Isotopic composition (238U/235U) of some commonly used uranium reference materials

We have determined U-238/U-235 ratios for a suite of commonly used natural (CRM 112a, SRM 950a, and HU-1) and synthetic (IRMM 184 and CRM U500) uranium reference materials by thermal ionisation mass-spectrometry (TIMS) using the IRMM 3636 U-233-U-236 double spike to accurately correct for mass fractionation. Total uncertainty on the U-238/U-235 determinations is estimated to be <0.02% (2 sigma). These natural U-238/U-235 values are different from the widely used 'consensus' value (137.88), with each standard having lower U-238/U-235 values by up to 0.08%. The U-238/U-235 ratio determined for CRM U500 and IRMM. 184 are within error of their certified values; however, the total uncertainty for CRM U500 is substantially reduced (from 0.1% to 0.02%). These reference materials are commonly used to assess mass-spectrometer performance and accuracy, calibrate isotope tracers employed in U, U-Th and U-Pb isotopic studies, and as a reference for terrestrial and meteoritic U-238/U-235 variations. These new U-238/U-235 values will thus provide greater accuracy and reduced uncertainty for a wide variety of isotopic determinations. (C) 2010 Elsevier Ltd. All rights reserved.