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 (³He/⁴He) is described. The instrument is capable of measuring ⁴He/²⁰Ne 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 ³He and ⁴He with a double collector system under a mass resolution power (M/ΔM) of >700. The presently attained specifications of He-MS are; (1) a mass resolving power of ca. 430, sufficient to separate ³He⁺ from interfering ions, HD⁺ and H3⁺, (2) ultra-high vacuum conditions down to 3×10⁻⁸ Pa, and (3) a sufficiently high sensitivity to permit amounts of ³He to be detected at levels as small as 10⁻¹³ cm³ STP (3×10⁶ atoms). Long term stability for ³He/⁴He analysis was examined by measuring the ³He/⁴He 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 cm³ STP introduced into the instrument. A dynamic range of measurable ³He/⁴He ratios with He-MS is greater than 10³ which was determined by measuring various types of natural fluid samples from continental gas (with a low ³He/⁴He ratio down to 2×10⁻⁸) to volcanic gas (with a high ³He/⁴He ratio up to 3×10⁻⁵). The accuracy and precision of ³He/⁴He and ⁴He/²⁰Ne 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.