The formation of NeH+ in static vacuum mass spectrometers and re-determination of 21Ne/20Ne of air

Air-derived neon is used for routine calibration of magnetic sector mass spectrometers, principally for determining sensitivity and mass discrimination for Ne isotope determinations. The commonly accepted 21Ne/20Ne ratio of air (0.002959 ± 0.000022; Eberhardt et al. (1965) does not take account of the contribution of 20NeH+ at m/z = 21. Honda et al. (2015) and Wielandt and Storey (2019) have recently re-determined the 21Ne/20Neair by resolving 20NeH+ from 21Ne+. The 21Ne/20Neair values of the two studies differ by 1.8%, beyond the uncertainty of the measurements (± <0.1%). We have developed a protocol for precise determination of NeH+ in air using a low-resolution Thermo Fisher ARGUS VI mass spectrometer and use it to re-determine the 21Ne/20Ne of air. 22NeH+/22Ne+ measured at different H2+ and Ne+ intensities reveal that (i) the partial pressure of H2+ in the instrument is the primary control on NeH+ production, and (ii) increasing Ne+ pressure suppresses the formation of NeH+. Calibration curves of 22NeH+/22Ne+ vs. 22Ne+ at constant H2+ are used to calculate the 20NeH+ production in aliquots of air-derived Ne and allow for hydride correction at m/z = 21. The fully isobaric interference-corrected Ne isotope compositions measured at different electron energy (eV) settings define a single mass fractionation line in 22Ne/20Ne vs. 21Ne/20Ne space. The 20NeH+/21Ne+ ratio varies between 0.4% (90 eV) and 2.3% (60 and 70 eV). Correcting for 20NeH+ assuming 22NeH+/20NeH+ = 22Ne/20Ne yields an over-correction of up to 0.7% and the data do not plot on a single mass fractionation line. Our study defines 21Ne/20Neair to be 0.002959 ± 0.14% (1σ) assuming 22Ne/20Ne = 0.102 (Eberhardt et al., 1965). This overlaps the value determined by Wielandt and Storey (2019), albeit with a slightly higher uncertainty. However, our value is statistically more robust and accounts for the dependency on hydride formation by Ne partial pressure. From this we conclude that high precision Ne isotope ratio determinations in future require the quantification of 20NeH+. The improved precision of air 21Ne/20Ne will result in more precise cosmogenic 21Ne surface exposure and (U + Th)/Ne ages