Ion mobility spectrometer. The spectrometer includes an enclosed region having a gas with a selected chemical species contained therein. An energy source ionizes the gas and the chemical species. Spaced apart electrodes generate high frequency and DC electric fields across the enclosed region and circuitry is provided for generating voltage waveforms on the electrodes. The voltage waveforms include a symmetric RF field to minimize ion loss and to prevent clustering of the ions with water molecules during an ion buildup phase. A DC and asymmetric, non-uniform RF field is provided to separate and focus the ions in the region during an ion separation phase. Finally, a changing DC or RF field causes the ionized chemical species to move to the electrodes and read-out circuitry responds to current in the electrodes to indicate the presence and/or amount of the chemical species.