This animation shows the operation of a mass spectrometer. Ions are formed at D and pass through the cathode C and then through a slit S1. They then travel between two plates, across which a potential (V) is applied. A magnetic field (strength B) is applied at right angles to the electrostatic field and so the electrostatic and electromagnetic forces act in opposite directions to each other.
Two sets of ions of different mass are shown. Those with the greater mass describe a circle with a larger radius than those of smaller mass.