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

We report the development of a method of carbon stable isotope ratio analys is based on 1-cm(-1) resolution Fourier transform infrared (FT-IR) spectros copy, 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 a ir 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. T he method requires no sample preparation other than optional drying of the sample and may be applied directly to ambient air samples containing simila r 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-base d analysis of human breath. A similar method could also be used to determin e the H/D ratio in atmospheric water vapor.