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PerMaSCal – add-on for advanced PTR-TOF mass-scale calibration

IONICON releases a new tool for all PTR-TOF systems for advanced mass-scale calibration.

The built-in PerMaSCal unit uses permeation to add a high mass peaks of stable intensity to every spectrum. These signals represent ideal references for accurate, automated TOF mass-scale calibration and other tuning tasks without affecting typical measurement set-ups.

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 PTR-TOFMS 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, frequent calibration of the mass scale using known peaks over an extended mass range is recommended for a high mass accuracy.

The key to this simple process is to find the appropriate peaks for mass scale calibration: Peaks have to be present in all mass spectra at suitable intensity levels, i.e. high enough to provide a sufficient signal-to-noise ratio but well below detector saturation. Moreover, they should not overlap with other compounds.

These criteria are met by some ions produced in the ion-source, which provide suitable calibration peaks, but only exist at low m/z. Using sample gas VOCs is inherently problematic since their concentration can vary to the point where they vanish completely in e.g. a 0-air measurement. This problem, to provide constant calibration peaks at high m/z, is now solved by IONICON’s PerMaSCal.

Permeation device: Several solutions have already been proposed, where a small amount of a compound is mixed with the sample gas stream, mostly requiring an additional, pressurized canister and manual valves. IONICON engineers have now developed an elegant solution which:

  • can be built-into the PTR-TOF
  • has a tunable emission rate
  • does not affect the measurement
  • allows an easy upgrade of existing PTR-TOFs.

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 pre-cursor ions (H3O+, NO+, O2+)
  • signals at high m/z (>200, > 300)
  • center mass of this halocarbon does not overlap with typical VOCs
  • compound of low toxicity – no health risk

An additional advantage is the fragmentation of this compound (with H3O+) to a mass at around 200, which can be used to check and tune E/N ratios.

Temperature control: 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.

Summary: The new IONICON Permeation device for Mass Scale Calibration (PerMaSCal) is built-into your PTR-TOF and provides a constant signal at a high mass. These peaks are an ideal reference for many tuning tasks such as MCP tuning and resolution optimization. Most importantly, this is a prerequisite for automated mass-scale calibration for high accuracy over the complete mass range.