Abstract: What is disclosed is a method and a system for (trace) gas analysis, specifically NH.sub.3 analysis in flue gases exhausted from power plants or in industrial waste gases. In this method and system, a laser is reversed between two of its intrinsic resonance lines whereof one wavelength corresponds to an absorption maximum and the other corresponds to an absorption minimum of the gas to be detected. The extinction E and thus the concentration of the gas under analysis is derived from the ratio of the values of intensity attenuation at the two wavelengths.