The optimization of working parameters controlling the transfer of an analyte from the separation into the spray capillary at the liquid junction interface is a complex problem The numerical models of hydrodynamic flow, electric field strength and the consequential mass transfer provide a valuable insight into, the function of the miniaturized device The results revealed that the most important parameter is the electric field strength inside the gap between the separation and spray capillaries. In a strong electric field, the analyte leaving the separation capillary is immediately transferred into the spray capillary at its maximum concentration Although the losses of analyte outside the interface are dominant in this case, the sprayed concentration determines the detection sensitivity Since only a small amount of the sprayed material enters mass spectrometer, the losses at the interface do not influence the sensitivity. At low electric field strength the total amount of analytes is transferred into the spray capillary In this case, however, the analytes enter the capillary slowly and, as a result, are significantly diluted. The electric field strength, pressure and dimensions deter-mine the mass transfer in the interface and must be considered when the optimum conditions of an analysis are chosen. Several fold improvement in sensitivity and efficiency of the method can be expected when working under the optimum conditions