Mobile Phase Design

A lot of different solvents, e.g. acetone, chlorinated alkanes and acetonitrile, cannot be used because they dissolve CTA more or less completely. In tetra-hydrofyran, CTA forms a gel. In some other solvents, like for example, 1,4 dioxane and dimethoxyethane, CTA swells too strongly. Different investigators found as a general rule that the retention factors of the investigated analytes increased when, respectively, methanol, ethanol or propanol was used as the eluent. However, the highest selectivity factors are often found with ethanol as the eluent. Successful separations are also obtained by using other low molecular weight alcohols, ethers, hydrocarbons and mixtures of these eluents. Examples are methanol-2-propanol (80:20; v/v); ethanol-tert-butyl methyl-ether-water (86:10:4, v/v) and «-hexane-2-propanol-water (70:27:3, v/v).

In general, a significant influence on the retention factor and enantioselectivity is observed when ethanol as the eluent is modified with methanol, 2-

propanol, different amounts of water, or is completely replaced by methanol or 2-propanol.

If compounds bearing an ionizable group have to be analysed, buffer systems should be tried. CTA seems to withstand the use of buffers in the range between pH 5 and 10 for a long time without any significant loss of chromatographic properties. Elu-ents with a high water content at high or low pH values should be avoided because CTA can be hydro-lysed under these conditions.

Empirically, it has been found that, on CTA successful enantiomer separations can be expected if the analytes possess an aromatic or nonaromatic ring close to the chiral centre or if the products have an asymmetric atom on a rigid ring structure.

Also it is known from experience that compounds bearing an ionizable group, like for example, a hy-droxyl, carboxyl or amino group, are generally poorly resolved on CTA.

Derivatization of these functional groups into the corresponding ester, amide or carbamate derivative often improves the selectivity. For alcohol, an esterifi-cation reaction with a P-substituted benzoic acid chloride (F, Cl, Br, CN, NO2, OCH3) has been successfully applied to solve a number of different separation problems.

CTA is attractive for chiral separations because it can be synthesized from a low cost natural product. The wide range of products that can be separated, together with its high loading capacity, certainly makes it a material that should be considered as a valuable tool in the development of chiral separation methods. Besides the usefulness of CTA in the field of enantiomer separations, this material is also suitable for the separation of positional isomers.

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