Sample molecules in vapor state are bombarded by fast moving electrons, conventionally 70 eV energy. This results in ion formation. One electron from the highest orbital energy is dislodged, and as a consequence molecular ions are formed. Some of this molecular ions decompose and fragment ions are formed. The fragmentation of a given ion is due to the excess of energy that it requires within the ionization.
Fragment ions can be odd electron or even electron.
Molecular ions formed in electron impact ionization are odd electron ions. Sample can be introduced to the EI source via a gas chromatography device, for example in the case of mixtures, or directly via a solids probe device. The quantities needed for an experiment is usually less than a microgram of material. EI mass spectra, in most of cases, contain intense fragment ion peaks and much less intense molecular ion peak. When the molecular ion peak is not observed in the mass spectrum, chemical ionization can be used in order to get molecular ion information.