Instrumentation and Method Development for On-Site Analysis of Helium Isotopes

Helium isotope determination may be useful in measuring volcanic activity and issuing earlier warnings of possible eruptions. A method is presented for measuring the ³He/⁴He ratio in a gas sample using a multiturn time-of-flight mass spectrometer "infiTOF". In contrast to conventional waveform averaging, peaks are determined by counting ion pulses from each time-of-flight trigger. Samples were also measured by conventional magnetic-sector mass spectrometry for comparison. Magnetic sector results were used to designate a standard for infiTOF measurement and to calculate a ratio for each sample measured by infiTOF. Mass assignment error for ultrapure ³He⁺ standard was 4.30 × 10^-5 Da. Mass assignment error of ⁴He²⁺ and ³He⁺ for sample cylinders was 3.00 × 10^-8 Da and 2.25 × 10^-4 Da, respectively. Abundance ratios determined by infiTOF were found to be within 2% of the abundance ratios determined by magnetic-sector mass spectrometry. Mass drift was <50 × 10^-6 Da over 10 h. Sample flow rate was not found to affect the results as long as the reference sample was analyzed under the same conditions. Results indicate that the infiTOF system may be a viable tool for measuring helium isotopes, which may eventually lead to earlier warnings of volcanic activity.