Frequency-stabilized cavity ring-down spectroscopy measurements of carbon dioxide isotopic ratios

Carbon dioxide (CO₂) isotopic ratios on samples of pure CO₂ were measured in the 1.6 μm wavelength region using the frequency-stabilized cavity ring-down spectroscopy (FS-CRDS) technique. We present CO₂ absorption spectra with peak signal-to-noise ratios as high as 28,000:1. Measured single-spectrum signal-to-noise ratios were as high as 8900:1, 10,000:1, and 1700:1 for ¹³C/¹²C, ¹⁸O/¹⁶O, and ¹⁷O/¹⁶O, respectively. In addition, we demonstrate the importance of utilizing the Galatry line profile in the spectrum analysis. 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 relatively low intensities of CO₂ transitions near λ=1.6 μm make this spectral region non-optimal, the sensitivity and stability of FS-CRDS enabled measurement precision of pure CO₂ samples which are comparable to those of other optical techniques which operate at far more propitious wavelengths. These results indicate that a FS-CRDS spectrometer designed to probe CO₂ bands near wavelengths of 2.0 μm or 4.3 μm could achieve significantly improved precision over the present instrument and likely be competitive with mass spectrometric methods.