Investigation of selenium speciation in in vitro gastrointestinal extracts of cooked cod by high-performance liquid chromatography–inductively coupled plasma mass spectrometry and electrospray mass spectrometry

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Abstract
As part of an ongoing study of Se bioavailability from the human diet, Se was determined in cooked cod and in enzyme extracts of cooked cod. Total Se was measured by direct nebulization ICP-MS and standard additions, following digestion with HNO3 in stainless steel pressure decomposition vessels. The concentration of Se in the cod was 1.52 mg kg–1 with an LOD of 0.008 mg kg–1. An in vitro gastrointestinal enzymolysis procedure was used to extract Se species from the cooked cod. HPLC, using an anion-exchange column, was used to separate Se standards (selenomethionine, selenocystine, sodium selenite and sodium selenate) and the species soluble in the gastrointestinal extract (pH 6.8). The 82Se signal was used to quantify the individual Se species. In the extracts of cooked cod, approximately 12% of total Se was thought to be selenite whilst the remaining Se eluted at a retention time different to that of any of the Se standards measured. However, as the retention time was close to that of selenomethionine, it was suggested that the Se-containing species was organic rather than inorganic. A similar peak was also found in the enzyme blanks. In order to gain further information as to the identity of the unknown species, the samples and standards were subjected to electrospray-MS (ES-MS) in both positive and negative ionization modes. Although the four standards were amenable to analysis by ES-MS, more work is required to improve the sensitivity of the system. This will involve modifying the chromatography for both ICP-MS and ES-MS.