Mass filter having reduced contamination

A mass filter (for example a quadrupole mass filter 4) has a DC resolving voltage applied thereto, wherein the polarity of the DC resolving voltage may be reversed automatically between different experiments but not reversed during an experiment. This may result in reversal of the direction in which any given ion outside the mass transmission window becomes unstable, and therefore spread the ion impacts over a greater surface area of the electrodes. The mass spectrometer may be operated with different operational parameters, or different mass to charge calibrations may be applied, when the polarity of the DC resolving voltage is in different orientations. Also disclosed is an AC-only mass filter 2 provided upstream of a mass filter 4. Mass pre-filtering using the AC mass filter 2 may reduce the rate at which contamination of the mass filter 4 builds up.