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.