3. Gas phase ionization methods at atmospheric pressure

The mechanisms that form positive or negative ions in an API source are classically grouped into two categories: gas-phase reactions in the API chamber, and liquid-phase reactions in the sample solution. Ionization in an APCI or APPI source is almost exclusively controlled by gas-phase ion-molecule reactions, unlike in an ESI source where liquid-phase and gas-phase ionization mechanisms may overlap.

Gas-phase ionization methods produce ions from neutral samples that can be vaporized by heat at atmospheric pressure. With the exception of a few salts, such as ammonium acetate or formate, which can be vaporized to produce ${\text{NH}}_4^{+}\text{,}{\text{HCO}}_2^{-}$...