High-precision helium isotope measurements in air

Helium has two natural isotopes which have contrasted, and variable sources and sinks in the atmosphere ( 3 He/ 4 He air = 1.382 ± 0.005 × 10 −6 ). Variations in the atmospheric helium isotopic composition may exist below typical measurement precision thresholds (0.2 to 0.5%, 2 σ ). 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% 2 σ ). 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% (2 σ ), with three repeat samples needed to reach 0.1% or better errors. The long-term reproducibility of our calibration sample is 0.033% (2 σ ). This paper details a method to make high precision (less than 0.2%) helium isotope measurements in air samples.