High-precision δ13CO2 analysis by FTIR spectroscopy using a novel calibration strategy

The 13C/12C ratio of atmospheric CO2 (δ13C–CO2) is a powerful tool to quantify CO2 flux strength of different ecosystem compartments, which is especially important in understanding the global carbon cycle and in predictions of the future uptake or release of CO2 in the global environment. To date, the majority of CO2 isotope studies have required air sample collection at remote locations, followed by laboratory analysis, which limits the number and frequency of measurements. In our study, we have developed a novel calibration strategy for the precise measurement of δ13C–CO2 in ambient air using a field-deployable Fourier transform infrared (FTIR) spectrometer. Quantification relies on a robust partial least squares (PLS) algorithm in combination with a set of multi-component standard spectra to determine δ13C–CO2 values from −8.7‰ to −19.0‰. Furthermore, the instrumental setup has significantly been improved to stabilize gas temperature and pressure. Thus we achieved an average accuracy for δ13C–CO2 of 0.4‰ and a precision of 0.15‰.