Topics - Exclusion
Exclusion chromatography is a separation technique that resolves mixtures of substances on the basis of molecular size. The stationary phase is chosen to have pores with a narrow range of pore diameters. The smaller molecules enter most of the pore and, thus, are exposed to the majority of the stationary phase (or static mobile phase) and are, thus, retained. Large molecules, are excluded from most of the stationary phase and static mobile phase (hence the term exclusion chromatography) and are eluted rapidly. Thus, solutes are eluted in order of the reduced molecular size, the largest are eluted first the smallest last. It should be pointed out, that all silica based stationary phases are porous and, thus, even when a solute is retained largely by interactive forces, its retention will also be affected by the exclusion properties of the solid phase. Consequently, retention will be the result of a combination of molecular interaction and exclusion. Retention by exclusion results from the standard entropy contribution to the standard energy and, if exclusion is the major factor controlling retention, the separation is said to be entropically driven.