Abstract: A system for incorporation in ion-beam equipment to provide ion species filtering and optional ion beam blanking. The system has four magnets arranged on an optical axis. In between the two center magnets is arranged a plate having an off-axis aperture. The ion beam is caused to converge to a focus on a midplane in which the aforesaid plate is situated. When the magnets are energized, the ion beam is caused to bend by an amount depending upon the charge-to-mass ratio of the ion species within the beam. If constructed correctly, the aperture can thus be arranged to pass only a single ion species, the remainder colliding with and being absorbed by the plate. Beam blanking may optionally be achieved by blanking plates which may be used to deflect the ion beam in such a way that the resultant focal point of the ion beam is displaced away from the aperture so that the whole ion beam is blanked off. Astigmatism introduced into the system may be reduced or eliminated by the use of octopole electrode sets.