Precise determination of 235U/238U isotope ratio in soil samples by using thermal ionisation mass spectrometry

Abstract

A rapid method based on extraction chromatography was developed for the separation of uranium from soil samples for TIMS. The isotope ratios were measured on three standards using a VG 54-30 thermal ionisation mass spectrometer in static mode with Faraday cup and Daly ion counting system. The use of a WARP (Wide Aperture Retardation Potential) energy filter improves abundance sensitivity by an order of magnitude over a conventional VG Sector TIMS. An abundance sensitivity of 28 ppb at mass 237 with respect to 238U can be achieved. By correcting mass bias as a function of ion beam intensity, precision of the 235U/238U ratio was enhanced with a sample load of around 300 ng U. The resulting reproducibility for standards and soil samples was better than 0.2% at two standard deviations. Uranium isotopic compositions have been determined in soil samples and compared between Chernobyl nuclear accident site and the criticality accident, which occurred in Japan at the JCO site.