Mass filter having reduced contamination

A mass spectrometer comprising:

• a first AC-only mass filter comprising a plurality of electrodes;
• a second mass filter arranged downstream of the first mass filter so as to receive ions transmitted by the first mass filter;
• one or more voltage supplies;
• and
• a control circuit configured to:
• control said one or more voltage supplies so as to apply a first AC voltage to electrodes of the first mass filter for radially confine ions between the electrodes, and apply at least one second AC voltage between electrodes of the first mass filter for radially exciting some of the ions such that these ions cannot be transmitted downstream into the second mass filter whereas other ions can be transmitted downstream into the second mass filter;
• and
• control said one or more voltage supplies so as to apply voltages to the second mass filter so that it mass filters the ions transmitted by the first mass filter;
• wherein at any given time the second mass filter only transmits ions having a first range of mass to charge ratios and filters out all other ions;
• and
• wherein the at least one second AC voltage is applied to electrodes of the first mass filter such that it radially excites ions such that at least some ions having mass to charge ratios above said first range are not transmitted into the second mass filter.