A method of monitoring detection of an analyte in a liquid sample using a measuring cell, the measuring cell comprising a working electrode for excitation of electrochemiluminescence in the liquid sample, an optical detector for detecting the excited electrochemiluminescence, the excitation and detection being performed in an measurement cycle, the measurement cycle comprising transporting the liquid sample via a transport path to the working electrode using a support liquid, the method comprising: coupling light of a light source into the transport path during part of the measurement cycle, the transport path forming a light guide between the light source and the optical detector, detecting the coupled light by the optical detector, analyzing the detected light for a gas bubble in the transport path, providing a measurement state if the result of the analysis deviates from a target state regarding the presence of a gas bubble in the transport path.