Tag-along effects

Figure 10.3b shows that in intermediate cases, when the concentration of the two components is similar, both the displacement and the tag-along effects appear simultaneously and the band profiles of both components are strongly influenced by the presence of the other component. 

From a process-engineering point of view, there is now a better understanding of the development of concentration profiles in chromatographic columns under overloaded conditions available. This includes in particular the quantitative description of displacement and tag-along effects caused by competitive adsorption. Since it is now possible (as mentioned before) to simulate concentration profiles on a personal computer, the choice of the appropriate mode... 

These effects stem from thermodynamic principles. If in a separation with a given purity requirement the peak resolution is not high enough due to these effects there is no way to overcome this problem by increasing column efficiency, which means optimizing the fluid-dynamic term of the resolution equation (Eq. 2.33). If a tag-along effect occurs the use of a smaller particle diameter has no effect, as long as the... 

Especially for feed mixtures with different ratios of the single components, the elution order must be considered. The major component should elute as the second peak, because in this case the displacement effect can be used to ease the separation (Chapter 2.6.2). If the minor component elutes as the second peak the tag-along effect reduces the purity and loadability of the system. 

One of the main advantages of equilibrium theory is the capability to predict some fundamental phenomena that occur in multi-component chromatography such as the displacement effect and the tag-along effect (Chapter 2.6.2). Another application is the use as short-cut methods for preliminary process design. As no effects causing band spreading are included, it is not possible to predict the system behavior exactly. 

The solution of the problem is long and complex. However, its derivation follows the same line as the one presented in the previous section, the simpler case of a wide rectangular injection pulse. The essential features of the solution are similar in both cases. They originate from the competition that takes place between the two components when they interact with the stationary phase. The major features, such as the displacement and the tag-along effects, are common to both solutions. Accordingly, it does not seem necessary to reproduce here the details of the derivations, which can be foimd elsewhere [12,14,15]. 

The interaction between the bands of different components of the feed eluted successively is due to the competition between these components for interaction with the stationary phase. Although the results of these band interactions are often important and can be spectacular rmder certain circumstances, their origin can be traced to the fact that the differences between the equilibrium constants of the two components are rather small. For example, if we have a 1 1 binary mixture of two components with a competitive Langmuir isotherm and a value of the relative retention x = fl2/ i = 1-2, which is not imusual in practice, the proportion of the second component in the stationary phase at equilibrium is only 20% larger than the proportion of the first component. Under such conditions, the molecules of the component present in large excess in the feed may crowd those of the other component out of the stationary phase, whether the major component is the more or the less retained one. Thus, the interactions that take place between these two components result in an important displacement effect of the first component, at the front of the second component band, and/or in a significant tag-along effect exhibited by the second component band. In this section, we discuss the former, the displacement effect. 

This effect results in a long plateau on the rear part of the elution profile of the second component when the colmim is strongly overloaded and the loading factor of the first component is much larger than the loading factor of the second one. Like the displacement effect, the tag-along effect is a consequence of the competition between the molecules of the two components for interaction with the stationary phase. At constant concentration, the second component is less retained in the presence of a finite concentration of the first one than when it is alone. 

It is convenient to choose the length of the plateau on the rear of the second component profile as a measure of the intensity of the tag-along effect. We shall define... 

As with the displacement effect, the intensity of the tag-along effect depends essentially on the ratio of the two loading factors. When the loading factor for... 

Thus, for a given value of the relative composition of the feed, the intensity of the tag-along effect, like the intensity of the displacement effect, depends strongly on the ratio of the two column saturation capacities. Note, however, that the solution of the ideal model for a binary mixture that is discussed in this chapter assumes that the Langmuir competitive model is valid. But the Langmuir competitive model is truly valid only if 5,1 = qsg-... 

Figure 11.24 Qualitative demonstrations of the displacement and tag-along effects. Left set Influence of the feed composition. Left column Chromatograms obtained with 200 mg of a mixture of two of the epimers of a 1,1,1-trisubstituted cyclohexanone on a 250x21.4 mm column packed with 12 im silica, with 40 mL/min of a solution of n-hexane and ethyl acetate (97.5 2.5). Composition as indicated. Right column results of computer calculations. Right set Influence of the column efficiency. Top two rows, experimental data under the same experimental conditions as in (a), except average particle size of silica particles, and mixture composition 1 3. Bottom row, results of computer calculations. Reproduced with permission from ]. Newburger and G. Guiockon,. Chromatogr., 484 (1989) 153 (Figs. 6 and 8).

Significant or even large differences between band profiles calculated with the two finite difference methods arise for aU mixture compositions at low column efficiencies. Such differences also arise at high efficiencies, when the relative concentration of the second component is low, i.e., when the tag-along effect is important [6-8]. In this last case, the only significant differences between the profiles calculated with the different methods are in the steepness of the shock layers in the mixed zone and in the retention time of the second component front [6-9,28]. The numerical problems have been discussed above, in Sections 11.1.3 and 11.1.4, and examples shown in Figures 11.5 and 11.6. 

In Figure 12.4c, the displacer concentration has been further lowered to 46.4 mg/mL, the watershed of the second component (Figure 12.4c). Therefore, the displacer front moves too slowly for the isotachic train to include the bands of the first two components, and in the case of this figme, the train begins with the third component. The first two bands are eluted as overloaded elution bands. The plateau on the top of the second band is the result of the tag-along effect between the first and second bands, as in Figure 8.6b. It has nothing to do with the displacement or the injection plateau. 

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