Pressure broadening of an infrared absorption band of gaseous CO 2 makes measurement of a band intensity very much easier and also much more precise in comparison with the results obtained by measuring the intensity of a single line in a gas-phase spectrum. The isotopic shift of the v 3 bands for carbon dioxide centered at 2350 cm −1 is sufficient to completely separate the P branch for 13 CO 2 from the two branches for 12 CO 2 . The intensity ratios can be measured accurately on a low-resolution FT-IR spectrometer when the CO 2 is pressurized to 10 bar (10 6 N m −2 ) with N 2 , permitting isotope ratio differences as small as 0.02 atom % above the natural abundance of 1.11 atom % to be detected.
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Kindness et al. (1997) studied this question.
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