Disclosed are an apparatus, system, and method for two dimensional electrophoresis of analytes of interest, particularly polypeptides. The apparatus includes a sample separation cavity comprising (1) an electrophoresis region located along an upper portion of the cavity for performing charge and/or size-based electrophoresis in a first dimension along the upper portion, and (2) below the first said electrophoresis region, a second electrophoresis region for performing electrophoresis in a second dimension in a direction substantially perpendicular to the first dimension. In one preferred embodiment, the second electrophoresis region contains an isoelectric focusing region containing a continuous pKa gradient immobilized on at least one of the major opposing surfaces of the cavity.