High-precision measurements of 17O/16O and 18O/16O ratios in CO2.

RATIONALE Measurements of δ17O and δ18O values of tropospheric CO2 are of great importance. However, to be useful, such measurements must be an order of magnitude more precise than in current published literature. With this purpose we developed a new method for high-precision mass spectrometric measurements of 17O/16O and 18O/16O ratios in CO2, which is presented in this study. METHODS The method is based on isotopic exchange equilibration between H2O and CO2 in sealed glass ampoules followed by water fluorination to produce O2. Dual inlet isotope ratio mass spectrometric (IRMS) measurements of the δ17O and δ18O values of this O2 allow δ17O values of CO2 to be obtained with very high precision (0.01 to 0.03 ‰). This method requires about 70 µmol of CO2. RESULTS Measurements of standard CO2 gas and atmospheric CO2 yield reproducibility of 0.01 to 0.03 ‰ for both δ17O and δ18O values, and 5 per meg for 17Oexcess. Fractionation factors 17α and 18α were determined in the H2O-CO2 equilibrium at 25 °C as 1.021254 ± 0.00004 and 1.041036 ± 0.00008, respectively, and the ratio ln17α/ln18α as 0.5229 ± 0.0001. CONCLUSIONS The results demonstrate that the new method is the most accurate analytical procedure ever presented for measurements of 17Oexcess of CO2 gas. It is suitable for measurements of CO2 extracted from relatively small samples of air (~5 L), and is thus useful for monitoring the 17Oexcess of CO2 on a broad global scale. Copyright © 2012 John Wiley & Sons, Ltd. [ABSTRACT FROM AUTHOR]