Liquid - Solid Chromatography:

The liquid - solid chromatographic technique is based on adsorption phenomenon. Consider a liquid solution of two compounds which has been brought into contact along with a porous adsorbent. A molecule from the solution enters the pores of adsorbent and become attached to its surface. Those molecules are held rather loosely through van der Waals' forces. A few pass back within the body of the solution, and their places in the pores of the adsorbent are taken through other molecules. Therefore, there is a continual interchange among the molecules within the body of the solution and those in the pores of the adsorbent. Typically, one elements tends to be more firmly held on the surface than the other, so that, when equilibrium is established, the concentration of this component in the pores will be higher than its concentration in the surrounding liquid. Supposing that the phases can be perfectly separated, the separation factor is defined by the equation

α = ( X / Y )_a / ( X/ Y)_l

i.e., α, the separation factor, is equal to the ratio of the mole fractions of the two components, X over Y, in the adsorbed phase, a, divided through their ratio in the liquid phase, l.