An improved method of high-precision determination of Δ(17)O of CO2 by catalyzed exchange with O2 using hot platinum.

Abstract

CO2 and O2 can exchange their oxygen isotopes rapidly in the presence of hot (~670 °C) platinum and this has led to a method for determining the δ(17)O value of a CO2 sample. We have improved the method to achieve a precision of 0.008 ‰ (1-σ standard deviation) in the determination of δ(17)O values. Such high precision is essential to identify the stratospheric component in tropospheric CO2 and use it for global carbon flux studies. The crucial issue in the accurate determination of the δ(17)O value is estimation of a correction factor, which depends on the amount ratio CO2/O2. An attempt was also made to investigate the mechanism of exchange with their controlling parameters. The oxygen isotopes of a CO2 sample gas are exchanged with those of an appropriate amount of tank O2 in the presence of hot platinum. The pre-exchange CO2 and O2 gas samples as well as the post-exchange O2 sample are analyzed by isotope ratio mass spectrometry. A mixing model was developed involving the δ(18)O value of the CO2 and δ(17)O and δ(18)O values of pre- and post-exchange O2 to obtain the δ(17)O value of the CO2 sample. A correction to the measured value was determined to obtain the actual value with high accuracy and precision. To obtain a precision better than 0.01 ‰ requires the amount ratio CO2/O2 to be controlled to better than ~15 %. We also find that the oxygen isotopes are nearly homogeneously distributed between the O2 and the CO2 molecules. In addition, determination of the (16) O(13)C(18)O/(16)O(12)C(16)O isotopologue ratio in the CO2 shows that the abundance of (16)O(13)C(18)O is close to that expected for random partitioning of the isotopes among the CO2 isotopologues. The isotopic scrambling between O2 and CO2 that occurs on hot platinum allows one to accurately determine the δ(17)O values of CO2 through isotopic analysis of O2.