Accurate measurement of atmospheric helium isotopes

We have developed an analytical system to measure atmospheric He-3/He-4 ratios precisely by using a static vacuum operation mass spectrometer and an ultrahigh-vacuum purification line. After purification of the sample, helium was introduced into the mass spectrometer. Ion beams of He-3 and He-4 were measured at the same time by a double collector system. A resolving power of about 700 at the 5% level of the peak height was attained for the complete separation of He-3 ions from those of HD and H-3. Repeated analysis of a sample calibrated against a standard showed a precision of about 0.2% error margin (2 sigma). This system has been applied to evaluating the He-3/He-4 ratio of He Standard of Japan (HESJ), and for the assessment of the atmospheric variation of latitude. The observed He-3/He-4 ratio of HESJ, 20.405 +/- 0.040R(air) (2 sigma) agrees well with the value of 20.408 +/- 0.044R(air) by Lupton and Evans, but is smaller than the recommended value of 20.63 +/- 0.10R(air) in a literature. Air samples collected from 45 to 20 degrees N in the Far East showed a slight decrease of the He-3/He-4 ratio with latitude. Even though the variation is attributable to an experimental artifact, it may not preclude the possibility that the anthropogenic release of crustal He-4 is significant in the high-latitude region of the northern hemisphere.