We propose a numerical model allowing to calculate the relative variations of isotopic ratios involved in a mass-independent isotopic fractionation (MIF) effect. This model is derived from classical trajectory simulation performed to reproduce the reactions yielding the isotopomers of ozone. In the ozone simulation, we did not introduce quantum mechanical selection rules for trajectories or the potential surface, but we separated instead exchange and non-exchange collisions, in order to introduce the fundamental quantum mechanical requirement according to which, for indistinguishable isotopes, the two possible reaction channels (elastic scattering or particle exchange) have to be superposed. The MIF effect is related to the molecular symmetry of the complex by the result that a different fraction of isotopically asymmetric complexes is stabilized than for symmetric ones. The model is applied on the results obtained experimentally for Mg and Ti isotopes in plasma. In plasma, Mg and Ti radicals resulting from the molecular dissociation of chlorides react with their parent molecules. In presence of hydrocarbons, isotope exchange rates are greatly enhanced when the intermediate activated complexes are adsorbed at the surface of the carbonaceous grains growing in the plasma. If a chemical reaction with the grain stabilizes the complex faster than its dissociation, MIF effects are observed. In such a chemical situation, the isotopic fractionation greatly exceed the usual theoretical predictions. Several characteristics of the MIF isotopic patterns are reproduced by the model.