Mass Spectrometer A simple mass spectrometer consists of a particle accelerator, anion velocity selector and a separator, as shown below. Velocity Selector Separ at or Particle Accelerator N Charged N ions S S Refer to the above diagram. Charged ions of only a certain speed pass through the velocity selection chamber undeflected because: the electric field strength is the same as the magnetic field strength only these ions possess the charge needed to be undeflected by the fields the net deflecting force from the electric and magnetic fields is zero for only these ions ions travelling at other speeds have insufficient kinetic energy to pass through the chamber