Speciation analysis of arsenic in seafood and seaweed: Part I—evaluation and optimization of methods

Abstract

Several extraction and chromatographic methods were evaluated to identify optimum conditions for arsenic speciation analysis in seafood and seaweed. The extraction systems, which include aqueous, aqueous-organic, acidic, basic, and enzymatic solutions, were examined for their efficiency in extracting arsenic from finfish, crustaceans, molluscs, and seaweed keeping the chemical forms of the native arsenicals intact. While dilute solutions of nitric acid, hydrochloric acid, and tetramethylammonium hydroxide (TMAH) extract high fractions of arsenic from most of the matrices, the extractants oxidized arsenite (As³⁺) to arsenate (As⁵⁺) and converted some arsenosugars and non-polar arsenicals to known and/or unknown forms. Hot water (90 °C) effectively maintained the integrity of the native arsenic species and enabled analysis of the extracts with no further manipulation than filtration and dilution. Stepwise extraction of water-soluble and non-polar arsenic with hot water and a mixture of dichloromethane and methanol, respectively, resulted in sufficiently quantitative (> 75%) arsenic extraction from seafood and seaweed. Anion and cation exchange chromatographic methods were optimized for separation and quantitation of the arsenicals extracted into hot water. The non-polar arsenicals were collectively determined after digesting the extract in acid. The application of the optimum extraction and chromatographic conditions was demonstrated by analyzing certified reference materials of tuna fish tissue (BCR 627), lobster hepatopancreas (TORT-2) and oyster tissue (SRM 1566b), and a sample of hijiki seaweed. For all the matrices, good agreement (80–92%) was found between the total water-soluble arsenic and the sum of the concentrations of the chromatographed species. Limits of quantification (LOQ) were in the range 4–11 ng g⁻¹ for 16 arsenicals.