The separation of ions of varying mass by a combination of electric and magnetic fields.

In the most common application, an isotopic mixture of ions is produced by either electron bombardment of a gas or thermionic emission.

The ionized particles are accelerated and collimated (adjusted the line of sight) into a beam by a system of electrodes, and the beam is projected into a magnetic field where the paths of the ions depend on their mass-to-charge ratio.