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CHROMATOGRAPHIC SEPARATION AND IDENTIFICATION OF PRODUCTS FROM THE REACTION OF DIMETHYLARSINIC ACID WITH HYDROGEN SULFIDE
Citation:
FRICKE, M., M. ZELLER, H. SUN, V. W. LAI, W. R. CULLEN, AND J. A. SHOEMAKER. CHROMATOGRAPHIC SEPARATION AND IDENTIFICATION OF PRODUCTS FROM THE REACTION OF DIMETHYLARSINIC ACID WITH HYDROGEN SULFIDE. CHEMICAL RESEARCH IN TOXICOLOGY. American Chemical Society, Washington, DC, 18(12):1821-1829, (2005).
Impact/Purpose:
To assess the stability of "FeOOHAs" residues in existing drinking water distribution systems and drinking water generated wastes.
Description:
The reaction of dimethylarsinic acid (DMAV) with hydrogen sulfide (H2S) is of biological significance and may be implicated in the overall toxicity and carcinogenicity of arsenic. The course of the reaction in aqueous phase was monitored and an initial product, dimethylthioarsinic acid, was observed by using LC-ICP-MS and LC-ESI-MS. Dimethylarsinous acid was observed as a minor product. A second slower-forming product was identified and the electrospray mass chromatograms for this species produced ions at m/z 275, 171 and 137 in positive mode. To aid in the identification of this slower-forming product, crystalline standards of sodium dimethyldithioarsinate and dimethylarsino dimethyldithioarsinate were prepared and re-characterized by using improved spectroscopic and structural analysis techniques. An aqueous solution of sodium dimethyldithioarsinate produced a single major chromatographic peak that matched the retention time (7.6 minutes) of the slower-forming product and contained similar molecular ions at m/z 275, 171 and 137 via LC-ESI-MS. The dimethylarsino dimethyldithioarsinate standard produced four aqueous phase species one of which co-eluted with the slower forming product. This co-eluting peak also produced the identical ESI-MS ions as the slower-forming product of DMAV + H2S. ESI-MS/MS experiments conducted on sodium dimethyldithioarsinate in deuterated water produced molecular ions at m/z 276, 173 and 137. Subsequent CAD experiments on m/z 276 did not produce a product ion at m/z 173. This data indicates that two different species are present in solution while NMR data indicates that only dimethyldithioarsinic acid exists in aqueous solutions. This discrepancy was investigated by conducting NMR studies on the acidic solution of sodium dimethyldithioarsinate after taking this solution to dryness. The resolubilized solution produced a proton NMR signal characteristic of dimethylarsino dimethyldithioarsinate. Therefore, it was concluded that the ESI-MS ion at m/z 275 associated with the slowly forming second reaction product and the sodium dimethyldithioarsinate standard is a product of the ESI desolvation process.