Technology
In commercial PTR-MS instruments water vapour is ionized in a hollow cathode discharge:
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After the discharge a short drift tube is used to form very pure (>99.5%) H3O+ via ion-molecule reactions:
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Due to the high purity of the primary ions a mass filter between the ion source and the reaction drift tube is not necessary and the H3O+ ions can be injected directly. The absence of this mass filter in turn greatly reduces losses of primary ions and leads eventually to an outstandingly low detection limit of the whole instrument. In the reaction drift tube a vacuum pump is continuously drawing through air containing the VOCs one wants to analyze. At the end of the drift tube the protonated molecules are mass analyzed (Quadrupole mass analyzer or Time-of-flight mass spectrometer) and detected.
Read more about this topic: Proton-transfer-reaction Mass Spectrometry
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