(d)  Chemical Ionization

Set-up for chemical Ionization is similar to electron impact ionization. However, in this method, a reagent gas like CH₄ is injected in the ion chamber. Due to electron impact, the reagent gas in the chemical ionization source gets ionized. This follows injection of analyte molecule. Analyte molecules undergo many collisions with the reagent gas. The reagent gas ions in this cloud react and produce adduct ions as shown in figure, which are excellent proton donors for analyte.