Quantitative selenium speciation in cod muscle by isotope dilution ICP-MS with a reaction cell:: comparison of different reported extraction procedures

Four different extraction procedures have been evaluated aiming at quantitative recovery of Se species in their extraction from cod muscle. ICP-MS with an octopole reaction system has been used to allow the elimination of interfering argon dimers in Se specific detection. Speciation analysis was performed by separating the species using both reversed-phase and size exclusion HPLC and the separated Se compounds were quantified on-line by post-column isotope dilution after converting intensity chromatograms into mass flow chromatograms (via adequate mathematical corrections). Se-78/Se-77 ratio was used, after correction of intensity signals for SeH+ formation, to quantify both total Se and each selenocompound. The highest Se recoveries were obtained when using enzymatic digestions whereas only 5% of total Se in cod was extracted when a "soft" extraction procedure (MeOH/HCl) was used. The main selenocompound found in enzymatic hydrolysates was selenomethionine. This selenocompound was absent in MeOH/HCl extracts indicating that Se is mainly incorporated into proteins. A number of unidentified Se species have also been detected in cod muscle tissue and their presence is documented here.