An operating method is disclosed for the selective detection of a target gas in a gas mixture to be measured by a field effect transistor with a gas-sensitive layer disposed on a carrier substrate, wherein the gas mixture to be measured is prepared by an electrochemical element such that the measured gas mixture includes minimal amounts of interfering gases that interfere with the measurement of the target gas, and/or at least one target gas is activated such that it is detected by the gas-sensitive layer.