An ion guide or ion trap is disclosed having an entrance electrode and an exit electrode. The potential of the exit electrode is periodically dropped for a relatively short period of time allowing some ions to escape from the ion guide or ion trap via an aperture in the exit electrode. The period of time that the potential of the exit electrode is dropped for is progressively increased and ions emerge from the ion guide or ion trap in a mass to charge ratio dependent manner. The ion guide or ion trap may be operated as a mass separator or low resolution mass analyzer.