A proposed experiment is modeled using a Monte Carlo simulation. In the simulation split gates define a nonlinear flashing potential within a semiconductor layer that contains two species of like-charged impurities with different mobilities. Flashing the external potential purifies the split gate region of the more mobile species, since impurities that have escaped are effectively unable to re-enter the split gate region. The model takes into account the Coulomb repulsion between the impurities. We demonstrate that the segregation efficiency and the purification rate are sensitive to the impurity density and strength of the confining potential, since Coulomb interaction aids the purification rate and also increases the unwanted leakage of the less mobile species.