Development of Helium Isotope Reference Materials near Ambient 3He/4He Ratio

Abstract

The 3He/4He ratio in the atmosphere is known to be 1.34 × 10−6. For accurate measurement of low 3He concentrations, a certified reference material (CRM) with a low 3He/4He ratio is necessary. In this work, we produced a new 3He CRM of helium isotopic ratio standard by the gravimetric method with a 3He concentration similar to that of the atmospheric air. Previously, a helium isotope CRM with a 3He concentration of 3He/4He = 4743 μmol/mol was developed. Because 3He/4He ratio of atmospheric helium is much lower than this, we decided to produce a new 3He CRM of helium isotopic ratio and the previously prepared 3He CRM was diluted by the gravimetric method. The concentration of 3He in the high purity helium gas used for dilution was measured by comparing it with the newly prepared low 3He concentration CRMs.