This is an apparatus and method for interfacing constant field drift tube ion mobility devices with oscillatory traveling wave ion mobility devices.  Constant field ion mobility is capable of providing accurate collision cross section (CCS) measurements but resolution is inherently limited by limitations on voltages and the length of the drift tube.  The resolution of a traveling wave ion mobility device, such as the Structures for Lossless Ion Mobility technology developed at PNNL, is theoretically unlimited, but collisional cross section measurements can’t be directly obtained and have to be inferred.  This invention provides a variety of embodiments for a device that can seamlessly and efficiently perform both constant field ion mobility separations and oscillatory field ion mobility separations.