A method is disclosed for analyzing ions by mass spectrometry by repeatedly executing a data acquisition cycle to acquire product ion data across a precursor mass range of interest. The data acquisition cycle comprises performing, for each of a plurality of isolation windows having different mass ranges, steps of (i) isolating precursor ions within the mass range of the isolation window, (ii) fragmenting the isolated precursor ions to generate product ions, and (iii) mass analyzing the product ions. The step of mass analyzing the product ions includes concurrently mass analyzing product ions corresponding to N isolation windows, N being an integer greater than or equal to one, wherein N is changed at least once across the data acquisition cycle.