Mahata Sasadhar, Bhattacharya S K, Liang Mao-Chang
Research Center for Environmental Changes, Academia Sinica, Taipei, Taiwan.
Graduate Institute of Astronomy, National Central University, Jhongli, Taiwan.
Rapid Commun Mass Spectrom. 2016 Jan 15;30(1):119-31. doi: 10.1002/rcm.7423.
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.
在热的（约670°C）铂存在下，二氧化碳（CO₂）和氧气（O₂）能迅速交换它们的氧同位素，这导致了一种测定CO₂样品δ(¹⁷)O值的方法。我们改进了该方法，在δ(¹⁷)O值的测定中达到了0.008‰（1-σ标准偏差）的精度。如此高的精度对于识别对流层CO₂中的平流层成分并将其用于全球碳通量研究至关重要。准确测定δ(¹⁷)O值的关键问题是校正因子的估计，该校正因子取决于CO₂/O₂的量比。我们还尝试研究了交换机制及其控制参数。
在热铂存在下，将CO₂样品气的氧同位素与适量的罐装O₂的氧同位素进行交换。通过同位素比率质谱法分析交换前的CO₂和O₂气体样品以及交换后的O₂样品。建立了一个混合模型，该模型涉及CO₂的δ(¹⁸)O值以及交换前后O₂的δ(¹⁷)O和δ(¹⁸)O值，以获得CO₂样品的δ(¹⁷)O值。确定对测量值的校正，以高精度和高准确度获得实际值。
要获得优于0.01‰的精度，需要将CO₂/O₂的量比控制在优于约15%的水平。我们还发现氧同位素在O₂和CO₂分子之间几乎均匀分布。此外，测定CO₂中¹⁶O¹³C¹⁸O/¹⁶O¹²C¹⁶O同位素异构体比率表明，¹⁶O¹³C¹⁸O的丰度接近同位素在CO₂同位素异构体之间随机分配时预期的丰度。
在热铂上发生的O₂和CO₂之间的同位素重排使得通过对O₂进行同位素分析能够准确测定CO₂的δ(¹⁷)O值。
