What is chemical ionization?

Chemical ionization (CI) is an alternate means to achieve ionization in GC/MS. In CI, reagent gas like methane or ammonia is allowed to enter into the ionization chamber. Here, the reagent gas is ionized to produce cation which undergoes further reaction to produce secondary ions.

Primary ion formation:

CH₄+ e^- ------------> CH₄^+. + 2e^-

Secondary ion formation:

CH₄^+. + CH₄ ------------> CH₅⁺ + CH₃^.

CH₄⁺ + CH₃⁺------------> C₂H₅⁺ + H₂

Product ion formation:

M+ CH₅⁺ ------------> CH₄ + [M+H]⁺

M+ C₂H₅⁺ ------------> [M+ C₂H₅]⁺

The secondary ion acts as a reagent and ionizes the analyte to produce product ion.  Since the ionization is brought by a secondary reagent gas, this ionization is very soft and produces M, M+1, M-1, M+29 as the ions, where M is the molar mass of the analyte.

Modern benchtop GC/MS are compatible with EI and CI  and can operate in either mode just by changing the ion source and adding reagent gas.