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Chemical Ionization




Ionization of sample (analyte) is achieved by interaction of its molecules with reagent ions. The analyte is ionized by chemical ion-molecule reactions during collisions in the source. The process may involve transfer of an electron, a proton or other charged species between the reactants. This is a less energetic procedure than .


HOW IT WORKS


Typical reagent gases such as Methane , Isobutane , or Ammonia are present in a millionfold excess with respect to the analyte. The analyte is ionized by ion-molecule Chemical Reaction s during collisions in the Ion Source . Some Chemical Reaction s of Methane reagent gas are shown below:

Primary Ion Formation:

:CH_4 + e^- o CH_4^+ + 2e^-

Secondary Reagent Ions:

:CH_4 + CH_4^+ o CH_5^+ + CH_3

:CH_4 + CH_3^+ o C_2H_5^+ + H_2

Product Ion Formation:

:M + CH_5^+ o CH_4 + + H ^+ (protonation)

:AH + CH_3^+ o CH_4 + A^+ (H^- abstraction)

:M + CH_5^+ o CH_5 ^+ (adduct formation)

:A + CH_4^+ o CH_4 + A^+ (charge exchange)

Chemical ionization in an atmospheric pressure electric discharge is called Atmospheric Pressure Chemical Ionization .


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