Chiral Separations of Ibuprofen and Propranolol by TLC. A Study of the Mechanism and Thermodynamics of Retention

Abstract

In contrast with gas and column liquid chromatography, both of which enable very efficient separation of enantiomers, thin‐layer chromatography (TLC) has never proved particularly successful in the same field. This can be regarded as puzzling because although the performance of TLC is substantially lower than that of the instrumental modes of chromatography, it still seems efficient enough to ensure even difficult separations of pairs of analytes. There is a steady demand for simple, inexpensive, and successful chiral separations, preferably executed with the aid of TLC. The best proof of this is a few documented and promising attempts reported by reliable laboratories in developing countries. Some of these reports, however, describe experiments performed with glass plates coated in the laboratory, which gives rise to questions regarding the accuracy and repeatability of the results obtained. Similar concerns are evoked by traditional visualization of the outcome of a separation by use of dyeing agents, rather than by densitometry (which furnishes concentration profiles of the bands and the possibility of in‐situ identification also). In this study, we have repeated chiral separations of two widely used drugs, ibuprofen and propranolol, adapting working conditions reported elsewhere to a system based on standardized and commercially available chromatographic plates. We also performed detection and identification on the developed chromatograms by means of densitometry. In our experiments with the modified chromatographic procedures, the results obtained proved at least as good as those reported in the original papers and occasionally somewhat better. For both ibuprofen and propranolol, preliminary thermodynamic evaluation of the standard chemical potentials of adsorption (Δμ_a) for each of the two enantiomers considered was also performed. The results obtained look promising and realistic; it seems that adsorption TLC can be used in the future for thermodynamic assessment of the racemization process.

Keywords:

• TLC
• Silica gel sorbent
• L‐Arginine impregnation
• Enantiomer separations
• S‐(+)‐ and R‐(−)‐ibuprofen
• S‐(−)‐ and R‐(+)‐propranolol

Acknowledgment

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