Analytical Sciences

Accurate Measurement of Atmospheric Helium Isotopes

Yuji SANO, Taichi TOKUTAKE, and Naoto TAKAHATA

Center for Advanced Marine Research, Ocean Research Institute, The University of Tokyo, Nakano, Tokyo 164-8639, Japan

We have developed an analytical system to measure atmospheric ³He/⁴He 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 ³He and ⁴He 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 ³He ions from those of HD and H₃. Repeated analysis of a sample calibrated against a standard showed a precision of about 0.2% error margin (2σ). This system has been applied to evaluating the ³He/⁴He ratio of He Standard of Japan (HESJ), and for the assessment of the atmospheric variation of latitude. The observed ³He/⁴He ratio of HESJ, 20.405 ± 0.040R_air (2σ) 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°N in the Far East showed a slight decrease of the ³He/⁴He 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 ⁴He is significant in the high-latitude region of the northern hemisphere.

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