11:00 AM - 1:00 PM
[MTT44-P01] Development of a portable mass spectrometer and preprocessing system for on-site analysis of helium isotope ratios of volcanic gases
Keywords:helium isotope, mass spectrometer, volcanic gas
The helium isotope ratio (3He/4He ratio) varies among the geochemical reservoirs, such as the atmosphere, crust, and mantle, depending on the ratio of the primordial component trapped in the Earth during its formation to the radiogenic component produced by radioactive decay of U and Th. Since volcanic gases are derived from magma and released to the earth's surface through various processes, the 3He/4He ratio reflects the mixing ratios of components originating from each reservoir. The 3He/4He ratio of volcanic gases is expected to be a new tool of volcano monitoring, because the 3He/4He ratio may change before an eruption due to increase in magmatic activity. For the measurement of the 3He/4He ratio, a magnetic field mass spectrometer with a total weight of more than 1 ton has been used because the sensitivity to detect trace amounts of 3He (generally less than 0.1 ppbv) and the mass resolution to distinguish 3He+ from HD+ are required. In addition, a vacuum line for purification and separation of noble gases is also required, so the 3He/4He measurements can only be performed in a laboratory. Therefore, it is necessary to bring the sample back to the laboratory to measure the 3He/4He ratio, and it is difficult to perform real-time monitoring, which is essential for volcanic observation. To make this possible, it was necessary to develop a method for on-site analysis using a portable mass spectrometer and a compact helium extraction system.
We used a portable multi-turn time-of-flight mass spectrometer (MULTUM), which is small enough to be carried around and has high mass resolution enough to discriminate 3He+ from HD+. However, the sensitivity of the original MULTUM was far below the level required to detect trace amounts of 3He in natural samples. One of the reasons is that in the case of commercially-available MULTUM, most of the sample gas introduced into the ion source is evacuated by a vacuum pump before being ionized. Therefore, we installed a valve between the ion source and the pump to enable static operation, in which the mass spectrometer volume is isolated from the vacuum pump during measurement and the sample gas is kept inside the spectrometer during analysis. In order to maintain a high vacuum in the spectrometer during the static operation, a getter pump was installed to absorb active gases other than noble gases. In addition, the aperture diameter was enlarged to improve ion transparency and pumping efficiency. As another way to improve the sensitivity, we adopted the ion counting method, that counts pulse signals that exceed a threshold value, enables detection of trace ions that could not be detected due to noise.
Since a helium extraction system using a quartz glass tube had been proposed, we combined this system with MULTUM. In order to verify the helium extraction using quartz glass tubes, helium permeation experiments were conducted. The glass tube was heated to 270-540°C and the gas permeation from the air in the laboratory was measured by MULTUM. The amount and rate of helium permeation were evaluated.
As a result of the ion counting and static operation, it was possible to detect 3He using MULTUM. The sensitivity of 2.4x10-10 cm3STP/cps was achieved, which is equivalent to the detection of 100 counts of 3He in a 10-minute measurement of a 0.4-4 cm3 volcanic gas sample, making the helium isotope ratio measurement of volcanic gases using MULTUM more realistic.
The helium permeation experiment using a quartz glass tube suggested that 4x10-5 cm3STP of helium could be extracted by permeation at 700°C for 15 hours. By combining this with the static operation of MULTUM, it is possible to measure the 3He/4He ratio with an accuracy of about 4% per hour.
We used a portable multi-turn time-of-flight mass spectrometer (MULTUM), which is small enough to be carried around and has high mass resolution enough to discriminate 3He+ from HD+. However, the sensitivity of the original MULTUM was far below the level required to detect trace amounts of 3He in natural samples. One of the reasons is that in the case of commercially-available MULTUM, most of the sample gas introduced into the ion source is evacuated by a vacuum pump before being ionized. Therefore, we installed a valve between the ion source and the pump to enable static operation, in which the mass spectrometer volume is isolated from the vacuum pump during measurement and the sample gas is kept inside the spectrometer during analysis. In order to maintain a high vacuum in the spectrometer during the static operation, a getter pump was installed to absorb active gases other than noble gases. In addition, the aperture diameter was enlarged to improve ion transparency and pumping efficiency. As another way to improve the sensitivity, we adopted the ion counting method, that counts pulse signals that exceed a threshold value, enables detection of trace ions that could not be detected due to noise.
Since a helium extraction system using a quartz glass tube had been proposed, we combined this system with MULTUM. In order to verify the helium extraction using quartz glass tubes, helium permeation experiments were conducted. The glass tube was heated to 270-540°C and the gas permeation from the air in the laboratory was measured by MULTUM. The amount and rate of helium permeation were evaluated.
As a result of the ion counting and static operation, it was possible to detect 3He using MULTUM. The sensitivity of 2.4x10-10 cm3STP/cps was achieved, which is equivalent to the detection of 100 counts of 3He in a 10-minute measurement of a 0.4-4 cm3 volcanic gas sample, making the helium isotope ratio measurement of volcanic gases using MULTUM more realistic.
The helium permeation experiment using a quartz glass tube suggested that 4x10-5 cm3STP of helium could be extracted by permeation at 700°C for 15 hours. By combining this with the static operation of MULTUM, it is possible to measure the 3He/4He ratio with an accuracy of about 4% per hour.