Abstract: A method for remotely monitoring the concentration of a particular gas species in a gaseous mixture. The remote detection of pollutants emitted in source effluents, e.g., gas from a smokestack, involves specific spectral measurements of infrared radiation and, in this method, includes ratioing the signals from two gas-reference cell pairs to eliminate dependencies upon the inlowering atmosphere and source effluent temperature. An instrument cell construction and assembly using a unique cells-within-a cell permits use of a single lens that insures common fields-of-view for both cell pairs; a triple chopping system uses tuning forks to alternately pass the incoming energy through the gas and reference cells eliminating the need for separate sync signal generators, and provides a high carrier frequency improving the signal-to-noise ratio of the sensor.