A method of mass spectrometry comprising: separating precursor ions using an ion mobility separator such that different precursor ions have different drift times through the ion mobility separator; mass filtering said separated precursor ions from a single separation cycle of the ion mobility separator 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, and wherein varying the mass to charge ratios of the precursor ions transmitted by the mass filter as a function of drift times does not cause only ions of selected charge states to be transmitted by the mass filter; and repeatedly alternating between performing a first mode of operation and performing a second mode of operation on the separated and mass filtered precursor ions from the single separation cycle of the ion mobility separator; wherein performing the first mode of operation comprises: fragmenting or reacting the separated and mass filtered precursor ions in a fragmentation or reaction device to form fragment or product ions; urging the fragment or product ions through the fragmentation or reaction device such that fragment or product 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 or reaction device; and detecting the fragment or product ions; and performing the second mode of operation comprises: the separated and mass filtered precursor ions bypassing the fragmentation or reaction device, or being transmitted to the fragmentation or reaction device but being substantially not fragmented, wherein the precursor ions are maintained separated according to their ion mobilities as they bypass the fragmentation or reaction device or as they are transmitted through the fragmentation or reaction device; and detecting the precursor ions; the method further comprising associating fragment or product ions detected in the first mode with precursor ions detected in the second mode. separating precursor ions using an ion mobility separator such that different precursor ions have different drift times through the ion mobility separator; mass filtering said separated precursor ions from a single separation cycle of the ion mobility separator 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, and wherein varying the mass to charge ratios of the precursor ions transmitted by the mass filter as a function of drift times does not cause only ions of selected charge states to be transmitted by the mass filter; and repeatedly alternating between performing a first mode of operation and performing a second mode of operation on the separated and mass filtered precursor ions from the single separation cycle of the ion mobility separator; wherein performing the first mode of operation comprises: fragmenting or reacting the separated and mass filtered precursor ions in a fragmentation or reaction device to form fragment or product ions; urging the fragment or product ions through the fragmentation or reaction device such that fragment or product 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 or reaction device; and detecting the fragment or product ions; and performing the second mode of operation comprises: the separated and mass filtered precursor ions bypassing the fragmentation or reaction device, or being transmitted to the fragmentation or reaction device but being substantially not fragmented, wherein the precursor ions are maintained separated according to their ion mobilities as they bypass the fragmentation or reaction device or as they are transmitted through the fragmentation or reaction device; and detecting the precursor ions; the method further comprising associating fragment or product ions detected in the first mode with precursor ions detected in the second mode.