A method and apparatus for detecting an analyte includes a sensor chamber for detecting an analyte, an analyte feed chamber, a distributor, and a controller for controlling the transport medium flow. The distributor includes an annular channel with four connections with a switchable isolating device between two connections. The controller controls the distributor for flushing the transport medium fed from the distributor to the sensor chamber without passing through the analyte feed chamber and for measuring the transport medium fed from the distributor to the sensor chamber while passing through the analyte feed chamber.