chemical ionisationA process of ionisation involving the reaction of a reagent ion and a neutral molecule to yield a charged ionic form of the molecule.
The first step in chemical ionisation is the creation of the reagent ion by electronically ionising reagent gas molecules, which are present in great excess. A stable population of reagent ions is formed through ion–molecule reactions, which eventually react with the neutral gas-phase molecules. In a common example, methane gas is ionised to form predominantly CH5, which then reacts with the neutral sample molecule in a process of protonation to form MH.
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