Thin-layer chromatography with liquid-phase coating

Tswett s initial column liquid chromatography method was developed, tested, and applied in two parallel modes, liquid-solid adsorption and liquid-liquid partition. Adsorption ehromatography, based on a purely physical principle of adsorption, eonsiderably outperformed its partition counterpart with mechanically coated stationary phases to become the most important liquid chromatographic method. This remains true today in thin-layer chromatography (TLC), for which silica gel is by far the major stationary phase. In column chromatography, however, reversed-phase liquid ehromatography using chemically bonded stationary phases is the most popular method. 

Precoated plates are also available for reversed-phase liquid-liquid partition thin-layer chromatography. Here the silica gel has been treated with an octadecyl silylating reagent thus coating the particles with a non-polar chemically-bonded thin film. The solvent employed is more polar than the film and chromatographic development results from partition between these two phases. 

Now that reversed phase column packing materials have been used successfully in column liquid chromatography for a number of years, success has also been achieved in thin-layer chromatography in that it is now possible to prepare HPTLC pre-coated plates with the same types of surface-modified sorbents and to use them for separations (2 8, 1, 20). 

Stationary phases can be solid surfaces or surfaces coated with liquids, while mobile phases can be liquids or gases. Standard forms that stationary phases take are often very fine granular powders spread evenly on planar glass or polymer supports (e.g., thin-layer chromatography) or packed in columns. Also common are porous polymeric solids usually containing a microcoating of water (liquid stationary phase) or capillary tubes whose inside walls are coated with a liquid stationary phase. 

Typical thin-layer separations are performed on a glass plate that is coated with a thin and adherent layer of finely divided particles this layer constitutes the stationary phase. The particles are similar to those described in the discussion of adsorption, normal- and reversed-phase partition, ion-exchange, and size-exclusion column chromatography. Mobile phases are also similar to those employed in high-performance liquid chromatography. 

In gas chromatography, a column is coated with a thin layer of a hquid stationary phase and a sample is separated by the relative abilities of its components to dissolve in it from the gas phase. The greater the interactions with the stationary phase, the slower the transit of the sample through the column. Combination of Abraham s solvation parameter model (Eq. (2.1)) [104] with GC measurements has been used to determine the nature of ionic liquid-solute interactions [105-107]. Given the experimental setup, this technique is likely to be particularly appropriate for understanding what ionic liquid-solute interactions could be important in SILP and other related surface-supported ionic liquid applications. 

The stationary phase in TLC is usually an adsorbent (Table 1) composed of very fine and highly porous particles coated as a thin layer onto a support. Today, the most widely used stationary phase is silica gel 60. With a solvent or solvent mixture as mobile phase, a liquid-solid chromatographic system is formed. Separation of sample components can be primarily described as adsorption chromatography. Sample molecules are retained due to specific interaction on the surface of the stationary phase while... 

Cyclodextrins have previously been successfully employed in separation science. For instance, the partial separation and enrichment of optical and structural isomers as well as routine compounds based on selective precipitation with CDs have been reported [5,7-8]. Additionally, solutions of CDs have served as the mobile phase in a few thin-layer and high performance liquid chromatographic separations [5,9,10]. However, their most widespread application in chromatography has been as part of the stationary phase [5,6]. Various polymeric CD materials, CD gels or resins, as well as CD coated columns have been utilized as the stationary phases in the separation of many important classes of compounds [5,6,11-13]. Unfortunately, the use of these CD phases has been largely restricted to column or gas chromatography due to their low efficiency and/or poor mechanical strength [14-16]. 

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