Method of measuring a ratio of concentrations of two isotopologues of a chemical species in a first gas mixture using an optical absorption spectroscopy based gas analyzer; the method includes measuring the line intensity of a rotational-vibrational line of one isotopologue and the line intensity of another rotational-vibrational line of another isotopologue. The method also includes determining the ratio of two line intensities and comparing it with another ratio measured using another gas analyzer for a gas mixture with known concentration of isotopologues. The second ratio can be measured for the same pair of lines or for different pair of lines. The second ratio can be measured at the same gas temperature or at different gas temperature. The method includes determining a ratio of concentrations of two isotopologues based on two ratios of line intensities and two temperatures.