A mid-infrared laser spectrometer was developed for simultaneous high-precision and isotope ratio measurements in carbon dioxide. A continuous-wave, liquid-nitrogen cooled, distributed feedback quantum cascade laser, working at a wavelength of , was used to probe , , and lines at . High sensitivity was achieved by means of wavelength modulation spectroscopy with second-harmonic detection. The experimental reproducibility in the short and long terms was deeply investigated through the accurate analysis of a large number of spectra. In particular, we found a short term precision of and , respectively, for and isotope ratios. The occurrence of systematic deviations is also discussed.
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