A method of improving the signal-to-noise using first and second mass spectrometers in tandem, with an ion detector and data system coupled to the second mass spectrometer, comprising selecting precursor ions with the first mass spectrometer, at least some of the parent ions being multiply charged, colliding or reacting the precursor ions in an intermediate chamber so that multiply charged parent ions produce product ions which have at least one fewer charge than the multiply charged precursor ions, and using the second mass spectrometer or the ion detector and data system to allow only those ions which have an m/z value higher than the multiply charged precursor ions to be recorded for analysis by the ion detector and data system, so that only a signal due to multiply charged precursor ions is obtained in said data system.