Chemical ionisation:

The chemical ionisation method uses ion/molecule reactions to produce ions from the analyte. For this reason a reagent gas like as methane, iso- butane, or ammonia is passed into the ionisation chamber where it gets ionised by electron ionisation. For example, methane gas gives mainly CH⁺₄ and CH⁺₃ ions as follows

CH₄ +e^- → CH₄  +2e^-

CH₄ → CH₃⁺ + H^•

These reagent gas ions then react along with the neutral molecules of the reagent gas as follows

CH₄ + CH₄ → CH₅⁺ + CH₃

CH₃⁺ + CH₄ → C₂H₅⁺ +H₂

The products of these ion-molecule reactions react along with the analyte molecules (M) to produce analyte ions. The reactions with the above ions can be shown as

M   +    CH⁺₅ →         M ⁺ +    CH₄

M   +    C₂H₅ ⁺ → MH⁺ +    C₂H₄

These give an ion at [M+1]. For the analyte M of RH type, we may have reaction that can be represented as

RH   +    CH⁺₅ → R⁺ +    CH₄    +   H₂

In this case the ions would be obtained at [M - 1]. Thus, the mass spectrum resulting from chemical ionisation method generally contains well defined [M+1]⁺ and [M   1]⁺ ion peaks. Further since the (M+1)⁺ ions do not undergo significant fragmentation, the spectra are simpler as compared to the ones obtained in EI method.