MC-ICP-MS analysis of non-natural U isotope ratios using a229Th/232Th external mass bias correction

Abstract

Civil and military uses of uranium lead to the possibility that U exists in the environment with non-natural isotope ratios. Therefore, characterisation of environmental U cannot rely on prior knowledge of any particular isotope ratio. For example, it is common to use the natural 235U/238U ratio to quantify mass bias effects in ICP-MS measurements and adjust 234U/238U accordingly. Such an approach cannot be applied to environmental samples whose 235U/238U is unknown. In this paper we demonstrate accurate and precise measurement of 235U/238U, 234U/238U and 236U/238U by multiple collector ICP-MS using a 229Th–232Th double spike to correct mass bias. This method has a major advantage over external correction of mass bias using bracketing standards because it can successfully correct small matrix-dependent changes in mass bias between standards and real environmental samples. However, care must be taken to avoid contamination of the analyte with Th from the sample which will differ in isotopic composition from the 229Th–232Th spike. Data are presented on non-natural U standards with a variety of 235U enrichment and depletion. The efficacy of the method for environmental samples is demonstrated with two analyses of soil samples taken from a military site where depleted uranium (DU) munitions have been tested. While the presence of DU in these soils is clearly indicated, detailed source apportionment is not possible because DU appears to be highly variable in isotopic composition.