0039.9140,81/060402-03502.OOiO Copyright 0 1981 Pergamon Press Ltd
Tulontu. Vol. 28. pp. 402 to 404. 1981 Printed in Great Britam. All rights reserved
GAS CHROMATOGRAPHIC DETERMINATION DIMETHYLARSINIC ACID IN AQUEOUS SAMPLES
SYOZO FUKUI,TERUHISAHIRAYAMA, MOTOSHI NOHARA and YOSHIHIKOSAKAGAMI Kyoto
(Received 11 August 1980. Revised 10 December 1980. Accepted 23 December 1980) Summary-Procedures are described for the determination of dimethylarsinic acid in aqueous samples by gas chromatography of iododimethylarsine. Dimethylarsinic acid is converted into iododimethylarsine rapidly and quantitatively by treatment with hypophosphorous acid and potassium iodide, the resulting iododimethylarsine is extracted with toluene and determined with a gas chromatograph equipped with an electron-capture detector. The detection limit is 0.005 ppm as As. Recoveries from urine are over 95%. Other arsenicals do not give any response in the chromatography. The method has been applied to the determination of dimethylarsinic acid in urine and in water extracts of sea-weeds.
Dimethylarsinic acid (DMAA) has been recognized as a major intermediate in metabolism of arsenicals in the body and environmental micro-organisms, and has been found as a ubiquitous arsenical in urine and the environment. l4 Most methods for determination of DMAA are based on its reduction to dimethylarsine and subsequent electric-discharge emission spectroscopy or atomic-absorption spectrometry, coupled with isolation of dimethylarsine from other arsenic hydrides.5-‘0 Some gas chromatographic methods based on the conversion of DMAA into volatile compounds such as diethyldithiocarbamate’ ’ or bromodimethylarsine” have been reported. A method involving the conversion of DMAA into iododimethylarsine (IDA) with hydriodic acid, followed by determination with electron-capture gas chromatography has also been reported.13 Because of the high sensitivity and selectivity of this method, a modification, mainly of the procedure for reduction and iodination of DMAA, has been attempted. It is based on the rapid and quantitative conversion of DMAA into IDA by treatment with hypophosphorous acid and potassium iodide; it also contributes toward stabilization of the resulting IDA, and simplification of the procedure.
EXPERIMENTAL Apparatus / A Shimadzu model 3BE gas chromatograph equipped with a 63Ni electron-capture detector and a 3 mm x 2 m glass column containing 10% SE-30 on 60-80 mesh Chromosorb AWA were used. The column was conditioned by preliminary introduction of IDA solution (prepared by treating stock DMAA solution by the procedure described below) to saturate IDA-adsorbing sites in the column. The oven and detector temperatures were both 80”. the carrier gas was nitrogen and its flow-rate 60 ml/min. The sensi402
tivity was set so that 0.1 ng of As (as IDA) gave [email protected]
”/; full-scale deflection. A Hitachi model M-80 GC-MS was used to confirm that IDA was formed. Reagents Hypophosphorous acid (Wako Pure Chemicals Co.) was 50% w/v, certified grade. DMAA (Wako Pure Chemicals Co.) was recrystallized from aqueous ethanol. IDA was prepared by the procedure described by Soderquist et al.” Monomethylarsonic acid was obtained from Ventron Corp. Toluene was distilled, and checked for purity by gas chromatography. Other chemicals were reagent grade. DMAA stock solution (100 ppm of As), prepared in distilled water, was reasonably stable for one week. Working solutions of 0.014.1 ppm (of As) were prepared by fresh dilution of the stock solution. Procedure Take 5 ml of aqueous sample in a glass-stoppered testtube, add 5 ml of hypophosphorous acid and 5 g of potassium iodide, and shake the mixture until the potassium iodide has dissolved. Add 5 ml of toluene and shake again for 2 mitt, then allow to stand for several minutes. Introduce 1 hl of the upper toluene layer into the gas chromatograph by microsyringe. If a thick emulsion occurs, centrifuge the mixture at 3000 rpm. Handle reagent blanks and the working solutions similarly.
