Non-mass-dependent 17O anomalies generated by a superimposed thermal gradient on a rarefied O2 gas in a closed system

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Cryogenic or heating methods have been widely used in experiments involving gas purification or isolation and in studying phase changes among solids, liquids, or gases for more than a century. Thermal gradients are often present in these routine processes. While stable isotopes of an element are known to fractionate under a thermal gradient, the largely diffusion-driven fractionation is assumed to be entirely mass-dependent. We report here, however, that distinct non-mass-dependent oxygen isotope fractionation can be generated when subjecting rarefied O2 gas in a closed system to a simple thermal gradient. The δ17O value, a measure of the 17O anomaly, can be up to -0.51‰ (standard deviation (s.d.) 1σ=0.03) in one of the temperature compartments. The magnitude of the 17O anomalies decreased with increasing initial gas pressures. The authenticity of this phenomenon is substantiated by a series of blank tests and isotope mass-balance calculations. The observed anomalies are not the result of H 2O contamination in samples or in isotope ratio mass spectrometry. Our finding calls attention to the importance of thermal gradient-induced isotope fractionation and to its implications in laboratory procedures, stable isotope geochemistry, and the physical chemistry of rarefied gases. Copyright © 2010 John Wiley & Sons, Ltd.

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Rapid Communications in Mass Spectrometry

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