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

We have developed a new, highly precise method for U isotopic analysis usin g a thermal ionization mass spectrometry (TIMS) by measuring UO2+ ions emit ted 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 keep ing the mass fractionation during measurement small and constant. U isotope analysis was carried out by a "dynamic multi collection" method using thre e Faraday cup collectors and a secondary electron multiplier in ion countin g mode with RPQ (RPQ-SEM). For precise isotope analysis, baselines of the F araday cups and a conversion factor between Faraday cups and the RPQ-SEM we re precisely determined simultaneously with U isotope collection. Analytica l 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 a nd 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.