The principle of the method specified is representative sampling of gases in a duct using two methods. In extractive sampling, the gases are conditioned to remove aerosols, particulate matter, and other interfering substances. In non-extractive sampling, the measurements are made in-situ, and no sample conditioning except filtering is required. The application is limited to the determination of O2, CO2, CO, SO2, NO, NO2. Although they are only mentioned briefly, detailed velocity measurements are required to determine the mass flow rates of gases.