Enantioseparation of S,R‐(±)‐Ketoprofen on Plain Silica Gel Layers with Achiral Mobile Phase

Abstract

In an earlier study, we provided sufficient experimental evidence in favor of the hypothesis that—contrary to the conviction widespread among the practitioners of thin layer chromatography (TLC)—silica gel used for the coating of chromatographic plates is not amorphous, but microcrystalline and chiral. This evidence was procured both by the spectroscopic measurements of the circular dichroism of the binder‐free silica gel samples (manufactured specially for coating of the TLC plates) and by TLC measurements with densitometric detection. From the chromatographic measurements, it was found that silica gel employed in the planar chromatographic mode enables two‐dimensional enantioseparation of the racemic (or scalemic) mixtures of the selected test profens in the one‐dimensional development mode, without using chiral mobile phases.

In our present study, this striking ability of the silica gel layers is further investigated with one more racemic mixture from the group of profens [i.e., S,R‐(±)‐ketoprofen]. Good separation was obtained of the racemic ketoprofen mixture on the chromatographic plates precoated with the plain silica gel, when using the achiral binary mobile phase acetonitrile‐water (5:1, v/v) plus several drops of glacial acetic acid. In this case, three different components of the investigated mixture were found, similar to the case observed earlier, when using a silica gel layer impregnated with L‐arginine. These three components were separated two‐dimensionally in the one dimensional development mode, i.e., their positions differed in terms of the R_F values and, moreover, the respective migration tracks of these three species all deviated to the right. It seems justified to conclude that the two‐dimensional enantioseparation in the one‐dimensional planar chromatographic mode on the microcrystalline silica gel layers is a promising option, enhancing the enantioseparative potential of planar chromatography that cannot be challenged by the column liquid chromatography.

Keywords:

• S,R‐(±)‐Ketoprofen
• TLC
• Chirality
• Chiral silica gel
• Two‐dimensional enantioseparation
• Racemic mixtures

Acknowledgment

The authors wish to thank Merck KGaA (Darmstadt, Germany) for supplying the TLC plates used in our experiments.