10:45 〜 11:00
[SGC32-01] マルチターン飛行時間型質量分析計を用いた火山ガス試料とHESJ標準ガスのヘリウム同位体分析
キーワード：質量分析、同位体比、ヘリウム同位体、火山ガス、イオンカウンティング
Helium isotope ratio (3He/4He ratio) shows different values in geochemical reservoirs such as the atmosphere, crust, and mantle, depending on relative contributions of primordial and radiogenic helium. 3He/4He ratios of volcanic gases vary between magmatic (up to 1.1 × 10−5 or lower) and crustal (less than 1 × 10−7) values. When magma becomes active, 3He/4He ratio of volcanic gas may increase due to the increased contribution of magmatic helium. Therefore, 3He/4He ratio of volcanic gas has the potential as a monitoring tool of volcanic activity. Such 3He/4He ratio increases preceding volcanic eruptions have been reported for El Hierro, Canary Islands[1], Mt. Etna, Italy[2], and Ontake, Japan[3].
Although continuous analysis of volcanic gas is necessary to monitor volcanic activity, it is difficult because a magnet-sector type mass spectrometer equipped with a massive electromagnet is currently used to analyze helium isotopes. There are two reasons why a magnet-sector type mass spectrometer is necessary. One is that adequate mass resolution is required to distinguish 3He+ from HD+, and the other is that high sensitivity is required to detect trace amounts of 3He. For these reasons, helium isotope analysis is limited to suitable laboratory and on-site, real-time measurement of 3He/4He ratio around a volcano is almost impossible.
We have been developing a new technique of noble gas analysis using the “infiTOF” (MSI TOKYO, Inc.), which is a portable mass spectrometer derived from the multi-turn time-of-flight mass spectrometer MULTUM-S II[4, 5]. The high mass resolution achieved by an infiTOF is more than enough to distinguish 3He+ from HD+. However, the sensitivity of a normal infiTOF was far lower than the requirement to analyze noble gases in volcanic gas because most of noble gas molecules admitted to the spectrometer were pumped out by vacuum pumps directly connected to the spectrometer before ionized by an electron ionization source. Therefore, we installed valves between the spectrometer and the vacuum pumps to operate the mass spectrometer while it is isolated from the pumps. Getter pumps, which absorb active gases but not noble gases, were also installed to keep high vacuum in the spectrometer during the operation. In addition, we introduced the ion counting method for signal processing of the secondary electron multiplier to detect weak signals of 3He ions.
As a result, it has become possible to detect a significant number of 3He ions during analyses of volcanic gas samples. The 3He/4He ratios of samples were calibrated by comparing with the analysis of the helium standard gas (HESJ) with a known 3He/4He ratio[6]. Besides, we investigated the pressure conditions, which enable accurate measurement of 3He/4He ratios, from the stability of observed 3He/4He ratios versus the pressure of sample gas admitted to the spectrometer. It was finally demonstrated that 3He/4He ratios of volcanic gas samples, which were measured under optimized conditions and calibrated with HESJ, were consistent with those measured with a magnet-sector type mass spectrometer within analytical errors.
[1] E. Padrón et al., Geology, 41(5), 539–542 (2013).
[2] A. Paonita et al., Geology, 44(7), 499–502 (2016).
[3] Y. Sano et al., Sci. Rep., 5:13069 (2015).
[4] M. Toyoda et al., J. Mass Spectrom., 38(11), 1125–1142 (2003).
[5] S. Shimma et al., Anal. Chem., 82(20), 8456–8463 (2010).
[6] J. Matsuda et al., Geochem. J., 36(2), 191–195 (2002).
Although continuous analysis of volcanic gas is necessary to monitor volcanic activity, it is difficult because a magnet-sector type mass spectrometer equipped with a massive electromagnet is currently used to analyze helium isotopes. There are two reasons why a magnet-sector type mass spectrometer is necessary. One is that adequate mass resolution is required to distinguish 3He+ from HD+, and the other is that high sensitivity is required to detect trace amounts of 3He. For these reasons, helium isotope analysis is limited to suitable laboratory and on-site, real-time measurement of 3He/4He ratio around a volcano is almost impossible.
We have been developing a new technique of noble gas analysis using the “infiTOF” (MSI TOKYO, Inc.), which is a portable mass spectrometer derived from the multi-turn time-of-flight mass spectrometer MULTUM-S II[4, 5]. The high mass resolution achieved by an infiTOF is more than enough to distinguish 3He+ from HD+. However, the sensitivity of a normal infiTOF was far lower than the requirement to analyze noble gases in volcanic gas because most of noble gas molecules admitted to the spectrometer were pumped out by vacuum pumps directly connected to the spectrometer before ionized by an electron ionization source. Therefore, we installed valves between the spectrometer and the vacuum pumps to operate the mass spectrometer while it is isolated from the pumps. Getter pumps, which absorb active gases but not noble gases, were also installed to keep high vacuum in the spectrometer during the operation. In addition, we introduced the ion counting method for signal processing of the secondary electron multiplier to detect weak signals of 3He ions.
As a result, it has become possible to detect a significant number of 3He ions during analyses of volcanic gas samples. The 3He/4He ratios of samples were calibrated by comparing with the analysis of the helium standard gas (HESJ) with a known 3He/4He ratio[6]. Besides, we investigated the pressure conditions, which enable accurate measurement of 3He/4He ratios, from the stability of observed 3He/4He ratios versus the pressure of sample gas admitted to the spectrometer. It was finally demonstrated that 3He/4He ratios of volcanic gas samples, which were measured under optimized conditions and calibrated with HESJ, were consistent with those measured with a magnet-sector type mass spectrometer within analytical errors.
[1] E. Padrón et al., Geology, 41(5), 539–542 (2013).
[2] A. Paonita et al., Geology, 44(7), 499–502 (2016).
[3] Y. Sano et al., Sci. Rep., 5:13069 (2015).
[4] M. Toyoda et al., J. Mass Spectrom., 38(11), 1125–1142 (2003).
[5] S. Shimma et al., Anal. Chem., 82(20), 8456–8463 (2010).
[6] J. Matsuda et al., Geochem. J., 36(2), 191–195 (2002).