Extracts of 11 samples of shrimp, crab, fish, fish liver, shellfish and lobster digestive gland (hepatopancreas), including five certified reference materials, were investigated for their contents of arsenic compounds (arsenic speciation). The cation-exchange high-performance liquid chromatography procedure was optimized to separate six cationic arsenicals present in the samples with internal chromatographic standardization by the trimethylselenonium ion, which was detected at m/z 82 pSe), in addition to arsenic at m/z 75, by inductively coupled plasma mass spectrometry. The content of each species (as arsenic atom) relative to the total arsenic extracted from the samples were: arsenobetaine 19-98%, arsenocholine and trimethylarsine oxide O-O.6% and the tetramethylarsonium ion 0-2.2%. Additionally, an unknown arsenic species (Ul) was present at 3.1 -1 8% in the shellfish and in the lobster digestive gland, and another unknown (U2) was present at 0.2-6.4% in all samples. The contents of arsenite and arsenate were 0-1.4%, dimethylarsinate 8.2-29% while monomethylarsonate was detected only in oyster at 0.3% of the total extracted arsenic. Finding tetramethylarsonium ion and arsenocholine in a variety of samples indicates steps of a biosynthetic pathway of arsenic leading to arsenobetaine in the marine environment. The intake of inorganic arsenic via ingestion of the seafood samples that were analysed did not represent a toxicological problem to humans. The limits of detection (LOD) were in the range 10-50 ng g-' (dry mass) with the exception of arsenobetaine for which the LOD was 360 ng g-l.

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