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 C-13/C-12 ratio of CO2 (i.e., delta(13)CO(2)) in air with an analytical precision of the order of +/-0.1 parts per thousand (i.e., +/-0.01%), The ET-IR method relies on calibration using synthetically calculated absorbance spectra and st 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 similar to 350 mu mol mol(-1) CO2 (molar mixing ratio). It may also be applied to samples more concentrated in CO2, such as human breath, similar to 5% CO2. We demonstrate the utility of the technique to the analysis of delta(13)CO(2) 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.