1:45 PM - 2:00 PM
[SGC54-01] Method Development and Evaluation of the infiTOF Time-of-Flight Mass Spectrometer for On-site Helium Isotopes Analysis
Keywords:Helium Isotope, On-site Analysis, Volcanic Activity, Mass Spectrometry, Ion Counting, Time-of-Flight
The “infiTOF” is a small, portable, time-of-flight (TOF) mass spectrometer capable of high mass resolution and high mass accuracy. The applicability of infiTOF for helium isotope monitoring was investigated by using software-based ion counting and a high-speed digitizer (commonly used in modern TOF instruments instead of a traditional time-digital-converter (TDC)), to measure extremely low-level signals. This configuration is advantageous compared to a TDC-based system because the averaged profile waveform can be used to monitor the overall spectrum, including high concentration ions. The concentration ratio of 3He compared to 4He in the expected sample is in the range of 10-6 to 10-8, and because of this large difference, they can not be monitored together without saturating the detector. Therefore, 4He2+ was measured as a quantitative reference for 4He+. The 3He+/4He2+ ratio of a sample was measured using the infiTOF MS by counting ion peaks from each TOF trigger waveform. A 3He standard was measured to verify the 3He peak and measure mass accuracy, which was observed with an error of 4.30 x 10-5 Da. The 3He+/4He2+ ratio was measured for three different helium gas cylinders by infiTOF. Mass accuracy for 4He2+ and 3He+ was also determined for sample cylinders with errors of 3.00 x 10-8 Da and 2.25 x 10-4 Da respectively. All cylinders were also measured by magnetic sector MS at University of Tokyo using standard helium gas HESJ (Helium standard of Japan, Matsuda et al., Geochem. J., 36, 2002). Using one cylinder as a secondary standard, the 3He+/4He2+ ratios for the other cylinders were determined using infiTOF measurements, which were then compared to the magnetic sector MS measurements and found to be in agreement with less than 5% error. Mass drift was also investigated and found to be less than 50 x 10-6 Da over ten hours. Results indicate that this method is accurate, stable, and has enough resolving power to differentiate helium isotopes, and may be a viable tool in future on-site analysis and prediction of volcanic activity.