Ion source

An ion source is a device that creates atomic and molecular ions. Ion sources are used to form ions for mass spectrometers, optical emission spectrometers, particle accelerators, ion implanters and ion engines.

Electron ionization is widely used in mass spectrometry, particularly for organic molecules. The gas phase reaction producing electron ionization is

where M is the atom or molecule being ionized, ${\ce {e^{-}}}$ is the electron, and ${\ce {M^{+\bullet }}}$ is the resulting ion.

The electrons may be created by an arc discharge between a cathode and an anode.