Chemistry

Isotopes Spinning Apart?

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Science  15 Apr 2011:
Vol. 332, Issue 6027, pp. 285
DOI: 10.1126/science.332.6027.285-c

The fractionation of a mixture of stable isotopes is typically governed by mass differences. A few processes, however, exhibit a mass-independent effect, such as the impact of molecular symmetry on oxygen isotope fractionation during the formation of ozone in the atmosphere. The effects of an external gradient on isotopic fractionation are not as well understood, but recent experiments have demonstrated that under certain conditions, O2 gas shows signs of mass-independent fractionation along a thermal gradient. Building on these efforts, Sun and Bao systematically examined possible factors controlling this phenomenon in O2 and SF6 gases to determine a physical mechanism. In both cases, the experimental data are consistent with the temperature gradient manipulating the different isotopes in accord with their associated differences in nuclear spin (as distinct from mass). Because the mechanism operates for two gases with very different molecular and nuclear configurations, it could be common among low-density gases with spin differences, such as those abundant in molecular clouds or solar nebulae (e.g., CO and H2O).

Rapid Commun. Mass Spectrom. 25, 765 (2011).

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