New arsenic compound identified in rice grain: dimethylarsonyldimethylarsinic acid

Environmental context The arsenic content and its speciation in harvested rice are dependent on the genetics of the rice variety, the agricultural practise as well as the speciation of arsenic in the soil. The geochemical conditions and therefore the microbiological regime in the paddy fields can influence the biomethylation of the arsenic in the porewater and therefore can influence the extent of arsenic uptake and its speciation in rice plants.Rationale Rice is routinely monitored throughout the world for compliance with regulatory limits of inorganic arsenic. Rice grains are well known for their relatively high arsenic concentration in the form of dimethylarsinic acid, methylarsonic acid and inorganic arsenic with other arsenic containing compounds rarely occurring. During a routine survey by the US FDA, an unexpected arsenic compound was noted.Methodology The compound was eventually identified using a combination of liquid chromatography with elemental (HPLC–ICPMS) and molecular mass spectrometry (HPLC–ESMS) as well as chemical derivatisation.Results Here we report the identification of the previously unknown arsenical in rice grain, dimethylarsonyldimethylarsinic acid; its origin is unknown.