A method and apparatus for performing differential mobility spectrometer (DMS) which includes decreasing the amount of time that ions spend inside fringing fields generated by the DMS. The apparatus includes an entrance electrode plate sealingly engaged to the entrance of the DMS, and is electrically separated from the parallel plate electrodes of the DMS, the entrance electrode plate has an aperture for allowing the traversal of ions into the DMS; wherein the cross-sectional area of the aperture is less than the cross-sectional area of the ion path, the ion path being located between the two parallel plate electrodes of the DMS. The entrance electrode plate may also have a focusing potential applied to it for focusing of ions.
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