A method and mass spectrometer for filtering ions are provided. The mass spectrometer generally comprises an ion guide, a quadrupole mass filter, a collision cell and a time of flight (ToF) detector, and is enabled to transmit an ion beam through to the ToF detector. The mass spectrometer is operated in MS mode, such that ions in the ion beam remain substantially unfragmented, the quadrupole mass filter operating at a pressure substantially lower than in either of the ion guide and the collision cell. The quadrupole mass filter is operated in a bandpass mode such that ions outside of a range of interest are filtered from the ion beam, leaving ions inside the range of interest in the ion beam. The ions inside the range of interest are analyzed at the ToF detector.