Separation selectivity, displacement chromatography

As discussed earlier in this chapter, one of the characteristic features of the competitive Langmuir isotherm is that the selectivity between any pair of components is constant, independent of their concentrations. As a consequence, the requirements for their successful separation by displacement chromatography would merely be that the selected displacer is retained more strongly than either feed... 

Note that actual chromatograms will look like those shown in Figures 12.1 to 12.4 only if the concentrations of each of the feed components are plotted versus the volume of eluent passed. This requires either the use of a selective detector e.g., LC/MS), or of fraction collection and analysis. The chromatogram in Figure 12.17 illustrates the difficulties encountered in developing a separation by displacement chromatography. Unless the isotachic train is fully formed and the response factors of all feed components are similar, the recording of the response of a nonselective detector is very difficult to accoxmt for. 

While much has been learned about the role and selection of the operation parameters in displacement chrcmatography (60-66), little is known yet about the rules of displacer selection and the means available to control the selectivity of the separation. The paucity of well characterized displacers and the lack of knowledge of the solute adsorption isotherms hinder most strongly the wider acceptance and use of displacement chromatography. In most cases, displacer selection is still done by trial-and-error. In the majority of modem displacement chromatographic publications a reversed-phase system was used to separate small polar molecules, antibiotics, oligopeptides and small proteins (52-66). 

The development of displacement separations has historically been an empirical process and even though chromatographic theory may guide the selection of operating conditions the final stage must involve experimental validation. Typically, several experiments will be carried out at or near the conditions determined by the theory. The final stage in the procedure is either experimental or numerical optimization of the displacement process to produce optimal yields, purities and productivities. At this point, the relative efficacy of selective and conventional displacement chromatography can also be evaluated. 

Thin-layer chromatography has been used as a selective, sensitive, reliable and simple separation method, and it has been proven a very useful method for optimisation of displacement chromatography 122), and in the determination of lipophilicity 123-25). 

Active ester formation by the mixed anhydride method is accompanied by the side reaction of esterification at the carbonate moiety of mixed anhydride 51 which generates mixed carbonate 52 (Scheme 12).This decreases the yields, but is more of a nuisance than an obstacle as the side products do not interfere with crystallization of the esters as the former are soluble in the crystallizing solvent. More mixed carbonate is formed from derivatives of the hindered amino acids and proline none is formed from a-unsubstituted acids. A-Hy-droxysuccinimide gives rise to much less byproduct than 4-nitrophenol other phenols generate intermediate amounts. Less byproduct is generated when the reagent is isopropyl chloroformate. The impurity can be readily removed from a solution of the ester by adsorption of the compounds on reverse-phase chromatography beads followed by separation by selective displacement. ... 

Experimental work of Kalasz et al. resulted in the statement of the characteristics and basic rules of displacement chromatography. They conceived properties of the fully developed displacement train, factors affecting displacement development, efficacy of separation, analysis of displaced fractions, determination of displacement diagrams from Langmuirian isotherms, as well as selection of the column, carrier, and displacer for displacement chromatography. Concentration of the sample is a particular feature of displacement chromatography. However, the displacer in the carrier is also definitely concentrated through the development of the displacement train. 

Antia and Horvath [32] have provided an analysis of the isotachic patterns in displacement chromatography that take into accormt the effects of a selectivity reversal. They used for this purpose the multicomponent isotherm generated by the IAS theory. In order to examine the possibility of performing separations even when the single-component isotherms of two components intersect and cross over, they analyzed the stability of the displacement train and the criteria for displacement and for the stability of the resulting botmdaries that are established in an isotachic train. 

Displacement chromatography. A sample mixture is applied to one end of a stationary phase, and then a mobile phase is introduced. The mobile phase selectively binds to the stationary phase competing for the same binding sites as the sample. Separation is established as a relative function of the intermolecular interactions of the sample and mobile-phase species with the stationary phase. 

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