Mass-independent fractionation

Light isotopes are kinetically easier to active than heavy isotopes. During evaporation of water, the lighter Isotope 16O is enriched in the vapour phase and the heavier isotope 18O is enriched in the not yet evaporated water. The same for chemical reactions, when the light isotopes diffuse faster than the heavy isotopes. These are classic mass-dependent fractionation. There are observations, like e.g. in meteorites that O-isotopes do not follow these principles. This is then called mass-independent fractionation. These can e.g. explained if grains from two different reservoirs with different O-isotopic compositions are mechanically mixed.

A kinetic system involving at least three isotopes of a given element is needed to identify a so-called mass independent isotope effect. Take for example 16O, 17O and 18O. Normally one would expect the rate of reaction of a system involving 17O to be about halfway between the rates of reactions of 16O and 18O. If that is not the case, it is termed a mass-independent effect. Isotopic substitution can affect the symmetry of the system, and it is possible that the nuclear spin of the isotopes will be different. Other than these two effects, the only effect that isotopic substitution could have on reaction rate would be one arising from mass, and since the mass independent effects are observed in some systems where these two causes can be ruled out, some have questioned the wisdom of calling these effects 'mass independent'. Nonetheless the effects are real as evidenced by 'three isotope plots.'

The anomalous distribution of 17O and 18O in ozone was discovered in the laboratory by Thiemens, and Mauersberger has documented these distributions in stratospheric ozone. Recently the Nobel Prize-winning chemical theorist Rudy Marcus has described how these distributions arise based on symmetry and changes in zero point energy.