Determination of (Oxy)thioarsenates in Sulfidic Waters

Although it has long been known that soluble arsenic−sulfur (As−S) compounds exist in sulfidic waters and may play significant roles in several important processes in the biogeochemical arsenic cycle, no suitable analytical methods exist for their determination. We provide evidence that the four homologue (oxy)thioarsenates, mono-, di-, tri-, and tetrathioarsenate (AsO3S3-, AsO2S2 3-, AsOS3 3- and AsS4 3-), can be formed in geochemical model reactions between arsenite and sulfide under anoxic conditions (through currently unknown reaction mechanisms) and that these compounds appear to be major As species in natural sulfidic waters. These As−S species are quantified by anion-exchange chromatography−inductively coupled plasma mass spectrometry (AEC-ICPMS) with instrumental detection limits of ∼0.1 nmol of As L-1 in undiluted samples; arsenite, arsenate, and monomethylarsenate are quantified as well, but dimethylarsenate cannot be analyzed by this technique. Sulfur in the eluting peaks can be measured as SO+ with detection limits of ∼0.1 μmol of S L-1. The (oxy)thioarsenates were synthesized in solution and characterized by electrospray-tandem mass spectrometry (ES-MS-MS). In geochemical model solutions, we confirmed that both the AEC-ICPMS retention times and the ES-MS-MS spectra of the reaction products of sulfide and arsenite matched the synthesized (oxy)thioarsenate standards; for natural waters, the mass spectrometric confirmation was unsuccessful, due to matrix interferences.