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High-precision helium isotope measurements in air

Abstract : Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere (3He/4Heair ¼ 1.382 0.005 106). Variations in the atmospheric helium isotopic composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2s). In order to investigate this possibility, it is necessary to be able to consistently measure helium isotopes in air with high precision (below 0.2% 2s). We have created an air purification and measurement system that improves the helium isotope measurement precision. By purifying a large quantity of air at the start of a measurement cycle we can make rapid standard-bracketed measurements. Controlling the amount of helium in each measured aliquot minimizes pressure effects. With this method we improve the standard errors by 2 over measuring the same amount of gas in a single step. Individual measurements have standard errors of 0.2 to 0.3% (2s), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2s).
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J Mabry, T.F. Lan, P.G. Burnard, Bernard Marty. High-precision helium isotope measurements in air. Journal of Analytical Atomic Spectrometry, Royal Society of Chemistry, 2013, ⟨10.1039/c3ja50155h⟩. ⟨hal-01346316⟩



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