18.104.22.168 Ionization Techniques for Volatile Analytes
Entering the MS from a GC 22.214.171.124b Chemical Ionization
(CI): Today, most mass spectrometers can perform both electron
ionization and chemical ionization, with different
interchangeable ionization units. The CI unit is less open to
diffusion of the reagent gas in order to contain the reagent gas
longer and promote chemical ionization. Several reagent gases
are used including methane, propane, isobutane, and ammonia,
with the most common being methane. CI is referred to as a soft
ionization technique since less energy is transferred to the
original analyte molecule, and hence, less fragmentation occurs.
In fact, one of the main purposes of using CI is to observe the
molecular ion, represented by M.+ or M.-, or a close adduct of
it, such as MH+, MH+2, or M plus the chemical ion (i.e. M+CH3
with methane as the reagent gas or M+NH3 with ammonia as the
reagent gas). Notice again that neutral, negative, and positive
fragments are produced but only the