A schematic diagram showing the kinetic isotope effect. The isotopic compositions of unconsumed reactant (blue, solid line), instantaneous product (black, broken line), and cumulative product (red, solid line) are shown as a function of the fraction of unutilized substrate (f) for a unidirectional reaction. Mathematical formulae for these reactants and products are also shown. Subscripts r, ip, and p represent reactant, instantaneous product, and accumulated product, respectively. Modified from Mariotti et al. (1981).