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optimize, to ensure that the best possible data is generated for structure elucidation. If a more gentle
form of ionization is required, however, chemical ionization should be used.

Chemical Ionization

Chemical ionization was first observed by Munson and Field [4], who introduced it as a technique for
ionization in mass spectrometry in 1966. The procedure involves first the ionization of a reagent gas
such as methane in a simple electron impact ion source. The partial pressure of the reagent gas is
arranged to be about two orders of magnitude greater than that of the sample. The reagent ions collide
with the sample molecules and produce ions. The process is considered to be a gentle form of
ionization, because the energy of the reagent ions never exceeds 5 eV, including those reagent ions that
are considered to have relatively high energies. As a consequence there is little fragmentation, and the
major sample ion produced usually has a m/z value close to that of the singly charged molecular ion.
The spectrum produced by chemical ionization depends strongly on the nature of the reagent ion and
thus different structural information can be obtained by choosing different reagent gases. This adds
another degree of freedom in the operation of the mass spectrometer. The reagent ion can take a number
of forms. Employing methane as the reagent ion the following reagent ions can be produced

Other reactions can also occur that are not useful for ionizing the solute molecules but, in general, these
are in the minority. The interaction of positively charged ions with the uncharged sample molecules can
also occur in a number of ways, and the four most common are as follows:

1. Proton transfer between the sample molecule and the reagent ion,