Use of Video Densitometry and Scanning Densitometry to Study an Impact of Silica Gel and L‐Arginine on the Retention of Ibuprofen and Naproxen in TLC Systems

Abstract

In our earlier articles, we demonstrated the phenomenon of the two‐dimensional enantioseparation of the selected 2‐arylpropionic acids (2‐APAs) in the one‐dimensional thin‐layer chromatographic mode, yet our measuring results originated from the classical scanning densitometry only. A basic precondition of the discussed phenomenon is the omnipresence of the two‐dimensional effective diffusion in planar chromatography. However, one should anticipate an action of the two different–and apparently chiral–elements, one responsible for the vertical enantioseparation (in the direction of the mobile phase migration) and another one responsible for the horizontal enantioseparation (inducing deviation of the chiral analytes' migration tracks from the vertical). In the investigated chromatographic systems two chiral elements are in fact present. One chiral element is the microcrystalline silica gel layer and the other one is L‐arginine as the impregnating chiral selector. Upon our earlier results originating from the scanning densitometry, it could be deduced that the crystalline chirality of silica gel is responsible for the horizontal enantioseparation, and the molecular chirality of L‐arginine is responsible for the vertical separation. In this study, we confirm our earlier findings in a more immediate way, using the flatbed video densitometry to record the pictures of the whole chromatograms and also those of the individual chromatographic spots of ibuprofen and naproxen. We combine these results with the data obtained from the classical scanning densitometry to monitor the discussed enantioseparations by means of the concentration profiles of the respective antimers. Combination of the results originating from the video and the scanning densitometry allows a deep enough insight in the chromatographic behavior of the two selected 2‐APAs on the plain silica gel layers and on those impregnated with L‐arginine.

Keywords:

• Ibuprofen
• Naproxen
• TLC
• Chirality of silica gel
• 2‐Arylpropionic acids (2‐APAs)
• Enantioseparations
• Video densitometry
• Scanning densitometry

Acknowledgment

The authors wish to thank Merck KGaA (Darmstadt, Germany) for supplying the TLC plates used in our experiments. The authors wish to thank the AR2i Company (Le Plessis Robinson, France) for supplying the Chromimage video densitometer and the Galaxie‐TLC software used in this study.