An ion mobility spectrometer instrument has a drift tube that is partitioned into a plurality of cascaded drift tube segments. A number of electric field sources may each be coupled to one or more of the plurality of drift tube segments. A control circuit is configured to control operation of the number of the electric field sources in a manner that sequentially applies electric fields to the drift tube segments with a magnitude of at least one being different than the others to allow only ions having a predefined ion mobility or range of ion mobilities to travel through the drift tube. Alternatively, the length of at least one of the drift tube segments may be made different from those of the others to produce the same result. The drift tube segments may define a linear drift tube or a closed drift tube with a continuous ion travel path.