Measurement of carbon-13: carbon-12 ratios by Fourier transform infrared spectrometry

An approach to the measurement of 13C: 12C ratios in isotopically enriched samples is described. The carbon in samples is converted into CO2 gas by either combustion of organic material or acidification of carbonates. The gas is then measured by FTIR spectrometry. When spectra are recorded at 0.25 cm–1 resolution the magnitude of the isotopic shift is such that the areas of 12C and 13C rotational lines of the ν3 vibrational band for CO2 can be measured and the isotopic composition determined by reference to a standard calibration graph. The relative standard deviation at natural abundance is 1.2%, which gives an absolute limit of detection of 0.026 atom-%13C. Details of the decomposition procedures and a comparison of the results with those obtained by mass spectrometry are also presented.