In an EI source, electrons are produced by heating a wire filament which has electric current running through it. The electrons are accelerated through the ionization space towards an anode; in the ionization space, they interact with analyte molecules in the gas phase, causing them to ionize, and frequently causing numerous cleavage reactions that give rise to fragment ions, which can convey structural information about the analyte.
The efficiency of ionization and production of fragment ions depends strongly on the chemistry of the analyte and the energy of the electrons. At low energies (around 20 eV), the interactions between the electrons and the analyte molecules do not transfer enough energy to cause ionization. At around 70 eV, the de Broglie wavelength of the electrons matches the length of typical bonds in organic molecules (about 1.4 Å), and energy transfer to organic analyte molecules is maximized, leading to the strongest possible ionization and fragmentation. Under these conditions, about 1 in 1000 analyte molecules in the source are ionized. At higher energies, the de Broglie wavelength of the electrons becomes smaller than the bond lengths in typical analytes; the molecules then become "transparent" to the electrons, and ionization efficiency decreases.