A method of mass spectrometry is disclosed comprising separating precursor ions using an ion mobility separator such that different precursor ions have different drift times through the on mobility separator; mass filtering said separated precursor ions with a mass filter, wherein the mass to charge ratios of the precursor ions transmitted by the mass filter vary as a function of the drift times of the precursor ions through the ion mobility separator; performing a first mode of operation comprising fragmenting the separated and mass filtered precursor ions in a fragmentation device to form fragment ions; urging the fragment ions through the fragmentation device such that fragment ions derived from different precursor ions that have been separated by the ion mobility separator are maintained spatially separated from each other as they are urged through the fragmentation device; and detecting the fragment ions. The method enables precursor ions to be associated with their related fragment ions more accurately.