Measurement of helium isotope ratio using multi-turn time-of-flight mass spectrometer

Abstract

The helium isotope ratio (³He/⁴He ratio) of volcanic gas has a potential as a monitoring tool of volcanic activity because that value varies depending on the contribution of magmatic and crustal helium. Continuous analysis is necessary to monitor volcanic activity; however, it is difficult because a large magnetic-sector mass spectrometer is currently used to analyze helium isotopes for requests of high mass resolution and sensitivity. We have been developing a new technique of noble gas analysis using multi-turn time-of-flight mass spectrometer (MULTUM). When we adopted the “static operation” (i.e. the spectrometer is isolated from the vacuum pumps so that sample gas molecules are closed off within the spectrometer envelope) and pulse counting method to MULTUM, significant number of ³He in the helium standard gas has been successively detected. Although as high sensitivity as that of the magnetic-sector mass spectrometer has not yet been achieved, precision of ³He/⁴He ratio will be improved enough to distinguish the origin of helium in volcanic gas sample by accumulating number of ³He signals.