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

Abstract

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.