Skip to main content
U.S. flag

An official website of the United States government

Official websites use .gov
A .gov website belongs to an official government organization in the United States.

Secure .gov websites use HTTPS
A lock ( ) or https:// means you’ve safely connected to the .gov website. Share sensitive information only on official, secure websites.

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



Joseph T. Hodges, David A. Long, Mitchio Okumura, Charles E. Miller


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.
Applied Physics B


carbon dioxide, isotope ratios, cavity ring-down


Hodges, J. , Long, D. , Okumura, M. and Miller, C. (2011), Frequency-stabilized cavity ring-down spectroscopy measurements of carbon dioxide isotope ratios, Applied Physics B, [online], (Accessed July 15, 2024)


If you have any questions about this publication or are having problems accessing it, please contact

Created May 15, 2011, Updated February 19, 2017