A method of mass spectrometry or ion mobility spectrometry is disclosed in which analyte ions of a desired charge state are isolated. The method comprises: separating analytes according to their electrophoretic mobility; ionizing the analytes; and mass filtering the resulting analyte ions, wherein the mass to charge ratios of the ions transmitted by a mass filter are varied as a function of the electrophoretic mobility and according to a predetermined relationship such that substantially only ions having said desired charge state are transmitted by the mass filter.