Several new electrofocusing techniques.

The segregation and analysis of low-abundance proteins from complex biological fluids requires serial application of separation techniques that can simultaneously fractionate and concentrate solutes. In general, these techniques belong either to the family of displacement methods, e.g., ITP, or to the gradient methods, e.g., gradient-elution HPLC. IEF is a member of the subset of the gradient methods referred to as equilibrium gradient methods (EGM) and has the important property that, starting from an arbitrarily distributed initial state, evolves over time to a self-sharpening, stationary steady state. Until the introduction of counteracting chromatographic electrophoresis by O'Farrell in 1985, IEF was the only known electrokinetic technique with this property. Today, the sub-family of electrokinetic EGMs has at least half a dozen members and is slowly growing. This review describes some of the essential properties of the displacement methods, the EGMs and the non-EGMs, showing how they can be applied in microelectromechanical systems platforms, how their performance can be predicted and how new members with orthogonal properties may be added to the EGM family.