Analyte Ion Formation in PTR-MS

On its way from the Venturi-type inlet to the downstream end of the drift section, HsO ions undergo nonreactive and reactive collisions with any one of the conponents of air [51,52]. The water reagent gas PICI process causes the analyzed VOCs to be converted into [M+H] ions. 

The flow tube, which is about 20 cm long, acts as a reaction region that is maintained at about 1 mbar by action of a vacuum pump attached to its exit. These conditions afford thermalization and effective proton transfer as the hydronium ions drift along together with the VOC-loaded air. As PTR-MS does not necessitate an additional buffer gas, the analyte air is not diluted any further, a feature that contributes to the high sensitivity of the method. Furthermore, the original mole fraction of the analytes in the air is maintained. 

Ion Source Drift Tube Transfer Lens System TOF - MS 12 3 

---