High-precision helium isotope measurements in air

J Mabry (1) , T.F. Lan (1) , P.G. Burnard (1) , Bernard Marty (1)

1 CRPG - Centre de Recherches Pétrographiques et Géochimiques

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).

Domaines: Géochimie

J Mabry, T.F. Lan, P.G. Burnard, Bernard Marty. High-precision helium isotope measurements in air. Journal of Analytical Atomic Spectrometry, 2013, ⟨10.1039/c3ja50155h⟩. ⟨hal-01346316⟩