Identification of Monomethylmonothioarsonic Acid as the Major Thioarsenical Generated During Extraction Processes for Arsenic Species Analysis.

Acid extraction is commonly used to analyze arsenic species in rice. During the extraction process, spiked monomethylarsonic acid (MMA) is often transformed into different compounds. A similar phenomenon is observed in the arsenic speciation analysis of seafood. To identify these compounds, we analyzed a previously prepared extract using liquid chromatography-time-of-flight/mass spectrometry in differential analysis and liquid chromatography-inductively coupled plasma-MS. The compound was identified as monomethylmonothioarsonic acid (MMMTA), a thioarsenical, which is estimated to be more cytotoxic than MMA. As MMMTA was readily produced by bubbling hydrogen sulfide through MMA, this suggests that MMA reacts with sulfur in rice during the extraction process. Our data also suggested that dimethylarsinic acid could be transformed into another compound, although the generation rate was low. For reliable arsenic speciation analyses, the transformation of arsenic compounds during extraction must be avoided. This study demonstrates that arsenic compounds can be transformed by dilute acid extraction.