[MTT48-P01] Noble gas isotope analysis using a multi-turn time-of-flight mass spectrometer
Keywords:noble gas, multi-turn time-of-flight mass spectrometer, volcanic gas, isotope ratio
We have been developing a new technique of noble gas analysis using “infiTOF” (infiTOF-UHV, MSI-Tokyo, Inc., Tokyo, Japan), which is a small, portable, time-of-flight (TOF) mass spectrometer derived from the MULTUM-S II multi-turn TOF mass spectrometer and is capable of high mass resolution [6, 7], in order to monitor 3He/4He ratio in volcanic gas on site. The high mass resolution achieved by infiTOF (>30,000) is more than enough to distinguish 3He+ and 20Ne+ from their interferences, HD+ and 40Ar++, respectively. However, sensitivity of normal infiTOF was not high enough to analyze noble gases in volcanic gas because most of the noble gas molecules admitted to the infiTOF were pumped out by a vacuum pump before ionized by an electron ionization source. To increase ionization efficiency by suppressing the number of exhausted gas molecules, we installed a gate valve between the ion source and pump. A getter pump, which absorbs active gases but not noble gases, was also installed to keep the pressure in the ion source low during the operation. Furthermore, we made an attempt to improve sensitivity by making a smaller aperture between the ion source and mass analyzer to reduce the number of noble gas molecules leaking out to the mass analyzer before ionized. Currently the 140 times sensitivity compared to that of normal infiTOF has been achieved. Although 3He in natural samples has not yet been observed with our infiTOF, of which ion current signals collected by a secondary electron multiplier are averaged for each flight cycle, it is expected to be detected in the future by processing the signals with pulse counting method [8, 9].
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