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Abstract. The atmospheric He/N2 ratio is expected to increase due to the emission of He associated with fossil fuels and isexpected to also vary in both space and time due to gravitational separationin the stratosphere. These signals may be useful indicators of fossil fuelexploitation and variability in stratospheric circulation, but directmeasurements of He/N2 ratio are lacking on all timescales. Here wepresent a high-precision custom inlet system for mass spectrometers thatcontinuously stabilizes the flow of gas during sample–standard comparisonand removes all non-noble gases from the gas stream. This enablesunprecedented accuracy in measurement of relative changes in the helium molefraction, which can be directly related to the 4He/N2 ratio usingsupplementary measurements of O2/N2, Ar/N2 and CO2.Repeat measurements of the same combination of high-pressure tanks using ourinlet system achieves a He/N2 reproducibility of∼ 10 per meg (i.e., 0.001 %) in 6–8 h analyses. This compares to interannual changesof gravitational enrichment at ∼ 35 km in the midlatitudestratosphere of order 300–400 per meg and an annual tropospheric increasefrom human fossil fuel activity of less than ∼ 30 per meg yr−1 (bounded by previous work on helium isotopes). The gettering andflow-stabilizing inlet may also be used for the analysis of other noble-gasisotopes and could resolve previously unobserved seasonal cycles inKr/N2 and Xe/N2.