Precise Measurement of Helium Isotopes in Terrestrial Gases

Helium isotope ratios of terrestrial natural gas samples were measured using a high-precision mass spectrometer and an ultrahigh-vacuum purification line. The line is equipped with a charcoal trap held at 40 K using a helium expansion refrigerating machine. To avoid interference, Ne was completely separated from He by the trap before the gas was admitted to the mass spectrometer. Ion beams of 3He and 4He were measured at the same time by a double collector system. A resolving power of about 550 at the 5% level of the peak height was attained for the complete separation of 3He ions from those of HD and H3. Precision and reproducibility of the helium isotope ratio were less than 1%, examined by repeated measurements of an air standard. Accuracy of the ratio was checked by comparing observed data with reports in the literature.