Abstract: A method for detecting very small concentrations of gases in gas mixtures passed as a gas stream through a measuring cell having a chamber through which the gas stream flows through, a drift zone provided between an inlet and an outlet of the chamber, a gas stream ionizing radiation source located at the inlet of the drift zone, a collector electrode located at the outlet of the drift zone, a grid consisting of two groups of parallel wires located in the drift zone current adjacent grid wires are supplied with different electrical voltages and whereat an AC voltage of adjustable frequency is fed to the two groups of grid wires, and a measuring apparatus for detecting the collector current, including the steps of inducing the movement of the ions through the drift zone by means of a ventilator which generates a controlled gas flow to the collector electrode, periodically changing the frequency of the AC voltage applied to the groups of grid wires either continuously or stepwise, simultaneously measuring the co