An instrument consisting of a reaction chamber with separate inlets for the sample and @R05190@ gas, an @O04307@, a photomultiplier and a signal processing device. The reactive gas is introduced in excess. The quantity of light produced is proportional to the sample flow rate and the concentration of the measured substance in the sample under specified temperature and pressure conditions. The @F02384@ limits the @W06659@ to the region of interest and helps to eliminate interferences.