High precision tungsten isotope measurement by thermal ionization mass spectrometry

Abstract

We describe a new technique for measuring the isotopic abundance of 182W 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 WO 3 − 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 186W/ 184W or 186W/ 183W, 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 183W/ 184W 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 182W/ 184W within ±4.5 ppm (2 σ SD). Repeated measurement of a gravimetrically prepared mixture of a natural W standard and a 182W enriched spike shows that differences in 182W/ 184W of ∼10 ppm can be well resolved using this method. The external reproducibility of ±4.5 ppm is ∼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.