High-precision determination of the (CO2)-C-13/(CO2)-C-12 isotope ratio using a portable 2.008-mu m diode-laser spectrometer

A compact and portable diode-laser spectrometer is developed for high-precision measurements of the (CO2)-C-13 /(CO2)-C-12 isotope ratio. Ro-vibrational transitions belonging to different combination bands, around 2.008 mum, are detected for the two isotopomers, using wavelength-modulation spectroscopy with first-harmonic detection. The determination of the isotope ratio is performed through a comparison between the spectra, simultaneously acquired in two different multiple-reflection cells designed to contain the sample and reference gases. The reproducibility of our method is extensively investigated over different time scales. In particular, the short-term precision of the isotope ratio determination is found to be 0.03%. We discuss the presence of possible systematic shifts in the delta-value and optimise the operating conditions to ensure accuracy within the precision level. A possible use for in-situ measurements in volcanic gases is also discussed.