Authors:Heinz Hauck, Olga Bund, Walter Fischer, and Michael Schulz
Miniaturization is general trend in modern analytical methods, with the main targets of increasing sensitivity, shortening analysis times drastically, and reducing the quantity of consumables per analysis. In planar chromatography a first step in this direction was the development of modern high-performance TLC (HPTLC). Compared with conventional TLC, the layers are characterized by smaller adsorbent material particle sizes and a slight reduction in layer thickness. A major final step in the direction of miniaturized planar chromatography are the new ultra-thin layers. In contrast with those on TLC and HPTLC plates, these layers are no longer based on granular adsorbents, but have a monolithic structure based on a silica gel matrix. This means there are no longer separate particles, as in TLC and HPTLC. Furthermore, no binder is needed to fix the layer on the glass plate. To be suitable for chromatographic purposes, the monolithic silica gel of these new ultra-thin layers has meso- and macro pores, with fine capillaries penetrating the layer. The layer thickness of approximately 10 μm is considerably less than that of conventional TLC and HPTLC layers. The exciting properties of these new UTLC silica gel plates, especially their selectivity and separation efficiency, are demonstrated by separations of steroids, pesticides, and some dyestuffs.
We have previously described unexpected two-dimensionality in the thin-layer chromatographic separation of pairs of enantiomers of chiral 2-arylpropionic acids (2-APA) by one-dimensional development on a chiral stationary phase prepared by impregnating silica gel layers with l
-arginine. The two-dimensionality of this separation was because in planar chromatography all analytes undergo two-dimensional effective diffusion and thus — theoretically at least — can migrate in two mutually perpendicular directions, i.e. in the direction of the mobile phase flow and in a direction perpendicular to this. Demonstration (and quantification) of this striking behavior was possible by use of densitometric detection. Migration of analytes in a direction perpendicular to that of the mobile-phase certainly is not commonplace in thin-layer chromatography and, as far as we are aware, the example described in our previous paper was the first of its kind reported. It was apparently induced by the chirality of the analytes and the stationary phase.In the work discussed in this paper we demonstrated another example of deviation from the vertical of the migration tracks of the same analytes, optically pure
-(+)-naproxen, when chromatographed on unmodified commercial precoated silica gel plates. Keeping in mind that (i) such deviations can occur only with a chiral analyte developed in an asymmetric chromatographic system, and that (ii) silicon dioxide (chemically identical with chromatographic silica gel) can crystallize as quartz (i.e. as rock crystal) in two asymmetric — left-handed and right-handed — forms, we assumed the working hypothesis that chromatographic silica gel layers can also seem to have an asymmetric microcrystalline structure responsible for deviation from the vertical of the migration tracks of chiral analytes. To verify our hypothesis we acquired circular dichroism (CD) spectra and UV absorption spectra of samples of the silica gel used for coating commercial TLC plates. The results obtained from these measurements provide convincing evidence of the correctness of our hypothesis and it seems most likely that the commercial silica gel layers are characterized by the crystalline asymmetry of the silicon dioxide particles.