Abstract: Method and apparatus for spectroscopic measurement of the concentration of a gas in a sample where you detect the intensity of light from a light source (1) passed through the sample (4) and through a reference cell (5) and generates a signal which represents the concentration of the gas. A laser diode (1) constitutes the light source which is locked to the absorption line of the gas at known concentration and pressure contained in a reference cell (5). The laser light with the properly selected wavelength is distributed via optical fibres (2, 3, 6) and/or glass prisms (19, 20) to the reference cell (5) and the sample (4). Non-gas-related transmission variations in the measurement path or in the optics is automatically compensated for by a special laser modulation which generates a time multiplexed reference.