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On the mechanisms of enantiomer separations by chiral thin-layer chromatography on silica gel, and implications when densitometric detection is used. A mini review

Sajewicz M., Kowalska T.

Acta Chromatographica

2010

Vol. 22, no. 4

499--513

Enantiomer separations have been one of the most important and, simultaneously, one of the most difficult to accomplish analytical (and technological) tasks, present at the top of separation scientists’ agenda since the early sixties of the last century. Awareness of their importance has been awakened by an infamous case of the racemic drug thalidomide, a widely advertised sedative drug which had unexpected teratogenic activity in pregnant women that resulted in thousands of ‘flipper babies’ born in the late fifties and the early sixties in many countries around the world. Since that time, separation scientists have developed numerous methods for enantiomer separation, basically by use of gas chromatography (GC), high-performance liquid chromatography (HPLC), and capillary electrophoresis (CE). In this respect, planar chromatography has remained to a large extent an undervalued enantiomer separation technique, despite separation performance sufficient to separate a pair of enantiomers. The large number of GC, HPLC, and CE enantiomer separation strategies and methods developed are evidence that — once confronted with this particular and no doubt very important challenge — instrumental chromatographic techniques have lost if not face, then, to a large extent, their reputation as robust, universal, and efficient separation tools. In these circumstances, planar chromatography on silica gel seems a very promising and tempting alternative, basically because of the advantageous properties of microcrystalline silica gel and the 2D effective diffusion available only in planar chromatographic mode. Enhancement of the enantiomer separating power of the silica gel by simple mechanical impregnation with a properly chosen chiral selector, and additional coupling of this with efficient instrumental detection (e.g. densitometric, DAD, or mass spectrometric) can yield in a simple, robust, and universal tool for separation of enantiomers comparable with the long-established chromatographic enantiomer-separation techniques. In this mini review, favourable preconditions for silica-gel-based planar chromatographic separation of enantiomers which can elevate planar chromatography to the status of leading tool for separation of enantiomers are discussed. Further improvements which can enhance the enantiomer separation performance of chiral planar chromatography are also indicated.

TLC study of the separation of the enantiomers of lactic acid

Sajewicz M., John E., Kronenbach D., Gontarska M., Kowalska T.

Acta Chromatographica

2008

Vol. 20, no. 3

367-382

Enantiomer separation by TLC is still much less frequent than with other, mostly instrumental, chromatographic techniques. From a literature survey it is apparent that separation of the enantiomers of D,L -lactic acid is primarily of interest to the diary industry and that this particular separation is less frequently performed by chromatographic than by membrane techniques. As far as we are aware, before our studies only one report of TLC separation of the enantiomers of D,L -lactic acid was available in the literature; this is dated 1991 and describes the use of non-instrumental TLC only. In this study, we started by reproducing the TLC procedure originating from 1991, for this purpose using TLC with automatic sample application and densitometric detection. We managed to repeat the earlier procedure and to achieve full, i.e. baseline, separation of the enantiomers, with a remarkable distance between the two antimers. However, we revealed a significant drawback of this separation procedure - D -(-)-lactic acid was transported almost with the mobile-phase front and its densitometric quantification was barely possible because of the relatively high UV absorption of the mobile-phase front line. The reference method for separation of the enantiomers of D,L -lactic acid consisted in preliminary impregnation of commercial silica gel TLC plates with copper(II) acetate. In-situ formation of bidentate complexes of the D,L -lactic acid antimers with the Cu 2+ cation resulted in different mobilities of these complex cations in the planar chromatographic system. The objectives of this study were twofold - to investigate separation of the enantiomers of D,L -lactic acid with other transition metal cations (i.e., Co 2+, Ni 2+, and Mn 2+) used to impregnate the silica gel (to achieve resolution that might enable quantification of the two lactic acid antimers and not only the L -(+) enantiomer) and to gain deeper insight into the mechanism of separation with these metal cations. For purposes of comparison, we chromatographed D,L -lactic acid on non-impregnated silica gel layers. As a result, we managed to establish efficient separation conditions with the Ni 2+ and Co 2+ cations that outperformed the earlier established procedure involving the Cu 2+ cation, and - partially at least - to elucidate the mechanism of separation of the enantiomers of D,L -lactic acid by these TLC systems. The Mn 2+ cation proved unsuitable for the purpose. Finally, we managed to separate the enantiomers of D, L -lactic acid on non-impregnated silica gel layer also, which seems yet more proof of the microcrystalline chirality of silica gel used as stationary phase and of its substantial contribution to the enantiomer separation investigated.