Preparation of Helium Isotope-certified Reference Materials by Gravimetry

An advanced gravimetric system was used to produce certified reference materials of mixtures of helium isotopes, traceable to the Système International d'Unitès (SI unit) of moles. Critical factors in developing isotope reference mixtures include the purity assessment of the source gases before preparation, the weighing system used for gravimetric mixing, a well‐developed gas filling system, and internal consistency among the mixtures. Three artificial helium isotope reference mixtures were prepared with absolute isotope ratios, R( 4He/ 3He), of 18.905 ± 0.036, 98.78 ± 0.21, and 209.82 ± 0.44 (k = 1), respectively. The internal consistency between the mixtures was verified using a precision single‐focusing magnetic sector‐type mass spectrometer with a dynamic inlet system. The results showed that the proportions of the three reference isotope mixtures were well within the measurement uncertainty. The isotope reference mixtures were produced with the highest metrological uncertainty level.