Equilibrium fractionation is strongest at low temperatures, and (along with kinetic isotope effects) forms the basis of the most widely used isotopic paleothermometers (or climate proxies): D/H and 18O/16O records from ice cores, and 18O/16O records from calcium carbonate.
[1] Isotopic fractionations attributed to equilibrium processes have been observed in many elements, from hydrogen (D/H) to uranium (238U/235U).
In general, the light elements (especially hydrogen, boron, carbon, nitrogen, oxygen and sulfur) are most susceptible to fractionation, and their isotopes tend to be separated to a greater degree than heavier elements.
In a reaction involving the exchange of two isotopes, lX and hX, of element "X" in molecules AX and BX, each reactant molecule is identical to a product except for the distribution of isotopes (i.e., they are isotopologues).
[2] When water vapor condenses (an equilibrium fractionation), the heavier water isotopes (H218O and 2H2O) become enriched in the liquid phase while the lighter isotopes (H216O and 1H2O) tend toward the vapor phase.
(2001) Equilibrium oxygen, hydrogen and carbon isotope fractionation factors applicable to geologic systems.
(1994) Liquid-vapor fractionation of oxygen and hydrogen isotopes of water from the freezing to the critical temperature.