A method of guiding or mass filtering ions comprising: providing an ion guide or mass filter device comprising a plurality of electrodes or rods (2a,2b); applying an AC or RF voltage to said plurality of electrodes or rods (2a,2b); supplying a plurality of signals (13,14,15) to said plurality of electrodes or rods (2a,2b), wherein said step of supplying said plurality of signals (13,14,15) comprises at least the steps of: (i) supplying a first signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said first signal (13,14,15) also comprising a plurality of frequency notches (16a, 16b, 16c), and obtaining a first set of data; characterised by (ii) supplying a second different signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said second signal (13,14,15) also comprising a plurality of frequency notches (16a,16b,16c), and obtaining a second set of data; and deconvoluting, decoding or demodulating said first set of data and/or said second set of data to determine the intensity of ions having a plurality of different mass to charge ratios. providing an ion guide or mass filter device comprising a plurality of electrodes or rods (2a,2b); applying an AC or RF voltage to said plurality of electrodes or rods (2a,2b); supplying a plurality of signals (13,14,15) to said plurality of electrodes or rods (2a,2b), wherein said step of supplying said plurality of signals (13,14,15) comprises at least the steps of: (i) supplying a first signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said first signal (13,14,15) also comprising a plurality of frequency notches (16a, 16b, 16c), and obtaining a first set of data; characterised by (ii) supplying a second different signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said second signal (13,14,15) also comprising a plurality of frequency notches (16a,16b,16c), and obtaining a second set of data; and (i) supplying a first signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said first signal (13,14,15) also comprising a plurality of frequency notches (16a, 16b, 16c), and obtaining a first set of data; characterised by (ii) supplying a second different signal (13,14,15) to said plurality of electrodes or rods (2a,2b) in order to resonantly or parametrically excite undesired ions within or from said ion guide or mass filter device, said second signal (13,14,15) also comprising a plurality of frequency notches (16a,16b,16c), and obtaining a second set of data; and deconvoluting, decoding or demodulating said first set of data and/or said second set of data to determine the intensity of ions having a plurality of different mass to charge ratios.