A method of analysing ions in a Data Dependent Acquisition (DDA) experiment comprising: performing an initial multi-dimensional survey scan comprising separating parent ions according to ion mobility and separating said parent ions according to mass to charge ratio; determining a plurality of parent ions of interest to be selected for the DDA experiment from said initial multi-dimensional survey scan; and then: sequentially selecting and transmitting said plurality of parent ions of interest based upon ion mobility and mass to charge ratio during a single cycle of ion mobility separation; fragmenting said plurality of parent ions of interest to derive one or more product ions; and analysing the one or more product ions derived from said plurality of parent ions of interest; wherein parent ions from said initial multi-dimensional survey scan that are not of interest are not subsequently isolated or selected for fragmentation; and wherein said step of sequentially selecting and transmitting said plurality of parent ions of interest comprises operating a quadrupole rod set with a broadband noise signal for ion rejection, said signal comprising multiple overlapping frequency notches to onwardly transmit parent ions of interest having more than one mass to charge ratio at any given ion mobility drift time. performing an initial multi-dimensional survey scan comprising separating parent ions according to ion mobility and separating said parent ions according to mass to charge ratio; determining a plurality of parent ions of interest to be selected for the DDA experiment from said initial multi-dimensional survey scan; and then: sequentially selecting and transmitting said plurality of parent ions of interest based upon ion mobility and mass to charge ratio during a single cycle of ion mobility separation; fragmenting said plurality of parent ions of interest to derive one or more product ions; and analysing the one or more product ions derived from said plurality of parent ions of interest; wherein parent ions from said initial multi-dimensional survey scan that are not of interest are not subsequently isolated or selected for fragmentation; and wherein said step of sequentially selecting and transmitting said plurality of parent ions of interest comprises operating a quadrupole rod set with a broadband noise signal for ion rejection, said signal comprising multiple overlapping frequency notches to onwardly transmit parent ions of interest having more than one mass to charge ratio at any given ion mobility drift time. sequentially selecting and transmitting said plurality of parent ions of interest based upon ion mobility and mass to charge ratio during a single cycle of ion mobility separation; fragmenting said plurality of parent ions of interest to derive one or more product ions; and analysing the one or more product ions derived from said plurality of parent ions of interest; wherein parent ions from said initial multi-dimensional survey scan that are not of interest are not subsequently isolated or selected for fragmentation; and wherein said step of sequentially selecting and transmitting said plurality of parent ions of interest comprises operating a quadrupole rod set with a broadband noise signal for ion rejection, said signal comprising multiple overlapping frequency notches to onwardly transmit parent ions of interest having more than one mass to charge ratio at any given ion mobility drift time. <EOS>