By Monte Carlo simulation, we study the preferential sputtering of isotopic systems at low fluences. The code is tested against several results known from analytical theory. A recent experiment on the sputtering of a Mo sample by a 5 keV Ar beam is simulated. The resulting enrichment of the lighter isotope in the simulated sputtered flux can be understood quantitatively by taking into account a realistic interaction cross section and the influence of the projectile species. We show the dependence of the enrichment on emission angle and energy. Particles sputtered at high energies are excessively enriched in the light species. The simulation result is in better agreement with the experimental data than the previous analytical prediction. The origin of the remaining discrepancy is discussed.