What is fractionation and why is it important for environmental science?
Fractionation refers to a process that separates the components of a mixture into fractions. Isotopic fractionation would result in a change in the isotopic composition of a substance. In our context, we focus on the isotopic fractionation of water. Water (H2O) is a mixture of 1H2 16O (molecular weight [MW] 18, the most abundant form), 1H2H 16O (MW 19), 1H 2 18O (MW20), etc. Different isotopes of the same element have different mass, and so different rates of diffusion, evaporation and chemical or biological reaction. Slower rates are associated with the heavier isotope. Suppose we start out with a mixture of (liquid) 1H216O and 1H218O and let it evaporate. The ratio of 1H218O : 1H216O in the vapor would be lower than the ratio in the original liquid because of the slower rates of the heavier isotope. Hence we say that there is isotopic fractionation associated with evaporation. Similarly for the other processes. Diffusion, evaporation and condensation rates are temperature dependent.
