Abstract: The present invention relates to an analysis apparatus for analyzing a gas sample comprising a concentration measurement path receiving the gas sample; a light transmitter for transmitting light signals into the concentration measurement path; a detector for detecting light signals exiting the concentration measurement path; an evaluation unit which is adapted to determine the concentration of at least one substance present in the gas sample on the basis of the intensity of the detected light signals and on the basis of the length of the concentration measurement path; and a measurement device which is configured to determine the length of the time of flight path of the transmitted light signals optically using the light transmitter, with the evaluation unit being adapted to determine the length of the concentration measurement path on the basis of the determined length of the time of flight path.