Ionization/ionization source

The first step in an MS experiment is to obtain the ions in gas phase. The mass spectrometers, therefore have an ionization chamber (also called ionization source) where the samples are introduced to achieve ionization. Ions are generated through one of the several methods that have their own merits and limitations. Some of the ionization methods are:

Electron Ionization (EI)

In electron ionization method (Figure 11.5), a heated filament is used to emit the electrons. The electrons are accelerated through the ionization chamber under the influence of a strong electric field. The sample in gas phase is introduced into the ionization chamber. A high energy electron can knock off an electron from an analyte molecule, M giving a molecular radical cation.

         .............................................................................................(11.9)

M^.+ is referred to as the molecular ion. Loss of electron is a miniscule loss of mass; therefore mass of M^.+ equals the mass of the molecule.

Figure 11.5 Design of an electron ionization source