Theoretical and experimental evaluation of the isotope effect of NDIR analyzer on atmospheric CO2 measurement

Nondispersive infrared (NDIR) CO2 analyzer could produce erroneous CO2 mole fraction measurements for an air sample when CO2-in-air mixtures having different isotopic compositions than atmospheric CO2 are used as the NDIR calibration gases. This is because (1) an optical band-pass filter equipped in a typical NDIR analyzer to minimize the interference effect from the other infrared-active species is basically designed to transmit only the absorption band of (CO2)-C-12-O-16 and (2) absorption bands for the other CO2-related isotopologues, for example, (CO2)-C-13-O-16, are shifted to lower wave numbers depending on their isotope effects. To evaluate the effect of the isotopic composition on the NDIR response, we computed the theoretical relative molar response of the instrument to each isotopologue based on the infrared absorptance by the individual isotopologues. We then prepared a gravimetric (CO2)-C-13-in-air mixture with CO2 mole fraction of 380 ppm to experimentally determine the optical filter property. The apparent mole fractions of the (CO2)-C-13-in-air mixture determined by three NDIR analyzers used in this study were 46, 94, and 27 ppm, indicating that the optical filters in these instruments substantially reduced the response to (CO2)-C-13-O-16. Based on these theoretical and experimental analyses, we evaluated the apparent difference in the CO2 mole fraction determined by the three NDIR analyzers from the true value when isotopically lighter CO2-in-air mixtures (delta C-13 = -32.4 parts per thousand and delta O-18 = +11.7 parts per thousand), as compared to atmospheric CO2 (delta C-13 = similar to -8 parts per thousand and delta O-18 = similar to +40 parts per thousand), are used as calibration gases. The estimated difference varied with NDIR analyzers, ranging from -0.04 to -0.08 ppm for air samples with 380 ppm CO2.