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A fractionation factor is a ratio of ratios: the isotope ratio of one element or compound with that of another. The isotope ratio of two isotopes of a compound or element is the ratio of the heavier isotope with the lighter.
αA-B = RA/RB
Given a mixture or compound, its fractionation factors can be determined, but can change when it undergoes isotope fractionation. For example, evaporation of water changes the isotope ratios (since water including deuterium is less likely to evaporate), i.e., the resulting water vapor has different isotope ratios and fractionation factors than the initial liquid water.