[en] Method and apparatus for detecting the presence of very small concentrations of certain vapors and gases in air or other gaseous backgrounds. A gas sample is ionized by a source of ionizing radiation. The ionized gas is then directed through a first drift region where the ions are subjected to a drift potential. Electrical biased grid means are provided to allow only heavier ions of lower mobility to pass through the first drift region into a second drift region, where electrical shutter means are provided to allow discrete packets of ions to pass and to drift at speeds depending upon their mobilities and be detected. In an alternate embodiment, the first draft region is positioned downstream from the second drift region