Displacement chromatographic separations

Displacement Chromatographic Separations on / -Cyclodextrin—Silica Columns... 

The development of a displacement chromatographic separation generally involves the following main steps ... 

In the initial experiments reported here we did not attempt to optimize the separation in terms of yield and production rate. Rather, cur intent was to demonstrate that displacement chromatographic separations are feasible on a chiral stationary phase, cyclodextrin-silica, and gather preliminary information regarding the structure of displacers which cam De used with cyclodextrin-sil icas. The method development sequence described in the previous paragraph will be followed in the discussion of the results. 

Thus, in conclusion, it can be stated that retention studies such as the ones outlined above permit the selection of operation conditions (type and concentration of the organic modifier, concentration and pH of the buffer, temperature of the eluent) which lead to sufficient initial solute retention (k >10) and maximum separation selectivity necessary for a successful displacement chromatographic separation. 

Since the chloroanilines are sufficiently retained (k >5) in a 10 % v/v methanol water eluent, and the Ibuprofen enantiomers are sufficiently retained in a 30 % v/v acetonitrile buffer eluent, these solvents were selected as carrier solvents for the displacement chromatographic separations. Also, these solvents were used to determine the adsorption isotherms of p-nitrophenol and 4-t-butylcyclohexanol on beta-cyclodextrin silica. The isotherms were determined from frontal chromatographic measurements as described in (56). The isotherms are shown in Figs. 7 and 8. Since both isotherms are downwardly convex, p-nitrophenol and 4-t-butylcyclohexanol might prove useful displacers for our test solutes, provided that they are more strongly adsorbed that the solutes. 

Many trial-and-error experiments can be avoided during the development of a displacement chromatographic separation, when the isotherm of at least the most strongly adsorbed sample component is known. Therefore, as the next step, the adsorption isotherms of the most retained iscmers of chloroaniline and Ibuprofen, the examples discussed above, were determined as shown in Figs. 8 and 9. It can be seen by cxnparing the isotherms of the solute and prospective displacer pairs that indeed p-nitrophenol can be used as a displacer for the separation of the chloroaniline iscmers. The situation is more complicated with Ibuprofen and 4-t-butylcyclchexanol because their isotherms cross each other at 1.5 irM. This indicates that successful separations can be expected only below this concentration level. Other examples of crossing isotherms were also reported (69). 

Once the composition of the carrier solution and the identity of the premising displacer are determined, the actual displacement chromatographic separation can be attempted. As an example for a... 

Similar displacement chromatographic separations have been obtained for other samples as well including naphthol iscmers (67), nitroaniline iscmers (67), nitrophenol iscmers (67), the cis- and trans-iscmers of 3-hexen-l-ol (68), and the enantiomers of mephobarbital (69), hexobarbital (69), dansyl leucine (69) and dansyl valine (69). Sample loadings on the 4.6 mn ID. analytical columns varied between 0.1 milligram and 60 milligram the concentration of the separated solutes in the collected fractions ranged from 0.1 nM to 10 nM. 

Quintero, G., Vo, M., Farkas, G., and Vigh, G. (1995). Series of homologous displacers for preparative chiral displacement chromatographic separations on Cyclobond II columns. J. Chromatogr. 693, 1-5. 

Streptomycin A and neomycin B have found novel use as effective displacer compounds in the ion-exchange displacement chromatographic separation of proteins. A paper on the H-nmr assignments of the kanamycins and butirosin A is mentioned in Chapter 21. 

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