Precision trace gas analysis by FT-IR spectroscopy. 2. The 13C/12C isotope ratio of CO2

We report the development of a method of carbon stable isotope ratio analysis based on 1-cm-1 resolution Fourier transform infrared (FT-IR) spectroscopy, deployable in both laboratory and field applications. We demonstrate the determination of the 13C/12C ratio of CO2 (i.e., delta 13CO2) in air with an analytical precision of the order of +/- 0.1/1000 (i.e., +/- 0.01%). The FT-IR method relies on calibration using synthetically calculated absorbance spectra and a multivariate calibration algorithm. The method requires no sample preparation other than optional drying of the sample and may be applied directly to ambient air samples containing approximately 350 mumol mol-1 CO2 (molar mixing ratio). It may also be applied to samples more concentrated in CO2, such as human breath, approximately 5% CO2. We demonstrate the utility of the technique to the analysis of delta 13CO2 in air during an experimental field campaign and to the laboratory-based analysis of human breath. A similar method could also be used to determine the H/D ratio in atmospheric water vapor.