Electric field gradient focusing.

Electric field gradient focusing (EFGF) is a relatively new separation technique with promising attributes, particularly for protein analysis. The fundamental fractionation mechanism in EFGF involves a gradient in electric field along the length of a separation column. The electrophoretic force that drives charged analytes in a region of changing electric field is opposed by a constant, pressure-driven bulk fluid flow. When the electrophoretic velocity of a particular moiety is equal and opposite to the velocity of the fluid flow, the analyte focuses into a stationary band. Thus, EFGF can both concentrate and separate charged species according to electrophoretic mobility. To date, the electric field gradients needed for EFGF have been established using a number of different approaches, including channels having changing cross-sectional areas, conductivity gradients caused by the diffusion of buffer ions across a membrane, electrode arrays, and temperature gradients in buffers whose conductivities change as a function of temperature. EFGF has proven particularly effective for sample enrichment, with concentration factors of 10,000 reported. In this article we review advances in EFGF technology and discuss prospects for further improving EFGF for chemical analysis.