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 H
3. Repeated analysis of a sample calibrated against a standard showed a precision of about 0.2% error margin (2
σ). 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 ± 0.040
Rair (2
σ) agrees well with the value of 20.408 ± 0.044
Rair by Lupton and Evans, but is smaller than the recommended value of 20.63 ± 0.10
Rair 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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