A method of mass spectrometry is disclosed comprising separating a first population of ions according to their ion mobility and selectively attenuating ions in the first population of ions in one or more ion mobility and/or drift time ranges so as to form a second population of ions. The step of selectively attenuating ions comprises applying a varying, increasing or decreasing attenuation factor to ions in different ion mobility and/or drift time ranges. The method further comprises separating the second population of ions according to their ion mobility.