A photo acoustic detection cell (6) is located within the optical cavity (3) of a cavity enhanced absorption spectroscopy apparatus (3,4,5). When a sample in the cell (6) absorbs radiation from a pulsed radiation beam coupled into the cavity (3) pressure waves are generated that are detected by a microphone (9). A detected signal (10) output by the microphone (9) may be processed to determine a value for the concentration of an absorber in the sample.