A method of mass spectrometry or ion mobility spectrometry comprising: providing ions towards an ion trap mass analyser; selecting a target maximum charge desired to be stored within the ion trap mass analyser; and reducing the ion current passing to the ion trap mass analyser such that the ions entering the ion trap mass analyser do not cause the total charge within the ion trap mass analyser to rise above said target maximum charge, wherein said step of reducing the ion current passing to the ion trap mass analyser comprises: temporally separating the ions according to their ion mobility in an ion mobility separator; mass filtering the ions according to mass to charge ratio with a mass filter; and attenuating the ions using an attenuation device; wherein said steps of separating and mass filtering the ions result in target ions having selected combinations of ion mobility and mass to charge ratio being transmitted towards the ion trap mass analyser; and wherein said method further comprises: accumulating the target ions within the ion trap mass analyser during an ion accumulation period; and the ion trap mass analyser mass analysing the accumulated ions; wherein the ion mobility separator performs separation cycles; wherein for each separation cycle, ions are pulsed into the ion mobility separator, the ions separate according to their ion mobilities, and the ions then exit the ion mobility separator; and wherein attenuating the ions comprises transmitting ions to the ion trap mass analyser from only some of said separation cycles and not others. providing ions towards an ion trap mass analyser; selecting a target maximum charge desired to be stored within the ion trap mass analyser; and reducing the ion current passing to the ion trap mass analyser such that the ions entering the ion trap mass analyser do not cause the total charge within the ion trap mass analyser to rise above said target maximum charge, wherein said step of reducing the ion current passing to the ion trap mass analyser comprises: temporally separating the ions according to their ion mobility in an ion mobility separator; mass filtering the ions according to mass to charge ratio with a mass filter; and attenuating the ions using an attenuation device; temporally separating the ions according to their ion mobility in an ion mobility separator; mass filtering the ions according to mass to charge ratio with a mass filter; and attenuating the ions using an attenuation device; wherein said steps of separating and mass filtering the ions result in target ions having selected combinations of ion mobility and mass to charge ratio being transmitted towards the ion trap mass analyser; and wherein said method further comprises: accumulating the target ions within the ion trap mass analyser during an ion accumulation period; and the ion trap mass analyser mass analysing the accumulated ions; wherein the ion mobility separator performs separation cycles; wherein for each separation cycle, ions are pulsed into the ion mobility separator, the ions separate according to their ion mobilities, and the ions then exit the ion mobility separator; and wherein attenuating the ions comprises transmitting ions to the ion trap mass analyser from only some of said separation cycles and not others. accumulating the target ions within the ion trap mass analyser during an ion accumulation period; and the ion trap mass analyser mass analysing the accumulated ions; wherein the ion mobility separator performs separation cycles; wherein for each separation cycle, ions are pulsed into the ion mobility separator, the ions separate according to their ion mobilities, and the ions then exit the ion mobility separator; and wherein attenuating the ions comprises transmitting ions to the ion trap mass analyser from only some of said separation cycles and not others.