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.
Permeation is the constant diffusion of a compound through a membrane. This rate depends on the compound’s and the membrane’s properties and on the temperature. These parameters have been optimized in the PerMaSCal to provide a constant rate of a compound, which is directly fed into the drift tube to provide concentrations tailored for TOF mass scale calibration without affecting the sampling. The compound provided in permeation device has several advantages:
- ionizable with all SRI selective reagent ions (H3O+, NO+, O2+)
- signals at high m/z (> 300)
- center mass of this halocarbon is distinct from isobaric VOCs
- compound of low toxicity – no health risk
Permeation rates are strongly temperature dependent. The PerMaSCal is mounted outside the PTR-TOF’s climate chamber and has a separate heating system. This makes the permeation rate independent of the drift temperature and allows adjusting the permeation rate to adapt the peak intensity. Connected to the PTR-TOF’s additional inlet temperature controller, the temperature can be set in the software. At ambient temperature, e.g. when the instrument is turned off, the permeation rate becomes negligible.
- Constant signals at high m/z
- Reliable and accurate mass-scale calibration
- Ideal reference for optimization of the MCP setpoint
- Ideal reference for optimizing the mass resolution
- Works with all SRI selective reagent ions
PTR-TOF systems that are not yet equipped with PerMaSCal can be upgraded by IONICON. For experienced user IONICON offers a PerMaSCal upgrade kit including installation instructions. Please contact us for a quote!