SELENIUM ISOTOPE FRACTIONATION IN NATURE AND IN THE LABORATORY

Abstract

A method was developed for the accurate comparison of selenium isotope ratios for various samples using selenium hexafluoride gas in a mass spectrometer. Calculations were made to determine the extent of selenium isotope fractionation that can be expected in isotope exchange processes. The selenium isotope ratios for 16 naturally occurring samples have been compared using meteoritic selenium as a standard. These results showed small variations for samples obtained from massive sulphide ores, including those of igneous origin. However, in selenium samples extracted from plant material and soil, variations of up to 1.5 percent were found in the Se82 /Se76 ratio. Undoubtedly, these large variations are due to oxidation and reduction processes. A kinetic isotope effect of 1.5 percent was found in the laboratory reduction of selenite to elemental seleniun. This effect is of the same order of magnitude as that predicted by a simple model of the reduction mechanism.