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Chromatographic Separation and Identification of Products from the Reaction of Dimethylarsinic Acid with Hydrogen Sulfide
journal contribution
posted on 2005-12-19, 00:00 authored by Michael W. Fricke, Matthias Zeller, Hongsui Sun, Vivian W.-M. Lai, William R. Cullen, Jody A. Shoemaker, Mark R. Witkowski, John T. CreedThe 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 min) 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 coeluted
with the slower forming product. This coeluting 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 collisionally activated dissociation (CAD) experiments on m/z 276 did
not produce a product ion at m/z 173. These data indicate that two different species are present
in solution, while NMR data indicate 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 compound is a product of the
ESI desolvation process.