High-precision analysis of 182W/184W and 183W/184W by negative thermal ionization mass spectrometry: Per-integration oxide corrections using measured 18O/16O

- Oxygen isotopic compositions are measured in situ for every integration, allowing correction for in-run oxygen fractionation.
- Average of newly determined oxygen isotopic compositions is used to make first-order oxide corrections to interfering oxides.
- Long-term external precisions are 5.7 ppm and 3.7 ppm (2SD) for 182W/184W when mass bias corrected using 186W/184W and 186W/183W, respectively.
- Long-term external precision is 6.6 ppm (2SD) for 183W/184W when mass bias corrected using 186W/184W.
- Measurement of IVB iron meteorite Skookum using this method is shown to be accurate and precise.

Here we describe a new analytical technique for the high-precision measurement of 182W/184W and 183W/184W using negative thermal ionization mass spectrometry (N-TIMS). We improve on the recently reported method of Trinquier et al. (2016), which described using Faraday cup collectors coupled with amplifiers utilizing 1013 Ω resistors to continuously monitor the 18O/16O of WO3− and make per-integration oxide corrections. In our study, we report and utilize a newly measured oxygen mass fractionation line, as well as average 17O/16O and 18O/16O, which allow for more accurate per-integration oxide interference corrections. We also report a Faraday cup and amplifier configuration that allows 18O/16O to be continuously monitored for WO3− and ReO3−, both of which are ionized during analyses of W using Re ribbon. The long-term external precision of 182W/184W is 5.7 ppm and 3.7 ppm (2SD) when mass bias corrected using 186W/184W and 186W/183W, respectively. For 183W/184W mass bias is corrected using 186W/184W, yielding a long-term external precision of 6.6 ppm. An observed, correlated variation in 182W/184W and 183W/184W, when mass bias corrected using 186W/184W, is most likely the result of Faraday cup degradation over months-long intervals.

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