Identification of dimethylarsinic acid as the major arsenic compound in fish sauce by liquid chromatography/electrospray ionization-mass spectrometry

ABSTRACT:   Arsenobetaine, the major arsenic compound in marine animals, is substantially non-toxic. It is, however, possible that arsenobetaine undergoes bacterial transformation during manufacturing of fermented fishery products. In the present study, therefore, three types of fish sauce were examined for arsenic concentrations and species compared with those in the raw materials (sardine, Japanese sandfish and Japanese common squid). Arsenic concentrations of the three types of fish sauce were almost equivalent to those in their raw materials, suggesting no accumulation of arsenic during fermentation. Arsenic speciation was performed by a combination of cation-exchange liquid chromatography (LC) and electrospray ionization (ESI)-single quadrupole mass spectrometry (MS). Optimal conditions of LC/ESI-MS were established to analyze seven arsenic compounds (arsenate, monomethylarsonic acid, dimethylarsinic acid, arsenobetaine, trimethylarsine oxide, arsenocholine and tetramethylarsonium ion) found in biological samples. When analyzed by LC/ESI-MS, the major arsenic compound in the raw materials was arsenobetaine as expected, while not arsenobetaine but dimethylarsinic acid was identified as the major arsenic compound in the three types of fish sauce. These results suggest that arsenobetaine in the raw materials is converted to dimethylarsinic acid but not to arsenate with high toxicity by bacterial actions during manufacturing of fish sauce.