ABSTRACT

An attempt was made to determine both elemental and isotopic abundances of rare gases in natural gases by means of a conventional quadrupole mass spectrometer (QMS). Elemental abundances and isotopic ratios of Ar, Kr and Xe on the atmospheric concentration level were successfully measured by the present method. Reproducibility of 10–20% was obtained for ⁴He, ²⁰Ne, ³⁶Ar, ⁸⁴Kr and ¹³²Xe measurements. Isotopic ratios of Ar, Kr and Xe were determined within about 2% experimental errors, except for ⁷⁸Kr/⁸⁴Kr, ¹²⁴Xe/¹³²Xe and ¹²⁶Xe/¹³²Xe. Sensitivity and reproducibility of the measurements were examined by repeating runs of the atmospheric air. In order to verify the applicability of the present method to terrestrial natural gas analyses, eight typical samples of natural gases in Japan, with differences in major compositions, were measured. The values observed by a QMS in this work are generally in good agreement with those obtained by magnetic deflection mass spectrometers. Enrichment of radiogenic ⁴⁰Ar was observed in some CO₂-rich natural gas samples. Observed Kr and Xe isotopic ratios in N₂-rich gases and CH₄-rich gases coincided with the atmospheric ratios within experimental error.