The invention discloses a method and device for improved interfacing of Differential Mobility Spectrometers (DMS) and, in particular, Field Asymmetric waveform Ion Mobility Spectrometers (FAIMS) to subsequent instrument stages. Those stages may include mass spectrometry (MS), ion mobility spectrometry (IMS), and their combination (IMS/MS) to which FAIMS has been coupled in the prior art, but not limited thereto. The invention has been reduced to practice and is particularly intended for coupling FAIMS devices of planar geometry well-known in the art, where an overall sensitivity gain due to improved efficiency of ion transmission to a following MS or IMS stage has been demonstrated. The invention is primarily designed for coupling to subsequent slit aperture and particularly slit aperture/ion funnel interfaces, but may also be used in conjunction with other MS interfaces and/or with apertures of rectangular, circular, and other geometries. In another aspect, the present disclosure allows a more compact design and convenient operation of FAIMS/MS and FAIMS/IMS systems.