Abstract. The atmospheric He/N2 ratio is expected to increase due to the emission of He associated with fossil fuels and is expected to also vary in both space and time due to gravitational separation in the stratosphere. These signals may be useful indicators of fossil fuel exploitation and variability in stratospheric circulation, but direct measurements of He/N2 ratio are lacking on all timescales. Here we present a high-precision custom inlet system for mass spectrometers that continuously

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... tabilizes the flow of gas during sample–standard comparison and removes all non-noble gases from the gas stream. This enables unprecedented accuracy in measurement of relative changes in the helium mole fraction, which can be directly related to the 4He/N2 ratio using supplementary measurements of O2/N2, Ar/N2 and CO2. Repeat measurements of the same combination of high-pressure tanks using our inlet system achieves a He/N2 reproducibility of ∼ 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changes of gravitational enrichment at ∼ 35 km in the midlatitude stratosphere of order 300–400 per meg and an annual tropospheric increase from human fossil fuel activity of less than ∼ 30 per meg yr−1 (bounded by previous work on helium isotopes). The gettering and flow-stabilizing inlet may also be used for the analysis of other noble-gas isotopes and could resolve previously unobserved seasonal cycles in Kr/N2 and Xe/N2.