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A fractionation factor is a ratio of ratios: the ratio heavy versus light isotopes in a compound produced by some kind of isotope fractionation. For example, when water evaporates, fractionation that leaves behind compounds mixed with the water): the evaporated water molecules are less likely to include 2H than those of the liquid water. The fractionation factor is the ratio of the before and after percentages of isotopes:
αA-B = RA/RB