Particular Effects Related to Column Overloading

For the sake of simplicity, the following discussion is limited to a binary mixture, but the phenomena described are the same for more complex mixtures. It is also assumed that the adsorption isotherms are Lang-muirian. This is the case for most practical situations in chromatography. The total band broadening occurring in the column is the combination of two contributions: that of the intrinsic column efficiency (kinetic term - the column efficiency under an infinitely small injected quantity) and that of the finite mass of product injected (thermodynamic term - related to the amount injected). The equipment itself, besides the column, can also contribute to band broadening, but this is not considered in the present discussion.

When a sufficiently large quantity of product is injected, the thermodynamic term (which is proportional to the injected quantity, to a first approximation) becomes larger than the kinetic term and the total band broadening is primarily controlled by the quantity injected. It has been concluded from this observation that, under high mass overload, there is no reason to use efficient columns in PLC since the contribution of the intrinsic column efficiency is masked by the thermodynamic contribution. This is true as long as pure products are injected, which is obviously not the case when PLC is used.

When a mixture of two products is injected, the elution bands occupy the same section of the column during a certain fraction of their migration through the bed. When this happens, the molecules of each component compete for the same retention sites. When the injected quantity becomes sufficiently large, the molecules that have more affinity for the stationary phase prevent the other ones from interacting with the adsorbent. As a result, the weakly retained product is eluted faster than expected. This is known as a displacement effect. This effect is particularly strong when the weakly retained product is present at a smaller concentration than the strongly retained one. Under such conditions, the peak for the weakly retained component is not only eluted more quickly than if it were injected alone, but it is also narrower and has the typical shape shown in Figure 1A. Better separation is achieved than would be found if the displacement effect were not effective. When the more strongly retained compound is at lower concentration than the weakly retained one, the peak for the second compound is broader than it would be if injected alone. It also starts to be eluted earlier, an effect known as the tag-along effect (see Figure 1B).

It is clearly preferable to optimize the choice of the chromatographic conditions so that the first product is the one at the smaller concentration (typically, the impurity to be removed). Under these conditions, the region where the two products interfere is strongly

controlled by the column efficiency (see Figure 2) and increasing the column efficiency results in the possibility of injecting (much) more product or recovering more product for a given injected quantity. The possibility of using displacement effects and injecting large quantities is thus associated with columns of sufficient efficiency. Such columns enable higher production rates, reduce solvent consumption and purification times, and thus decrease purification costs.

The displacement and tag-along effects, as well as the role of the various operating parameters, have been modelled by several groups. Unfortunately, it is not yet possible (and it is questionable if it will be in a near future) to make accurate predictions of the properties of a given chromatographic system under moderate and severe overloading conditions, and thus optimize the operating conditions to minimize the purification cost, for instance. The rigorous treatment of Guiochon and co-workers, based on the simultaneous resolution of the mass balance equations for the various species involved in the chro-matographic process, involves numerical integration of a set of differential equations. This treatment is based on the model of competitive Langmuir isotherms and the approach is not simple since it requires knowledge of the composite adsorption isotherms. Major difficulties lie in experiments to measure these composite isotherms and the lack of suitable models to describe them with sufficient accuracy.

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