Precise analysis of 234U/238U ratio using UO2+ ion with thermal ionization mass spectrometry for natural samples

We have developed a new, highly precise method for U isotopic analysis using a thermal ionization mass spectrometry (TIMS) by measuring UO2+ ions emitted from a very small amount (100-10 ng) of U. A new ionization activator, a silicic acid-diluted phosphoric acid solution, was found to be effective not only for producing a strong and stable UO2+ ion beam, but also for keeping the mass fractionation during measurement small and constant. U isotope analysis was carried out by a dynamic multi collection method using three Faraday cup collectors and a secondary electron multiplier in ion counting mode with RPQ (RPQ-SEM). For precise isotope analysis, baselines of the Faraday cups and a conversion factor between Faraday cups and the RPQ-SEM were precisely determined simultaneously with U isotope collection. Analytical precision and reproducibility of U-234/U-238 ratio ranged from 0.10% to 0.38% (2 sigma (mean)) and from 0.13% to 0.31% (2 sigma), respectively, for 100-10 ng of U extracted from a natural rhyolitic obsidian. The precision and reproducibility were improved compared with previous studies, and thus, our method is especially useful for samples with very low abundance of U. (C) 2001 Elsevier Science B.V. All rights reserved.