Ionization, chemical selective-ejection

When the device is operated as a mass spectrometer, ions produced by an electron-impact or chemical ionization source are admitted through a grid in the upper end cap. The ionization source is pulsed so as to create a burst of ions. The ions over a large mass range of interest are trapped simultaneously. The ion trap can store ions for relatively long times, up to 15 minutes for some stable ions. A technique called mass-selective ejection is then used to sequentially eject the trapped ions in order of mass by increasing the radio-frequency... 

The first PSIT work at PNNL showed how important such reactions can be. Adventitious water in the ion trap was shown to be responsible for rapid reaction of Ar+ to form H2O+ and H30 . With the low-mass cut-off of the ion trap set to m/z 19 or more, H3O+ ions are ejected from the trap as they are formed, thereby interrupting the mechanism by which H2O+, and H3O+ chemically ionize background species. Unfortunately, H2O also reacts with a wide variety of cations that are of great analytical interest, in particular, metal atomic cations. A more selective reagent gas is needed. 

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