Electron ionisation:

This is the oldest and probably the best-characterized of all the ionisation methods. In that technique, a high energy beam of electrons passes through the sample in the gas-phase. These electrons commonly have energy of 10-150 eV. The electrons on colliding with the neutral analyte molecule knock off an electron from it and generate a positively charged ion. This procedure generates either a molecular ion or one of its fragments. This technique is good for volatile compounds but the molecular ion peak is either weak or absent.