Carbon dioxide (CO2) isotopic ratios were measured at 1.6 μm through the use of frequency-stabilized cavity ring-down spectroscopy (FS-CRDS). We report the highest spectrum signal-to-noise ratios to date for CO2 transitions, with values as high as 28,000:1 achieved. Measured single-spectrum precisions were 0.11, 0.09, and 0.59 for the 13C/12C, 18O/16O, and 17O/16O ratios, respectively. In addition, the importance of utilizing the Galatry line profile is demonstrated. The use of the Voigt line profile, which neglects the observed collisional narrowing, leads to large systematic errors which are transition-dependent and vary with temperature and pressure. While the low intensities of CO2 transitions at 1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS have enabled isotope ratio measurement precisions comparable with other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer constructed at 2.0 or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.
Citation: Applied Physics B
Pub Type: Journals
carbon dioxide, isotope ratios, cavity ring-down