MASS FILTER HAVING REDUCED CONTAMINATION

A method of mass filtering ions is disclosed comprising: providing a first, AC-only, mass filter 2; providing a second mass filter 4 downstream of the first mass filter; applying a first AC voltage 8 to electrodes of the first mass filter so as to radially confine ions between the electrodes, and applying a second AC voltage 10 between electrodes of the first mass filter 2 so as to radially excite some of said ions such that these ions are not transmitted; and using the second mass filter 4 to mass filter ions; wherein at any given time the second mass filter 4 only transmits ions having a first range of mass to charge ratios and filters out all other ions; and wherein the step of applying the at least one second AC voltage 10 to electrodes of the first mass filter 2 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.