New Method and Detection of High Concentrations of Monomethylarsonous Acid Detected in Contaminated Groundwater

Monomethylarsonous acid (MMA(III)) was detected in groundwater from a former herbicide production plant in the USA. The site has total arsenic concentrations up to thousands of mg/L, representing one of the most severe cases of arsenic contamination ever reported. Structure-specific detection of MMA(III), along with arsenite (As-III), arsenate (As-V), monomethylarsonic acid (MMA(V)), and dimethylarsinic acid (DMA(V)), was achieved using liquid chromatography separation with electrospray ionization tandem mass spectrometry detection (HPLC-ESI-MS/MS). To enable the electrospray of MMA(III) and As-III, dimercaptosuccinic acid (DMSA) was used to derivatize these trivalent arsenicals online, so that their complexes with DMSA could be detected using negative ionization ESI-MS/MS. The presence of MMA(III) was verified using high resolution mass spectrometry to measure accurate mass, tandem mass spectrometry to monitor fragmentation, and three different separation techniques to resolve arsenic species. The measured accurate mass of the suspected MMA(III) compound in a groundwater sample was 122.9607 +/- 0.0003 amu, which was in good agreement with the theoretical value and that of the MMA(III) standard. Simultaneous monitoring of As0(+) at m/z 91 and SO+ at m/z 48 using HPLC-ICPMS operating in dynamic reaction cell mode ruled out possible confounding from any sulfur-containing arsenic compound. The concentrations of MMA(III) found in the groundwater samples from a contaminated site were as high as 3.9-274 mg/L, the highest ever observed in the environment