with the increase of its (NH4)2S208 concentration, it was not a linear increase. In fact, when the (NH4)2S208 concentration exceeded 5%, the increased tendency iin the catalyzed oxidizing capacity became less pronounced, meanwhile the reagent blank would become very great. This will certainly result in a decrease in the upper limit of detectable V content. The author, therefore, considers it is reasonable that, the original method specified 5% concentration. In addition, as the concentration of (NH4)2S208 affects the analysiai result, it is hence required that the weighing and the preparing should be particularily accurate and reproducible. Gallic Acid. Figure 4 shows the effect of the gallic acid concentration on thie analysis result. One can see, the net absorbance given by 20 ng of V almost linearly increased with the increase of the gallic acid concentration. The concentration used in the original method is 2 % , but by the present method, at 2% concentration the absorbance of the reagent blank reached 0.358 already. Thus, upon such a blank value it is difficult to accurately detect V content up to 20 ng. Also, practice has already demonstrated that 2% gallic acid is very unstable (in fact, this greatly exceeds its solubility), and crystals are easily formed even by repetitively pipetting. Therefore the gallic acid concentration should be changed to 1% . Likewise, as the concentration of gallic acid quite affects the analysis result, it is hence required that the weighing and the preparing should be particularly accurate and reproclucible. The stability of 1%gallic acid has also been tested. Over 2 days its oxidation-coloration intensity essentially did not change and only on the third day did the intensity gradually reduce; after the fowth day the reagent itself began to colorize and crystallize. This result assures that 1% gallic acid is entirely stable for one day's use. V Standard Solution. The preservation of the V working standard solution has been tested. A prepared working standard solution was kept through the entire period of the test; meanwhile, a fresh working standard solution was prepared on each date of the test. Both solutions were simul-
2046
ANALYTICAL CHEMISTRY, VOL. 55, NO. 13, NOVEMBER 1983
Table 11. Precision and Accuracy Present ng of V
V standard 20 ng
19.5 20.3 79.7 78.8 6.3 6.9 21.3 27.1 5.5 5.8 52.8 54.2 7.3 7.6 89.2 89.2
V standard 80 ng water sample no. 6 water sample no. 6
+ V standard 20 ng
water sample no. 5 water sample no. 5 + V standard 50 ng water sample no. 2 water sample no. 2 + V standard 80 ng
f 0.3m
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Flgure 5. Representatlve calibration curves produced from data ac-
quired on different dates and at dlfferent room temperatures. taneously determined at different times over a half year, and the deviations between both were calculated. The result shows: these deviations are very small, their variation with time is also slight, and they are all within the standard deviation of the method itself. Reaction Temperature a n d Prethermostat Time. The design of the original method is to thermostat-react a t 25 "C for 60 min. But in order to allow a thermostat to continually work when the room temperature overruns 25 OC, the author tested the thermostat-react process at 30 "C for 40 min. The obtained absorbance results are on the same level as the former. Also, the latter method can save some time. In order to assure the reaction temperature is constant, the original method demands the placing of samples and standards in the thermostatic bath and keeping them there for 30-40 min before adding reagents. But a new experiment (at room temperature, 19 "C) demonstrated that the prethermostatic time from 5 rnin to 40 min did not affect the result at all. Nevertheless, considering different containers and lower room temperatures, the author suggests allowing the samples and standards to equilibrate for 15 rnin a t 30 "C. Calibration Curves a n d Reproducibility. Six standard series were prepared over nearly a one month period a t the lowest room temperature, 16 OC, through the highest, 26 O C . Three of them have been plotted in Figure 5. Those parts with low V content of these calibration curves nearly overlapped, only the absorbances of those standards with high V content increased slightly with increasing room temperature. This demonstrates that the reproducibility of the improved
