Editing Chemical ionization (section)

===Charge-exchange chemical ionization===
This is also similar to CI and the difference lies in the production of a radical cation with an odd number of electrons. The reagent gas molecules are bombarded with high energy electrons and the product reagent gas ions abstract electrons from the analyte to form radical cations. The common reagent gases used for this technique are toluene, benzene, NO, Xe, Ar and He.

Careful control over the selection of reagent gases and the consideration toward the difference between the resonance energy of the reagent gas radical cation and the ionization energy of the analyte can be used to control fragmentation.<ref name=":1" /> The reactions for charge-exchange chemical ionization are as follows.

:<chem> He{} + e^- -> He^{+\bullet}{} + 2e^- </chem>
:<chem> He^{+\bullet}{} + M -> M^{+\bullet} </chem>
[[File:Apci.png|thumb|Atmospheric pressure chemical ionization source]]