Thioarsenates have recently been reported to contribute substantially to arsenic (As) speciation in paddy-soil pore waters. Here, we show that thioarsenates can also accumulate in rice grains and rice products. The occurrence of thioarsenates in rice is typically not addressed because commonly used acid-based extractions and chromatographic separations transform thioarsenates.
For their detection, a method was developed using a pepsin-pancreatin enzymatic extraction followed by chromatographic separation at pH 13 coupled to a triple quadrupole inductively coupled plasma mass spectrometer (ICP-QQQ). Besides the commonly investigated species: inorganic As (iAs: arsenite and arsenate) and dimethylarsenate (DMA), this method enables the detection of two inorganic and four methylated thioarsenates. Dimethylmonothioarsenate (DMMTA) was the most frequently encountered thioarsenate species, detected in 115 out of 120 analyzed commercial samples including white, parboiled and husked rice, rice waffles, and rice flakes.
Food guidelines in Europe only control the content of iAs in rice grains and rice-based products. Commonly used acid-based extractions transform DMMTA to non-regulated DMA. Co-determination of high DMMTA concentrations observed in rice waffles can be critical from a food safety perspective since DMMTA is known to be highly cytotoxic. Further investigations are needed to understand how widespread thioarsenate occurrence in rice grains and rice products is and how agricultural managing practices or food processing treatments influence the accumulation of thioarsenates.