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 Ne-21/Ne-20 ratio of air (0.002959 +/- 0.000022; Eberhardt et al. (1965) does not take account of the contribution of (NeH+)-Ne-20 at m/z = 21. Honda et al. (2015) and Wielandt and Storey (2019) have recently re-determined the Ne-21/Ne-20(air) by resolving (NeH+)-Ne-20 from Ne-21(+). The Ne-21/Ne-20(air) 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 Ne-21/Ne-20 of air. (NeH+)-Ne-22/Ne-22(+) measured at different H-2(+) and Ne+ intensities reveal that (i) the partial pressure of H-2(+) in the instrument is the primary control on NeH+ production, and (ii) increasing Ne+ pressure suppresses the formation of NeH+. Calibration curves of (NeH+)-Ne-22/Ne-22(+) vs. Ne-22(+) at constant H-2(+) are used to calculate the (NeH+)-Ne-20 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 Ne-2(2)/Ne-20 vs. Ne-21/Ne-20 space. The (NeH+)-Ne-20/Ne-21(+) line in ratio varies between 0.4% (90 eV) and 2.3% (60 and 70 eV). Correcting for (NeH+)-Ne-20 assuming (NeH+)-Ne-22/(NeH+ )-Ne-20= Ne-22/(20) yields an over-correction of up to 0.7% and the data do not plot on a single mass fractionation line. Our study defines Ne-21/Ne-20(air) to be 0.002959 +/- 0.14% (1 sigma) assuming Ne-22/Ne-20 = 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 (NeH+)-Ne-20. The improved precision of air Ne-21/Ne-20 will result in more precise cosmogenic Ne-21 surface exposure and (U + Th)/Ne ages. (C) 2019 Elsevier Ltd. All rights reserved.