High-precision determination of 238U/235U isotope ratio in rocks by multicollector inductively coupled plasma mass spectrometry
T15N1
G.V. Mandzhieva, A.S. Sadasyuk, I.V. Chernyshev, K.N. Shatagin, A.V. Chugaev, B.I. Gareev
The paper describes high-precision U-isotope analysis of rocks. The analysis is based on the multi-collector inductively-coupled plasma mass spectrometry (MC-ICP-MS) and includes, as an essential part, a three-stage ion-exchange chromatography uranium separation from chemically complex solutions. In the analysis, the 235U and 238U mass-bias of the MC-ICP-MS is corrected against the reference ratio of 233U and 236U in a double-spike added. The double-spiking precedes sample chemical digestion. This approach is useful as it corrects isotope data for a possible U-isotope fractionation that could have occurred during chemical preparation of samples. The double-spike 236U/233U is 1.03183 ± 0.00005. The extraction procedure allows more than 80% uranium yield. Accuracy of the analysis was estimated by the study of 238U/235U in international standard sample IRMM-3184 as well as in monitor samples. The uncertainty is estimated to be ± 0.008% (2SD) which is comparable to the accuracy of the results previously published by foreign researches.The analysis was elaborated for studying natural 238U/235U ratio variations. It is used by the authors for high-precision U-isotope composition determination in rocks (granites, shales, volcanic rocks) and U-bearing minerals.
