IMPROVED INFRARED SPECTROSCOPIC METHOD FOR THE MEASUREMENT OF C-13 C-12 RATIOS/

Pressure broadening of an infrared absorption band of gaseous CO2 make s measurement of a band intensity very much easier and also much more precise in comparison with the results obtained by measuring the inten sity of a single line in a gas-phase spectrum, The isotopic shift of t he nu(3) bands for carbon dioxide centered at 2350 cm(-1) is sufficien t to completely separate the P branch for (CO2)-C-13 from the two bran ches for (CO2)-C-12, The intensity ratios can be measured accurately o n a low-resolution FT-IR spectrometer when the CO2 is pressurized to 1 0 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.