A typical column used in SEC has two substances called phases that interact with the mixture and these substances help force the separation of the mixture. The mobile phase is a solvent which helps carry the mixture down the column, and the stationary phase which does not move. Unlike other chromatography methods, the stationary phase used in SEC does not exploit the polarity of each component. It is worth noting there is no chemical interaction between the stationary phase and a component. A column could be packed with two types of stationary phase material. First the stationary phase could be a porous solid. A common porous solid is silica which has a diameter of 5-10µm and a pore sizes ranging from 100-1000Å. Silica provides excellent rigidity and will not move throughout the column. Porous solids are also stable at high pressures and temperatures which allows for the use of a vast variety of solvents. Unfortunately silica particles have a tendency to allow absorb soluble components and they have the capacity to break down components into atoms. For these reasons columns could also be packed with porous gels. The structure of gel is vastly different from a porous solid because gel’s have a polymer-like (a repeating structure of the same substance) structure. Gels have been shown to be more effective than porous solids at allowing components through its pores. Gels have pore sizes ranging from 100-100000Å. So how is it that larger molecules exit the column before smaller molecules? 