四重極型質量分析計と気体透過膜を用いた新しい地下水溶存ガスの元素・同位体比連続測定システム

抄録

We have developed a new system for continuous measurements of elemental and isotopic ratios of gases dissolved in groundwater using a quadrupole mass spectrometer and a gas-permeable membrane. Gases dissolved in water are extracted from the liquid phase using a gas exchange module made with silicone membrane tubes. Appropriate amounts of extracted gases are introduced into a quadrupole mass spectrometer through a variable leak valve and an orifice of 50μm in diameter. Selected gas species up to twelve different mass numbers can be repeatedly measured with a time interval as short as one second. This operation is fully automated using a personal computer. Temperature dependence of the gas extraction rate from tap water was examined using a constant temperature water bath, which demonstrated that the gas extraction rate through the silicone membrane is strongly dependent on temperature. We show some preliminary results of the continuous measurements of dissolved gas composition in the Ontake Myojin hot spring water, central Japan. CO₂/³⁶Ar and O₂/³⁶Ar ratios showed clear daily variations, which are probably explained by the difference in temperature dependence of gas extraction rate through the silicone membrane. In contract, daily variations in ⁴He/³⁶Ar, ⁴⁰Ar/³⁶Ar, and ⁸⁴Kr/³⁶Ar ratios were much less obvious, suggesting that relative gas exchange rates among noble gas species are smaller than those among different chemical species. We are preparing a comprehensive continuous observations at the Ontake Myojin hot spring, including data telemetry system, simultaneous continuous measurements of radon concentration and environmental parameters such as water temperature, barometric pressure, rain fall in addition to the system developed in the present study.