Construction of a Newly Designed Small-Size Mass Spectrometer for Helium Isotope Analysis: Toward the Continuous Monitoring Of 3He/4He Ratios In Natural Fluids

摘要

The construction of a small-size, magnetic sector, single focusing mass spectrometer (He-MS) for the continuous, on-site monitoring of He isotope ratios (~(3)He/~(4)He) is described. The instrument is capable of measuring ~(4)He/~(20)Ne ratios dissolved in several different types of natural fluids of geochemical interest, such as groundwater and gas from hot springs, volcanoes and gas well fields. The ion optics of He-MS was designed using an ion trajectory simulation program “TRIO,” which permits the simultaneous measurement of ~(3)He and ~(4)He with a double collector system under a mass resolution power (M /ΔM ) of >500. The presently attained specifications of the He-MS are; (1) a mass resolving power of ca. 490, sufficient to separate ~(3)He~(+) from interfering ions, HD~(+) and H_(3)~(+), (2) ultra-high vacuum conditions down to 3×10~(?8)?Pa, and (3) a sufficiently high sensitivity to permit amounts of ~(3)He to be detected at levels as small as 10~(?13) cm~(3)STP (3×10~(6) atoms). Long term stability for ~(3)He/~(4)He analysis was examined by measuring the ~(3)He/~(4)He standard gas (HESJ) and atmospheric He, resulting in ～3% reproducibility and ≤5% experimental error for various amounts of atmospheric He from 0.3 to 2.3×10~(?6) cm~(3)STP introduced into the instrument. A dynamic range of measurable ~(3)He/~(4)He ratios with He-MS is greater than 10~(3) which was determined by measuring various types of natural fluid samples from continental gas (with a low ~(3)He/~(4)He ratio down to 2×10~(?8)) to volcanic gas (with a high ~(3)He/~(4)He ratio up to 3×10~(?5)). The accuracy and precision of ~(3)He/~(4)He and ~(4)He/~(20)Ne ratios were evaluated by comparing the values with those measured using well established noble gas mass spectrometers (modified VG5400/MS-III and -IV) in our laboratory, and were found to be in good agreement within analytical errors. Usefulness of the selective extraction of He from water/gas using a high permeability of He through a silica glass wall at high temperature (700°C) is demonstrated.