System for treating molecules or particles of interest carried by a viscoelastic liquid, which comprises: - a means for establishing a laminar flow, during at least one portion, referred to as "concentration phase", of the operating time of the system, of the viscoelastic liquid in a concentration, pre-concentration by sample stacking and/or purification device (705, 1100), said device comprising a concentration area (110, 160, 210, 525, 1110) having, in the direction of said flow, an inlet cross-section surface that is larger than the cross-section surface of each outlet channel; and - a means (725, 1108) for applying an electric field between the inlet and the outlet of the concentration area during the concentration phase, the action of the electric field on the molecules or particles of interest being, in the concentration area, opposite to the direction of said flow and causing the molecules or particles of interest to be retained at least in the concentration area, the flow exerting, because of the significant shear at the scale of the molecules or particles of interest and because of the viscoelastic properties of the fluid, a counter-reaction on the molecules and particles of interest, perpendicular to the flow, which pushes the molecules and particles of interest towards the walls of the concentration area, such that the molecules and particles of interest are concentrated in the concentration area along lines of equal shear, the system also comprising a modulation means (730, 1108) configured to control the means for applying the electric field in order to apply, after the concentration phase, an electric field with intensity other than zero and lower than the intensity of the electric field applied during the concentration phase, the modulation means being configured to control the electric field application means to apply an electric field gradually decreasing over time after the concentration phase, for separating the molecules concentrated beforehand. - a means for establishing a laminar flow, during at least one portion, referred to as "concentration phase", of the operating time of the system, of the viscoelastic liquid in a concentration, pre-concentration by sample stacking and/or purification device (705, 1100), said device comprising a concentration area (110, 160, 210, 525, 1110) having, in the direction of said flow, an inlet cross-section surface that is larger than the cross-section surface of each outlet channel; and - a means (725, 1108) for applying an electric field between the inlet and the outlet of the concentration area during the concentration phase, the action of the electric field on the molecules or particles of interest being, in the concentration area, opposite to the direction of said flow and causing the molecules or particles of interest to be retained at least in the concentration area, the flow exerting, because of the significant shear at the scale of the molecules or particles of interest and because of the viscoelastic properties of the fluid, a counter-reaction on the molecules and particles of interest, perpendicular to the flow, which pushes the molecules and particles of interest towards the walls of the concentration area, such that the molecules and particles of interest are concentrated in the concentration area along lines of equal shear,