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

RATIONALE

Measurements of δ(17)O and δ(18)O values of tropospheric CO(2) 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 (17)O/(16)O and (18)O/(16)O ratios in CO(2), which is presented in this study.

METHODS

The method is based on isotopic exchange equilibration between H(2)O and CO(2) in sealed glass ampoules followed by water fluorination to produce O(2). Dual inlet isotope ratio mass spectrometric (IRMS) measurements of the δ(17)O and δ(18)O values of this O(2) allow δ(17)O values of CO(2) to be obtained with very high precision (0.01 to 0.03 ‰). This method requires about 70 µmol of CO(2).

RESULTS

Measurements of standard CO(2) gas and atmospheric CO(2) yield reproducibility of 0.01 to 0.03 ‰ for both δ(17)O and δ(18)O values, and 5 per meg for (17)O(excess). Fractionation factors (17)α and (18)α were determined in the H(2)O-CO(2) equilibrium at 25 °C as 1.021254 ± 0.00004 and 1.041036 ± 0.00008, respectively, and the ratio ln(17)α/ln(18)α as 0.5229 ± 0.0001.

CONCLUSIONS

The results demonstrate that the new method is the most accurate analytical procedure ever presented for measurements of (17)O(excess) of CO(2) gas. It is suitable for measurements of CO(2) extracted from relatively small samples of air (~5 L), and is thus useful for monitoring the (17)O(excess) of CO(2) on a broad global scale.