Retention, solutes on cyclodextrin silicas

Concentration of the Organic Solvent in the Eluent. As in other reversed-phase systems, the solute retention curves (log k vs. % organic modifier) on cyclodextrin-silicas are quasi-linear, at least over a limited concentration range. Such curves are shown in Figs. 2-4 for the positional isomers of nitrophenol (Fig. 2) and chloroaniline (Fig.3), and the enantiomers of Ibuprofen (Fig.4). Similar retention curves were observed and published for other positional iscmers (67), cis/trans isomers (68) and enantiomers (69). ... 

The Effects of Eluent Temperature. m agreement with previous studies (67), both solute retention and separation selectivity were found to increase significantly as the eluent temperature was reduced from 30 to 0 C, as shown in Fig. 6 for the enantiomers of tryptophan. The change is much larger than what is observed on an octadecylsilica reversed-phase column and the improved separation selectivity is attributed to more pronounced inclusion complex formation. Solute retention decreases precipitously when the temperature of the eluent is raised to or above 60 C, an important fact that can be utilized advantageously for the rapid removed, of strongly adsorbed components (such as displacers) from the cyclodextrin-silica column. 

Alak and Armstrong (107,108,112,113) investigated the influence of different silicas and binders on the separation behavior of P-cyclodextrin TLC plates. Besides nine racemates, three diastereomeric compounds and six structural isomers were separated. Wilson (109) impregnated silica plates with a 1% solution of P-CD in ethanol-dimethylsulfoxide (80 20 by volume) racemic mandelic acid was barely separated, and the antipode separation of P-blockers was not possible. Armstrong et al. (110) were the first to describe application of P-cyclodextrin as a chiral eluent additive for separations on reversed-phase TLC plates. The success of separation was strongly dependent on type and quantity of modifier applied, but above all on the concentration of P-CD. The low solubility of 3-CD in water (0.017 M, 2S C) can be improved by addition of urea sodium chloride stabilizes the binder of the RP plates. Compared to 3-CD bonded phases, a reversed retention behavior was noticed, the D-enantiomer eluting above the L-isomer. The separation of steroid epimers and other diastereomeric classes of compounds is also possible with this technique. Hydroxypropyl and hydroxyethyl P-... 

Sebille et al obtained a high performance liquid chromatographic support based on silica beads coated with a j -cyclodextrin-containing polymer, which allows the elution of solutes in order of their affinity for j -cyclodextrin. Binding constants were determined from the retention data for drugs. 

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