Investigation of the pH effects on the formation of methylated thio-arsenicals, and the effects of pH and temperature on their stability

Abstract

Thio-arsenicals have been discovered in both food and biological samples. During analysis, the potential exists for interconversion between the oxide and sulfide forms, raising questions regarding analytical factors which may influence the observed speciation. Within this context, the conversion of trimethylarsine oxide (TMAO), dimethylarsinic acid (DMA) and monomethylarsonic acid (MMA) to their respective sulfide forms, in the presence of sulfide (Na₂S) was investigated with respect to pH. All three arsenic oxides produced very little conversion to the corresponding sulfide above pH 8 while conversion to the sulfide form was observed at every pH ≤ 7. Less than 10% of the initial TMAO remained at every pH below eight, with the sulfide form trimethylarsine sulfide (TMAS) accounting for more than 90%. Likewise, the mono- and di-thiolated forms of DMA (dimethylthioarsinic acid (DMTA) and dimethyldithioarsinic acid (DMDTA) respectively) formed over the pH range 5–7, leaving less than 8% of the initial DMA in solution. In the pH range 5–7, MMA was converted to monomethylthioarsonic acid (MMTA), with more than half of the MMA converted to an unidentified peak. This peak is thought to be monomethyldithioarsonic acid (MMDTA) but a positive identification was not possible due to lack of a synthetic standard. The primary species detected at pH 3 were the mono-thiolated forms MMTA and DMTA. Given the range of extraction and separation procedures (specifically pH and temperatures) associated with speciation analysis, the stability of TMAS, DMTA and MMTA under such conditions is another area of concern. Room temperature sulfide to oxide conversion rates from 0.3% to 2.2% per week were observed in basic (pH 10), neutral (pH 7) and acidic (pH 3) solutions, demonstrating very little pH-induced instability for all three thio-arsenicals. Elevated temperature accelerated the sulfide to oxide conversion rates to ∼10% per week for all three thio-arsenicals, while refrigeration retarded it to <0.4% per week.