A mass spectrometer is disclosed comprising a quadrupole rod set ion guide or mass filter device (6). A broadband frequency signal (10) having one or more notches (11a, 11b, 11c) is applied to the rods of the quadrupole rod set (6). The notched broadband frequency signal (10) causes undesired ions to be resonantly ejected from the ion guide (6). The notched broadband frequency signal (10) has frequency components missing which correspond with the resonance frequency of ions which are desired to be onwardly transmitted. The ion guide or mass filter device (6) enables a plurality of desired ions having different mass to charge ratios to be simultaneously transmitted by the ion guide or mass filter device (6) whilst other ions are resonantly ejected from the ion guide or mass filter device (6).