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Improved Infrared Spectroscopic Method for the Measurement of 13C: 12C Ratios

Volume 51, Number 1 (Jan. 1997) Page 17-21

Kindness, Andrew; Marr, Iain L.


Pressure broadening of an infrared absorption band of gaseous CO2 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 upsilon3 bands for carbon dioxide centered at 2350cm-1 is sufficient to completely separate the P branch for 13CO2 from the two branches for 12CO2. The intensity ratios can be measured accurately on a low-resolution FT-IR spectrometer when the CO2 is pressurized to 10 bar (106N m-2) with N2, permitting isotope ratio differences as small as 0.02 atom % above the natural abundance of 1.11 atom % to be detected.