Size separation, steric exclusion mechanism

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 ... 

The two techniques differ in that HDC employs a nonporous stationary phase. Separation is affected as a result of particles of different size sampling different velocities in the interstitial spaces. Size exclusion chromatography is accomplished by superimposing a steric selection mechanism which results from the use of a porous bed. The pore sizes may vary over a wide range and the separation occurs as a result of essentially the same processes present in the gel permeation chromatography of macromolecules. 

Theory. The most widely accepted mechanism of size separation is based on steric exclusion (1). In terms of thermodynamic properties, the distribution coefficient consists of enthalpic and entropic contributions ... 

Di- and tricarboxylic acids such as oxalic and citric acid elute between the excluded and the total permeated volume. Apart from Donnan exclusion, the predominating separation mechanism is, in this case, mainly steric exclusion. The retention is determined by the size of the sample molecule. Since the pore volume of the resin is established by its degree of crosslinking, the resolution can only be improved by applying another or by coupling with another separator column, respectively. 

Because in gel chromatography steric exclusion is the dominating mechanism, the most common application of this type is the purification of a mixture of substances differing from one another by particle size. We think of purification in a broad sense depending on the degree which can be reached by the separation process itself or which is required. Various modes of GPC application will be discussed from this point of view. 

The size ratio of the fractionated macromolecules or particles to the channel thickness must be taken into account. A decrease in channel thickness can lead to an important contribution of steric-exclusion to the mechanism of separation whenever the distance of the center of gravity of the concentration distribution of the retained species from the accumulation wall, I, becomes commensurable with the size of the retained species. Whereas the elution order in normal (polarization mode) TFFF is from the small to the large species, it is inverted in purely steric-exclusion mode. Consequently, the fractionation deteriorates in the vicinity of the inversion point The dependence of the retention ratio R on the particle radius r can be... 

Unlike the pellicular packings used for ion exchange, the packings used in ion exclusion are derived from totally sulphonated polymeric materials. Separation is dependent upon three different mechanisms Dorman exclusion, steric exclusion and adsorption/partitioning. Dorman exclusion causes strong acids to elute in the void volumes of the column. Weak acids which are partially ionised in the eluent are not subject to Dorman exclusion and can penetrate into the pores of the packing. Separation is accomplished by differences in acid strength, size and... 

More precisely, the basic size exclusion separation mechanism of concentrated mixtures obviously incorporates the concentration gradient-driven exclusion of smaller species from the mobile phase into stagnant zones in micropores combined with a steric and osmotic exclusion of larger species into the larger pores and moving parts of the mobile phase in the... 

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