the invention relates to the operation of trapping ion mobility spectrometers based on pushing ions by a gas flow against a counter - acting electric dc field barrier , preferably in combination with a mass analyzer as ion detector . the invention provides an additional rf ion trap upstream of the trapping ion mobility spectrometer , wherein the rf ion trap is operated as an accumulation unit in parallel with the trapping ion mobility spectrometer such that a first group of ions can be analyzed in the trapping ion mobility spectrometer while a second group of ions from an ion source are simultaneously collected in accumulation unit .