Determination of selenium in seleno compounds and marine biological tissues using electrothermal atomization atomic absorption spectrometry

Abstract

The effect of copper, palladium and nickel nitrates, and combinations of these chemical modifiers with magnesium nitrate, on the thermal stabilization of selenium in the furnace was examined for both selenium compounds (Se^–II as selenocysteine, selenomethionine, selenocystamine dihydrochloride and trimethylselenium iodide, Se^IV as selenium dioxide and Se^VI as selenate) and marine biological digests. When compared in molar concentrations, the stabilizing effect of palladium was superior to that of the other metals. Palladium allowed higher pre-treatment temperatures to be used in the furnace programme. Addition of Mg(NO₃)₂ was found to increase significantly the magnitude of the selenium signal but to lower the thermal pre-treatment temperature that could be used. Digested biological tissues required higher levels of chemical modifier than reference selenium standards for full recovery of the analyte.

The influence of the ratio of elements in each modifier combination and the absolute amount of modifier was also examined for both selenium compounds and digested marine biological tissues (NIST SRM 1566a Oyster Tissue; NRCC DORM-1 Dogfish Muscle and TORT-1 Lobster Hepatopancreas). Three-dimensional integrated absorbance surface response diagrams showed that all selenium compounds, except trimethylselenium iodide, gave a similar optimum working range for both Cu–Mg and Pd–Mg when these modifiers were compared on a molar basis. With trimethylselenium iodide, only Pd–Mg was found to be effective in the thermal stabilization of selenium. The surface response diagrams for digested biological tissues, when compared with selenium compounds, showed a marked shift to higher levels of modifier for optimum recovery of the element. In general, Pd–Mg gave a 10% increase in signal magnitude relative to the optimum Cu–Mg modifier for Se in biological digests. The optimum signal response for selenium in biological tissue using Se^IV as an inorganic standard was obtained using Pd–Mg at a level of 0.1 µmol of Pd + 0.4 µmol of Mg (i.e., 10.6 µg of Pd + 9.7 µg of Mg). This value was selected as a compromise between the optimum amount of modifier needed for the stabilization of selenium compounds in aqueous reference standards and that required by the digested tissues.