Which is the most common ion source for GC and how does it work?

Electron Ionization (EI) ion source is the most common ion source for GC. The effluent from the column is allowed to enter into the heated ion source.  The electrons are drawn out from a tungsten filament by using a collector voltage of 70eV. This voltage applied to the filament defines the energy of the electron. These high energy electrons help in exciting the neutral analytes that are in vapor state. The applied voltage 70eV is very high and produces further fragmentation of the molecular ion. For the same reason EI source is considered as strong ionization source. In other words, the EI source does not give molecular ion as the base peak, instead it further fragments molecular ions. Most GC/MS libraries are based on EI spectra.