Figure 1.

The vertical electric field gradient pushes each protein down the separation chamber with a velocity (black arrows) proportional to the product of the field strength and the protein’s electrophoretic mobility. A counter-flow buffer pumped into the bottom of the separation chamber pushes the proteins back up the chamber at a uniform velocity (white arrows). Proteins focus at the height in the chamber where the two velocities are equal. The mobility of the solid gray protein exceeds the mobility of the striped protein, so the gray protein focuses at a lower height in the separation chamber.