RESULTS AND DISCUSSION The procedure using hypophosphorous acid and potassium iodide achieves rapid and quantitative conversion of DMAA into IDA even in presence of water, so the prior evaporation of the sample io dryness (as in the procedure bv Soderauist et al.‘? can be eliminated. Figure 1 shows that the optimum quantities of reagents are 5 ml of hypophosphorous acid and 5 g of potassium iodide. Tvnical aas chromatograms obtained by this method are shown in Fig. 2. The retention time of the peak in Fig. 2 closelv aarees with that of svnthetic IDA. The identity of the product was confirmed by GC-mass spectrometry, which gave m/e values of 232 [(CH&AsI], 217 [CH,AsI], 202 [AsI], 127 [I], I05 [(CH&As] and 90 [CHsAs], as shown in Fig. 3. The need for rapid sample work-up and gas chromatography on account of the hydrolytic instability of IDA I
(g for pomssium iadidq
ml for v
Fig. 1. Influence of quantities of hypophosphorous acid and potassium iodide on the conversion of DMAA into IDA.
Fig. 2. Gas chromatograms of: (A) blank, (B) 0.02 ppm DMAA working standard. 171
Fig. 3. Mass spectrogram of IDA obtained from DMAA solution (lo5 ppm as As), was noted by Soderquist et al.” However, in the present method the IDA is fairly stable in the toluene layer, even when this is in contact with the aqueous reaction mixture, the detector response to IDA in the toluene layer remaining constant for 12 hr. These results suggest that any hydrolytic degradation of the IDA is counteracted by resynthesis. A calibration graph of peak-height us. amount of DMAA was linear up to 0.1 ng (as As). Assuming a l-p1 injection volume, the lower limit of detection is 0.005 ppm. Monomethylarsonic acid and inorganic arsenate or arsenite did not yield any products giving a gas chromatographic response. Di-iodomethylarsine and tri-iodoarsine are probably formed from monomethylarsonic acid and arsenate or
Table 1. Recovery tests of DMAA from urine DMAA added, pgglml(of As) 0 0.020 0.040 0.060 0.080 0.100
DMAA found,* pg/ml (of As)
Recovery,* 0, ,”
0.030 0.049 (0047-0.050) 0.068 (0.06&0.071) 0.091 (0.087XW4) 0.111 (0.108-0.113) 0.130 (0.128-0.139)
95 (85-100) 95 (90-103) 102 (95-107) 101 (98-W) 100 (98-109)
* Averages of five runs, ranges in parentheses.
Fig. 4. Gas chromatograms of: (A) urine sample, (B) toluene layer washed with water.
arsenite respectively, but because of their high boiling points (128”/16 mm,‘* and 403”,” respectively; the b.p. of IDAI is 154”), are not likely to be eluted. The results of recovery tests for DMAA in urine are shown in Table 1. Typical gas chromatograms for DMAA in urine are shown in Fig. 4. The target peak shown in Fig. 4 (A) completely disappeared when the upper toluene layer was shaken with water for 40 min, indicating that the peak results from a single hydrolysable substance. Acknowledgement-The work was supported by a grant for Environmental Preservation Research from the Environmental Agency. REFERENCES 1. J.,M. Wood, Science, 1974, 189, 1049. 2. E. A. Woolson, Environmental Health Perspectives, 1977, 19, 73. 3. W. P. Rideley, L. D. Dizikes, A. Cheh and J. M. Wood, ibid., 1977, 19, 43.
4. R. S. Braman and C. C. Foreback, Science, 1973, 182, 1247. 5. R. S. Braman, L. L. Justen and C. C. Foreback, Anal. Chem., 1970,42, 1480. 6. F. E. Lichte and R. K. Skogerboe, ibid., 1977, 44, 180. 7. R. S. Braman, L. L. Justen and C. C. Foreback, ibid., 1972, 44, 2195. 8. R. S. Braman, D. L. Johnson, C. C. Foreback, J. M. Ammons and J. L. Bricker, ibid., 1977, 49, 621. 9. E. A. Crecelius, ibid., 1978, SO, 826. 10. C. Feldman, ibid., 1979, 51, 664. 11. E. H. Daughtrey Jr., A. M. Fitchett and P. Mushak. Anal. Chim. Acta, 1975, 79, 199. 12. B. J. Gudzinowicz and H. F. Martin. Anal. Chem.. 1962, 34,648. 13. 1 C. J. Soderquist, D. G. Crosby and J. B. Bowers, ibid., 1974,46, 155. 14. R. D. Feltham, J. Organometal. Chem., 1967, 7, 285. 15. I. T. Millar, H. Heaney, D. M. Heinekey and W. C. Fernelius, Inorganic Syntheses, 1960, VI, 113. 16. J. Bailar Jr., ibid., 1939, I, 103.