Formation of Ions in Positive-Ion Chemical Ionization

In chemical ionization, new ionized species are formed when gaseous molecules interact with ions, i.e., chemical ionization is based on ion-molecule reactions. Chemical ionization may involve the transfer of an electron, proton, or other ions between the reactants [8]. These reactants are i) the neutral analyte M and ii) ions from a reagent gas. 

Cl differs from what we have encountered in mass spectrometry so far because bimolecular processes are used to generate analyte ions. The occurrence of bimol-ecular reactions requires a sufficiently large number of ion-molecule collisions during the dwell time of the reactants in the ion source. This is achieved by significantly increasing the partial pressure of the reagent gas. Assuming reasonable 

Mass Spectrometry, 2nd ed., DOI 10.1007/978-3-642-10711-5 7, Springer-Verlag Berlin Heidelberg 2011 

Note It has been common to denote [M+H] and [M-H] ions as quasimolecu-lar ions because they comprise the otherwise intact analyte molecule and are detected in place of a true molecular ion, IVP, when soft ionization methods are employed. The term was also applied to [M+alkali] ions created by soft ionization methods other than Cl. Recent recommendations deprecate the term quasimolecular ion. Now, [M-hH] ions are addressed as protonated molecules, [M-H] as deprotonated molecules, and [M+Na] ions as sodiated molecules. 

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