Determination of uranium and thorium in basalts and uranium in aqueous solution by inductively coupled plasma mass spectrometry

Abstract

Methods are described for the simultaneous determination of uranium and thorium in basaltic rocks at concentrations from mg kg^–1(ppm) to µg kg^–1(ppb) levels and for uranium in solution at femtogram levels. The technique for basalts does not require preconcentration and samples are taken into solution using a conventional rapid acid digestion method. The results obtained agree, in general, within 10% of published values for eight basalt reference materials. In addition, optimum conditions with respect to acidic media and storage vessel were ascertained as 5% HNO₃ and high-density polyethylene, respectively. The method is applicable to a wide range of silicate rocks and other geological materials. For the determination of uranium at sub-ng ml^–1 concentrations, such as occur in natural waters, sample introduction by electrothermal vaporization was investigated using several matrices. Complexation of uranium with ethylenediaminetetraacetic acid (EDTA) prior to analysis proved successful at levels down to 0.5 fg ml^–1. Routine determination of such low concentrations is thus feasible.