19.5 20.2 80.8 80.0 6.7 7.3 28.0 28.0 5.2 5.6 53.2 53.3 7.9 7.9 87.5 87.5
standard deviation
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20.0 20.5 79.0 80.6 7.1 6.5 26.8 27.2 5.3 5.5 53.0 54.4 7.8
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method is quite good and adequate to satisfy the requirement of trace analysis. Therefore, the author proposes that for routine analyses a common calibration curve may be used if analyses are done within 10 days and the changes of room temperature are less than 5 "C, so that one need not prepare them a t each time of determining samples. Precision a n d Accuracy. Multiplicate determinations have been performed on different V standard concentrations, different water samples, and on each water sample supplemented with different V amount. The standard deviations of the method for different samples have been calculated as indicators of precision; meantime the recoveries of V in different water samples have also been calculated in order to evaluate the accuracy of the method (Table 11). The results show that the relative standard deviations varied between 1.1 and 5.4% and the range of the recoveries was 95.8 to 103%. Both can satisfy the requirements for routine analyses, and it can be considered that the precision and accuracy of the improved method are better. (The relative standard deviations of the original method are 0.9-25%; the recoveries are 85-129%.) The author has successfully determined V concentrations in tap water (0.25 pg/mL), well water (0.34-1.77 pg/L), lake water (0.56 pg/L), river water (0.34-3.92 hg/L), and pond water (0.21 wg/L) by the improved method. This proves the present method is completely suitable for determining trace V concentration in various freshwater samples. Most of above determined freshwater samples had certain background absorbances, although they were hardly found to have any color. So the author reminds users to measure the background absorbance of each original water sample and subtract it from the result. ACKNOWLEDGMENT I sincerely thank M. J. Fishman, J. E. Bonelli, and J. R. Garbarino for editing, typing, and redrafting my original manuscript, and R. F. Babcock, James Carr, and one other reviewer for their criticisms and corrections. Registry No. Vanadium, 7440-62-2; water, 7732-18-5. LITERATURE C I T E D (1) Jarabin, 2.; Szarvas, P. Acta Univ. Debrecen. 1961, 7 , 131-135; Chem. Abstr. 1962, 57,9192e. (2) Fishman. M. J.: Skouastad. M. W. Anal. Chsm. 1964, 3 6 , 1643-1646. (3) Costache, D. An. Univ. Bucuresti, Chim. 1972, 2 1 , 145-150; Chem. Abstr. 1973, 79,73233~. (4) Fuller, C. W.; Ottaway, J. M. Analyst (London) 1970, 95,41-46. (51 Kreingol'd, S. U.; Panteleimonova. A. A.: Pooonova. R. V. Zh. Anal. Khim. 1973, 28, 2179-2181. ~I
'
Anal. Chem. 1903, 55,2047-2050 (6) Il’icheva, I. A,; Degtereva, I. F.; Dolmanova, I. F.; Petrukhina, L. A. Metody Anal. Kontrolya Kach. Prod. Khim. Promsti. 1078. 4 , 65-66; Chem. Abstr. 1078, 8 9 , 190385n. (7) Lisetskaya, G. S.; Bakal, G. F. Ukr. Khim. Zh. (Russ. Ed.) 1970, 36, 709-71 2. (8) Motoharu, T.; Norio, A. Anal. Chim. Acta 1087, 3 9 , 485-490. (9) “Standard Methods for ihe Examination of Water and Wastewater”,
2047
14th ed.; APHA-AWWA-WPCF: Washington, DC, 1976; Sec. 3228,p
260.
RECEIVED for review February 28, 1983. Accepted July 15, 1983.
Simultaneous Determination of Arsenic, Antimony, and Selenium in Marine Samples by Inductively Coupled Plasma Atomic Emission Spectrometry Elisabeth de Oliveira,’ J. W. McLaren,* a n d S. 8. B e r m a n Analytical Chemistry Section, Chemistry Division, National Research Council of Canada, Ottawa, Ontario, Canada KIA OR9
A method is described for the determination of arsenic, antimony, and selenium In marine samples by continuous hydride generation inductively coupled plasma atomic emission spectrometry. A variety of sample dissolution procedures and hydride generation reaction conditions were evaluated in an attempt to eslabllsh optimal conditions for the simultaneous determination of all three elements. Detection limits of 1 fig L-’ for As and Sb and O.!; Fg L-’ for Se have been achieved. Results of analyses of NRCC and NBS reference materials demonstrate the applicability of the technique to biological and geological marine samples.
