Determination of small amounts of selenium in organic matter
Methods for determining small amounts of selenium in organic matter have been examined. Determination first requires oxidative destruction of organic matter. The usual oxidising systems employing sulphuric acid initially, with hydrogen peroxide, nitric and/or perchloric acids, gave very low recoveries of selenium when high proportions of fats were present in the samples. Continuous combustion and oxygen flask methods were also unsatisfactory. A wet-oxidation procedure in which nitric and perchloric acids (5 + 1) were followed by nitric and sulphuric acids gave satisfactory results. It was important to ensure that selenium was present as selenium(IV) by boiling the solution with hydrochloric acid after the oxidation. Colorimetric, gas-liquid chromatographic, fluorimetric and atomic spectroscopic methods were considered for the selenium determination; of these, the last two were selected for collaborative trials. For the fluorimetric finish, the solution, after oxidation, was treated with 2,3-diaminonaphthalene reagent, the complex was extracted with cyclohexane or dekalin and the fluorescence in the organic phase was measured in a spectrofluorimeter with excitation at 369 nm and emission reading at 525 nm. Atomic-absorption or fluorescence measurements were made after hydrogen selenide generation and atomisation. The precisions of analyses for samples with selenium contents from less than 0.1 to 10 µg g–1 by the fluorimetric and hydride generation methods are illustrated.