Mechanism of ideal gel chromatography

The term ideal gel chromatography denotes the process in which, exclusively, the differences in the dimensions of separated molecules are operative. 

Let us consider the bed of randomly packed porous gel particles within the column that is percolated by the mobile phase (eluent) carrying the sample components. The sizes of gel pores and separated molecules have mutually matched distribution so that the largest sample molecules are totally excluded from all pores of the gel. These molecules will not be retained in the column and are eluted by the volume of the eluent equal to the interstitial volume of the column, 1, . This means that the retention volume, of excluded molecules equals V . On the other hand, the smallest sample molecules with the sizes that are comparable with the size of eluent molecules permeate all pores of gel and diffuse back into the mobile phase. The consequence of this repeated process is the retention of the small molecules in the column and, for their elution, such volume of eluent is necessary that equals the interstitial volume, kj, plus pore volume, V,. The resulting retention volume is VR = yn.= K+yc 

The molecules of intermediate dimensions are partly retained in the gel bed by their permeation into pores large enough to accomodate them. Such molecules are eluted from the column in retention volume 

(1) is a formal analogy to equation (17) in Chapter 1 and shows important inherent restriction of gel chromatography the space in which separation takes place is confined to the volume of pores and, consequently, the peak capacity of the method is fairly limited. 

The above simple model of a steric exclusion mechanism was considered by several authors attempting to describe quantitatively the gel chromatographic separation process. Distribution coefficients were expressed on the basis of the model considerations of the dimensions of both the separated molecules and the pores of gel, as well as of the stochastic model approaches (for reviews see e.g.. Refs. 1, 3-6), and also of the thermodynamic reasoning on the changes of conformational entropy of macromolecules due to their transfer from the interstitial volume into the pores in the course of separation [7]. However, besides the steric exclusion from the pores, at least two other size-based mechanisms are operative in the ideal gel chromatography  

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