Accurate Measurement of Atmospheric Helium Isotopes
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We have developed an analytical system to measure atmospheric <sup>3</sup>He/<sup>4</sup>He 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 <sup>3</sup>He and <sup>4</sup>He 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 <sup>3</sup>He ions from those of HD and H<sub>3</sub>. Repeated analysis of a sample calibrated against a standard showed a precision of about 0.2% error margin (2<i>σ</i>). This system has been applied to evaluating the <sup>3</sup>He/<sup>4</sup>He ratio of He Standard of Japan (HESJ), and for the assessment of the atmospheric variation of latitude. The observed <sup>3</sup>He/<sup>4</sup>He ratio of HESJ, 20.405 ± 0.040<i>R</i><sub>air</sub> (2<i>σ</i>) agrees well with the value of 20.408 ± 0.044<i>R</i><sub>air</sub> by Lupton and Evans, but is smaller than the recommended value of 20.63 ± 0.10<i>R</i><sub>air</sub> in a literature. Air samples collected from 45 to 20°N in the Far East showed a slight decrease of the <sup>3</sup>He/<sup>4</sup>He 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 <sup>4</sup>He is significant in the high-latitude region of the northern hemisphere.
- Analytical Sciences
Analytical Sciences 24(4), 521-525, 2008-04-10
The Japan Society for Analytical Chemistry