TOF mass-scale calibration
In a Time-of-Flight (TOF) analyzer, the flight time of an ion is converted into mass information. This quadratic relation needs to be precisely determined for the instrument's capability to resolve the atomic composition of protonated VOCs by exact mass measurement. Temperature variations of the TOF-MS can cause a slow change in the length of the ion's flight path over time. Therefore, a frequent calibration of the mass scale using known peaks over an extended mass range is important.
Peaks for mass scale calibration have to be present in all mass spectra with suitable intensity, high enough to provide a sufficient signal-to-noise ratio and low enough to be not affected by detector saturation. Moreover, they should not overlap with other compounds. Only mass spectra with congruent mass scales can be added up without the smearing out of mass peaks. Such integration over several spectra may be desirable to increase the signal-to-noise ratio.
These criteria are met by some ions produced in the ion-source, which provide suitable calibration peaks, but only at low m/z. Sample gas VOCs are inherently problematic since their concentration cannot be known a priory and should even be vanishing in 0-air measurements. This problem, to provide constant calibration peaks at high m/z, is now solved by IONICON’s PerMaSCal.