Separation of enantiomers of chiral weak acids with polysaccharide-based chiral columns and aqueous-organic mobile phases in high-performance liquid chromatography: Typical reversed-phase behavior?

When polysaccharide-based chiral columns are used in combination with aqueous-organic mobile phases for the separation of enantiomers in high-performance liquid chromatography the separation mode is commonly called "reversed-phase" in analogy to achiral separations. In several earlier and recent studies on neutral and basic chiral analytes it was shown by our and other groups that due to multiple type of interactions involved in selector-selectand binding and enantioselective recognition with polysaccharide derivatives, the above mentioned separation system may not always behave like a reversed-phase system. In the present study additional examples of non-reversed-phase behavior are described for the first time for weak acidic chiral analytes. In addition, the reversal of enantiomer elution order was observed again for the first time for several analytes based on water-content in the mobile phase.

Effect of basic and acidic additives on the separation of some basic drug enantiomers on polysaccharide-based chiral columns with acetonitrile as mobile phase.

The separation of enantiomers of 16 basic drugs was studied using polysaccharide-based chiral selectors and acetonitrile as mobile phase with emphasis on the role of basic and acidic additives on the separation and elution order of enantiomers. Out of the studied chiral selectors, amylose phenylcarbamate-based ones more often showed a chiral recognition ability compared to cellulose phenylcarbamate derivatives. An interesting effect was observed with formic acid as additive on enantiomer resolution and enantiomer elution order for some basic drugs. Thus, for instance, the enantioseparation of several ß-blockers (atenolol, sotalol, toliprolol) improved not only by the addition of a more conventional basic additive to the mobile phase, but also by the addition of an acidic additive. Moreover, an opposite elution order of enantiomers was observed depending on the nature of the additive (basic or acidic) in the mobile phase.