A series of reports from this laboratory (1,344 have shown that inductively coupled plasma atomic emission spectrometry (ICP-AES) is a suitable technique for the simultaneous determination of major, minor, and trace elements in a variety of marine samples. In (addition, a number of publications (6-1 7) have demonstrated that hydride generation, followed by introduction of the gaseous hydrides into an ICP, is a suitable method for the determination of several trace elements, including arsenic, antimony, and selenium, for which detection limits with conventional pneumatic nebulization are inadequate for inarine samples. A variety of sample digestion procedures have been shown to be suitable for the determination of arsenic (8, 11, 12), selenium (lo), arsenic and antimony (9, 14), or arsenic and selenium (15) in various materials; however, rather few procedures permitting the simultaneous determination of arsenic, antimony, and selenium have been described (13,16,1;3. The optimal reaction conditions for the generation of the hydrides can be quite different for the various elements. The type of acid and its concentratialn in the sample solution often have a marked effect on sensitivity. Additional complications arise because many of the hydride-forming elements exist in two oxidation states which (are not equally amenable to borohydride reduction. For example, potassium iodide is often used to prereduce As(V) and Sb(V) to the 3+ oxidation state for maximum sensitivity, but this can also cause reduction of Se(1V) to elemental Eielenium from which no hydride is formed. For this and other reasons Thompson and co-workers found it necessary to develop a separate procedure for the Present address: Instituto de Quimica, Universidade de SBo Paulo, Caixa Postal 20780, SHo Paulo, Brasil.
determination of selenium in soils and sediments (10) although arsenic, antimony, aind bismuth could be determined simultaneously (9). Recently, a method for simultaneous determination of As(III), SSb(III),and Se(1V) in water samples was reported in which the problem of reduction of Se(1V) to Se(0) by potassium iodide was circumvented by adding the potassium iodide after the addition of sodium borohydride (16). Goulden et al. (15) have reported the simultaneous determination of As, Sb, Se, Sn, and Bi in water samples, but it appears that in this case the generation of AsH3 and SbH3 occurs from the 5+ oxidation state. This report describes the application of a simple continuow hydride generation system coupled to a low-power (1.4 kVV) ICP to the determination of trace concentrations of arseniic, antimony and selenium in marine samples. A variety of sample dissolution procedures and hydride generation reaction conditions were evaluated in an attempt to establish optimal conditions for the simultaneous determination of all three elements. In addition, the effect of the oxidation state of the elements on hydride formation in dilute hydrochloric acid solution was studied. EXPERIMENTAL SECTION Apparatus. The custom ICP-echelle spectrometer used in this work has been described in previous publications (1-5). Hydride generation was accomplished in a continuous mode by using two channels of a four-channel peristaltic pump (Gilson Instrument Co., Minipuls 11) to deliver sample and borohydride reagent to a phase separator modified from that of Thompson et al. (6): a schematic diagram of the assembly employed is shown in Figure 1. An air bubble maintained by the surface tension at the junction of the two horizontal arms of the “T” prevents mixing of tlhe reagent and sample until the two solutions begin to flow down the vertical arm into the phase separator. This results in a smooth and continuous generation of hydrogen which is not significantly disturbed by changeover from one sample to the next, or to tlhe blank. The gaseous hydrides and hydrogen are swept from tlhe phase separator and into the ICP by a continuous flow of argon. Reagents. Acids used in this work were purified by subboiliing distillation (18). High-purity water was produced by passing distilled water through a deionizing system (Cole Parmer Instrument Co., Chicago IL). Other reagents were analytical grade. A solution of 170sodium borohydride in 0.1 M sodium hydroxide was prepared every day from NaBH, powder (Alfa Inorganics, Danvers, MA, 99%). A stock solution (1000 mg L-l) of As(V) was prepared by dissolution of As203in aqua regia and dilution with 3 M hydrochloric acid. Stock solutions of Sb(V) and Se(V1)were prepared by dissolution of antimony and selenium metals in aqua regia followed by dilution with 3 M hydrochloric acid. Solutions
0O03-27Q0/83/0355-2047$01.5Q/O 0 1983 American Chemical Society
