A simple, effective accurate mass assignment procedure for a time-of-flight mass spectrometer is desirable. External mass calibration using a mass calibration standard together with an internal mass reference (lock mass) is a common technique for mass assignment, however, using polynomial fitting can result in mass-dependent errors. By using the multi-turn time-of-flight mass spectrometer infiTOF-UHV, we were able to obtain multiple time-of-flight data from an ion monitored under several different numbers of laps that was then used to calculate a mass calibration equation. We have developed a data acquisition system that simultaneously monitors spectra at several different lap conditions with on-the-fly centroid determination and scan law estimation, which is a function of acceleration voltage, flight path, and instrumental time delay. Less than 0.9 mDa mass errors were observed for assigned mass to charge ratios ( m/z) ranging between 4 and 134 using only Ar as a reference. It was also observed that estimating the scan law on-the-fly provides excellent mass drift compensation.
Helium isotope determination may be useful in measuring volcanic activity and issuing earlier warnings of possible eruptions. A method is presented for measuring the He/He 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 He standard was 4.30 × 10 Da. Mass assignment error of He and He for sample cylinders was 3.00 × 10 Da and 2.25 × 10 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 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.
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