High precision tungsten isotope measurement by thermal ionization mass spectrometry

We describe a new technique for measuring the isotopic abundance of W-182 with improved precision in natural silicate samples. After chemical purification of W through a four-step ion exchange chromatographic separation. the W isotopic composition is measured as WO3- by negative thermal ionization mass spectrometry using a Thermo-Fisher Triton instrument. Amplifier biases are cancelled by using amplifier rotation, and Faraday cup biases are monitored by using a cup configuration that allows two-line data acquisition. Data are initially corrected for oxide interferences, assuming a predefined O isotope composition, and for mass fractionation, by normalization to W-186/W-184 or W-186/W-183, using an exponential law. Despite these corrections, isotopic ratios exhibit small but strongly correlated variations. This second-order effect may reflect a mass dependent change of O isotope composition in the measured W (and Re) oxides, and is corrected by normalization to W-183/W-184 using a linear law. Repeated analysis of an Alfa Aesar W standard (n = 39), and of three dissolutions of a La Palma (Canary Islands) basalt, applying the double normalization procedure, demonstrate external reproducibility of W-182/W-184 within +/- 4.5 ppm (2 sigma SD). Repeated measurement of a gravimetrically prepared mixture of a natural W standard and a W-182 enriched spike shows that differences in W-182/W-184 of similar to 10 ppm can be well resolved using this method. The external reproducibility of +/- 4.5 ppm is similar to 5 times more precise than conventional W isotope measurements by MC-ICP-MS. The new technique constitutes an ideal tool for investigating the W isotope composition of terrestrial rocks for potential contributions from the core, and late accreted extraterrestrial materials. Published by Elsevier B.V.