Separation Mechanisms

According to Small, the separation in HC is governed by three contributing effects: hydrodynamic forces,

Figure 11 Schematic representation of the HC separation principle. Larger particles are excluded from the wall and can freely migrate only in a part of the volume of the capillary column. As a result, their elution times are shorter compared with the elution times of smaller particles.

electrostatic repulsions, and Van der Walls forces. The density of the separated particles influences only their mobility and rotational moments. Soft particles can be deformed due to the high shear stress and this effect can influence their retention volumes.

A model of the separation by flow was originally proposed by DiMarzio and Guttman to explain retention in SEC. Their model approximates the structure of a packed chromatographic column to a complex system of capillaries in which the separation is caused by the same steric exclusion phenomenon as shown in Figure 11. The average velocity of the carrier liquid in a cylindrical capillary is given by:

APR2 8pL

where AP is the pressure drop along the capillary of the length L and the radius R, p is the viscosity of the carrier liquid and r is the radial coordinate. The average velocity of uniform-sized particles is given by:

AP 4pL

ya where a is the radius of the separated particles. The last term of the equation represents the rotational moment of the particles which reduces the velocity of their axial migration. The resulting retention is defined, similarly as in FFF, by the ratio of both velocities:

Whenever HC is carried out in an open capillary, the separation is clearly dominated by this mechanism. Many authors consider that particles do not move within all the sterically accessible volume but in an ave annular volume which is determined by the radial forces generated by the flow of the carrier liquid. The particles carried by the flow undergo the effect of the radial force which concentrates them within the annular volume. This force is due to the combination of the rotational and translational movements of the particles and is analogous to the Magnuson effect.

The electrostatic double layer on the surface of the separated particles influences their effective sizes. The electrostatic double layer, on the surface of the chromatographic packing or of the wall of the capillary column, reduces the accessible volume of the column due to the repulsion of separated particles of the same charge. The increased concentration of ions (ionic force) in the carrier liquid causes the screening of the surface electric charges and, consequently, reduces all electrostatic interactions. On the other hand, the reduced repulsions allow the separated particles to approach within a small distance at which the attractive Van der Walls force become effective. As a result, the hydrodynamic phenomena and electrostatic repulsions dominate the separation mechanism at a low ionic force of the carrier liquid, while at a high ionic force, the separation is dominated by hydrodynamic forces and adsorption phenomena. The order of the elution can be inverted, the particles can form aggregates, and the separation can be completely perturbed by these effects.

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