Method and apparatus

An ion filter for a mass spectrometer includes: an ion modifier 6 comprising two electrodes; an ion selector 4 configured to select a subset of ions based on their mobility in a gas; and a controller. The controller is configured to operate the ion modifier 6 in: a first mode to modify the ions by subjecting them to an RF electric field in the region between the two electrodes to raise their effective temperature to provide daughter ions; and a second mode to output the ions without modification. The ion filter provides ions to a mass spectrometer. The modification of the ions may be before or after the selection of a subset of ions. The ion selector 4 may comprise the ion modifier 6.