A method of analysing ions in a MS 3 Data Dependent Acquisition experiment comprising: separating parent ions according to ion mobility or differential ion mobility and separating said parent ions according to mass, mass to charge ratio or time of flight; fragmenting specific parent ions of interest if said parent ions of interest have a specific predetermined ion mobility or differential ion mobility property and a specific predetermined mass, mass to charge ratio or time of flight property to generate a plurality of first generation fragment ions; sequentially selecting and transmitting a plurality of first generation fragment ions of interest based upon ion mobility or differential ion mobility of corresponding parent ions and mass, mass to charge ratio or time of flight of the first generation fragment ions during a single cycle of ion mobility separation; fragmenting said plurality of first generation fragment ions of interest to generate a plurality of second generation fragment ions; and analysing either: (i) said second generation fragment ions; and/or (ii) fragment, product or other ions derived from said second generation fragment ions; wherein the step of selecting said plurality of first generation fragment ions of interest comprises using a quadrupole rod set mass filter to filter ions according to mass, mass to charge ratio or time of flight; and wherein said method further comprises applying a broadband excitation voltage to said mass filter in order to attenuate or reject ions having a range of masses or mass to charge ratios and wherein said broadband excitation voltage includes one or more frequency notches so that ions having specific desired masses or mass to charge ratios are not substantially attenuated or rejected by said mass filter. separating parent ions according to ion mobility or differential ion mobility and separating said parent ions according to mass, mass to charge ratio or time of flight; fragmenting specific parent ions of interest if said parent ions of interest have a specific predetermined ion mobility or differential ion mobility property and a specific predetermined mass, mass to charge ratio or time of flight property to generate a plurality of first generation fragment ions; sequentially selecting and transmitting a plurality of first generation fragment ions of interest based upon ion mobility or differential ion mobility of corresponding parent ions and mass, mass to charge ratio or time of flight of the first generation fragment ions during a single cycle of ion mobility separation; fragmenting said plurality of first generation fragment ions of interest to generate a plurality of second generation fragment ions; and analysing either: (i) said second generation fragment ions; and/or (ii) fragment, product or other ions derived from said second generation fragment ions; wherein the step of selecting said plurality of first generation fragment ions of interest comprises using a quadrupole rod set mass filter to filter ions according to mass, mass to charge ratio or time of flight; and wherein said method further comprises applying a broadband excitation voltage to said mass filter in order to attenuate or reject ions having a range of masses or mass to charge ratios and wherein said broadband excitation voltage includes one or more frequency notches so that ions having specific desired masses or mass to charge ratios are not substantially attenuated or rejected by said mass filter.