Precise and rapid determination of 238U/235U and uranium concentration in soil samples using thermal ionisation mass spectrometry

Uranium isotopic composition and concentration have been determined on over 500 soil samples from the Greenham Common Air Base and surrounding Berkshire, UK, to detect potential contamination from nuclear sources. Due to the large number of samples involved in this study and the potentially subtle nature of the contamination a new method of analysis was developed which is both rapid and highly precise. Data are presented for international standard reference materials and soil samples for uranium content and 238U/235U measured by thermal ionisation mass spectrometry (TIMS) using a dynamic combination of Faraday and ion-counting detectors. Precision of the 238U/235U ratio was enhanced by correcting for mass bias as a function of ion beam intensity. In-run mass fractionation was controlled without recourse to double spiking by ensuring that sample loads were always in excess of 350 ng U. The resulting reproducibility for standards and soil samples was better than 0.2% at 2 standard deviations (sd). This precision means that an addition of ≥0.4 ng of enriched uranium (93 at% 235U) per gram of typical soil can be reliably identified. Subtle deviations from natural uranium composition have been recognised in a number of soil samples in the Aldermaston area.