Abstract: A method for operating an Ion Mobility Spectrometer in which an analyte sample is ionized in the spectrometer reaction chamber and the analyte ions are permitted to flow freely from the reaction chamber into the spectrometer drift chamber. The ion current output from the spectrometer ion current detector is observed during this time. The ion flow from the reaction chamber to the drift chamber is then momentarily interrupted and the time delay between the time of ion flow interruption and the appearance of a decrease in the ion current detector output is observed. Different substances are characterized by different delays. The concentrations of the different substances may be determined by measuring the change in detected ion current after the ion flow interruption.