The geometry of a Kingdon ion trap, in which harmonic ion oscillation in a potential well in a longitudinal direction is completely decoupled from ion oscillation in a direction transverse to the longitudinal direction, is arranged so that the oscillating ions introduced through the entrance tube cannot return to the entrance tube until they have performed several longitudinal oscillations during which time heavier ions can be introduced into the trap. In one embodiment, ions enter the trap via an entrance tube extending through, but electrically insulated from, one of the Kingdon trap housing electrodes and located outside the minimum of the potential well in the longitudinal direction.