ISOTOPIC FRACTIONATION

enrichment of one isotope relative to another in a chemical or physical process. Two isotopes of an element are different in weight but not in gross chemical properties, which are determined by the number of electrons. It can be predicted theoretically and demonstrated experimentally, however, that subtle chemical effects do result from the difference in mass of isotopes. Isotopes of an element may have slightly different equilibrium constants for a particular chemical reaction, so that fractionation of the isotopes results from that reaction. The extent of isotopic fractionation can be expressed by a fractionation factor, alpha (a), also known as a separation factor, or enrichment factor, that is the ratio of the concentrations of the two isotopes in one compound or phase divided by the ratio in the other compound or phase. If Nl and Nh stand for the relative abundances of the light and heavy isotopes, respectively, in the one compound and if nl and nh are the corresponding abundances in the other compound, then a = (Nl/Nh)/(nl/nh). The fractionation factor is the factor by which the abundance ratio of two isotopes will change during a chemical reaction or a physical process. The precipitation of calcium carbonate from water is an example of an equilibrium fractionation process. Oxygen-18 is enriched by a factor of 2.5 percent during this precipitation; the fractionation factor depends upon the temperature and, consequently, can be used as a means of determining the temperature of the water in which the precipitation occurred. This is the basis of the so-called oxygen isotope geothermometer. During the process of photosynthesis, carbon-12, the light isotope of carbon, is enriched relative to the heavier isotope, carbon-13; the cellulose and lignin in wood from trees is enriched by a factor of about 2.5 percent during this process. The fractionation in this case is not an equilibrium process but rather a kinetic effect: the lighter isotope proceeds faster through the photosynthetic process and, consequently, is enriched. Physical processes, such as evaporation and condensation and thermal diffusion, may also result in significant fractionation. For example, the light isotope of oxygen, oxygen-16, is enriched in water evaporating from the sea. On the other hand, precipitation is enriched in the heavy isotope, resulting in a further concentration of the light isotope in atmospheric water vapour. Because of the process of evaporation and condensation between the equatorial regions and polar regions, water in the polar regions may be enriched in the light isotope by as much as 5 percent. The fissile isotope uranium-235 has been separated from the more abundant, nonfissile isotope uranium-238 by exploiting the slight difference in the rates at which the gaseous hexafluorides of the two isotopes pass through a porous barrier.