A mass filter is disclosed having at least one electrode (42-48) comprising an aperture (43) or recess. Voltages are applied to the electrodes (42-48) of the mass filter such that ions having mass to charge ratios in a desired range are confined by the electrodes and are transmitted along and through the mass filter, whereas ions (47,49) having mass to charge ratios outside of said desired range are unstable and pass into the aperture (43) or recess such that they are filtered out by the mass filter. The aperture (43) or recess reduces or eliminates the number of ions that would otherwise impact the electrode surface facing the ion transmission axis and hence reduces degradation of the ion transmission properties of the mass filter.