Harnessing electrical forces for separation : Capillary zone electrophoresis, isoelectric focusing, field-flow fractionation, split-flow thin-cell continuous-separation and other techniques

Abstract

A simple analysis, first presented twenty years ago, showed that the effectiveness of a field-driven separation like electrophoresis, as expressed by the maximum number of theoretical plates ( N), is given by the dimensionless ratio of two energies ▪ in which −Δμ ext is the electrical potential energy drop of a charged species and RT is the thermal energy ( R is the gas constant and T is the absolute temperature). Quantity −Δμ ext is the product of the force F acting on the species and the path length X of separation. The exceptional power of electrophoresis, for which often N ≈ 10 6, can be traced directly to the enormous magnitude of the electrical force F. This paper explores the fundamentals underlying several different means for utilizing these powerful forces for separation, including capillary zone electrophoresis, gel electrophoresis, isoelectric focusing, electrical field-flow fractionation and split-flow thin continuous separation cells. Remarkably, the above equation and its relatives are found to describe the approximate performance of all these diverse electrically driven systems. Factors affecting both the resolving power and separation speed of the systems are addressed; from these considerations some broad optimization criteria emerge. The capabilities of the different methods are compared using numerical examples.