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Retention theory
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A large number of theoretical studies concerning the retention mechanism of reversed phase HPLC have been performed during the last thirty years. Despite all the efforts that have been made to understand the basics of the retention, an unified view has not been achieved.

 

The subject has therefore been under debate in the chromatographic community for many years. That there are differences in opinions is not surprising. The reason is that there are several parameters that determine the retention. These are:

 

- the manufacturing process of the silica and the type of alkyl ligands in the stationary phase.

- the type and composition of the mobile phase.

- the properties of the solute molecule that is used in the investigation.

 

 Since all these parameters can vary widely between different experimental studies, the interpretation of the reslults can also vary from study to study. .

 

Most of the discussion is concentrated on whether the retention is due to partition or adsorption of the solute to the stationary phase. These two retention processes are the two extremes of a spectrum of possible retention models

 

In the adsorption mechanism, retention is due to transfer of the solute from the mobile phase to the interface between the stationary phase and the mobile phase. The solute is partly in contact with the mobile phase and is not fully embedded in the stationary phase.

 

In the partition mechanism, retention is due to a process that resembles the partition of the solute between two homogenous liquid phases, e.g. water-octanol. In this mechanism, the solute is transferred from the bulk liquid phase to a homogenous non-polar stationary phase. The solute is considered to be fully embedded in the stationary phase.

 

 

 

Solvophobic theory

 

The first theory that was put forward to explain the retention pattern in reversed phase chromatography was the solvophobic theory, proposed by Professor Csaba Horvath, Yale University.

 

The main point of the theory is that, for a given mobile and stationary phase, the retention factor of different solutes depends on two parameters:

- the contact area between the solute and the mobile phase, the larger contact area the higher retention factor.

- the surface tension between the mobile phase and the solute.

 

The general idea here is that the free energy can be expressed as the product of surface area and surface tension.

 

The theory therefore predicts that e.g. toluene will have a higher retention factor than benzene because it has a higher contact area with the mobile phase.

 

Similarly, the theory predicts that an increase in the methanol content will decrease the retention factor because the surface tension between toluene and a methanol/water phase decrease with increasing content of methanol.

 

On the whole, the theory gives a physically reasonable way to think about how different mobile phase and solute parameters can influence the retention.

 

With time it became clear that the properties of the stationary phase also has influence on the retention. Furthermore, it was also shown that the composition of the mobile phase also influences the properties of the stationary phase.

 

The soplvophobic theory has therefore been criticized for its focus on the role of the mobile phase in determining retention. This seems to be true in the sense that this theory does not explicitly consider variations in the stationary phase properties in the equations. However, it is in principle possible to include such properties in the theory.

 

Lattice theories

 

Several attempts have been made to overcome the weaknesses of the solvophobic theory. The most serious theoretical approaches are based on statistical mechanical concepts which model the stationary as well as the mobile phase as lattices.

 

In this approach,the solute molecule, the mobile phase molecules as well as the carbon ligands are considered as occupying points in a lattice. The distribution of the solute is determined by statistical effects and the interactions with neighboring lattice points in the two phases.

 

The first lattice type model was developed by Martire and Boehm. In their theoretical treament the solute is assumed to partition into the stationary phase. The model incorporate changes in the properties of the stationary phase under varying mobile phase compositions.

 

Other lattice types of models have been proposed by Dill. The model resembles very much the model by Martire and Boehm, the principal difference is in the treatment of the stationary phase properties. Dill takes into account the limitation of the movement of the alkyl chains that are anchored to the solid support. This leads to a higher degree of freedom for the part of the chain that is at most distance from the support.

 

 

 

 

 

 

 

 

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