Thin Layer Chromatography - TLC Stationary Phases and Supports
TLC Stationary Phases and Supports
The TLC plate coating can serve two purposes in the separation process. The surface of the coated material can made to contain chemical groups that actually interact directly with the solutes themselves and, thus, determines the extent of their retention and the selectivity of the phase system in which case the material acts as the stationary phase proper. Silica gel would be the classic example of such material. Alternatively the material can behave merely as a support for other substances carrying the interactive groups that will establish the degree of retention and selectivity and, thus, make little or no direct contribution to solute retention. Cellulose fiber would be a typical example of this latter type of coating. Water or aqueous solvents can be absorbed in the cellulose, and the absorbed solvents will be the sole agents that interact with the solutes and control retention; the cellulose will only act as a benign support. In practice, however, the distinction between supports and stationary phases is not nearly as straightforward as it might appear. Silica actually holds a layer of water or solvent that is adsorbed on its surface and so acts to some extent as a support. Conversely, the hydroxyl groups on the cellulose will certainly act as a source for polar interactions with solutes and so as far as interactive capability with the solutes is concerned, cellulose will not be as benign as it might seem. Consequently, although the various materials discussed below may be defined as stationary phases or supports, none will behave exclusively as one or the other.
The production and properties of silica gel have already been discussed elsewhere (7). The chemical structure particle size, pore volume, pore size and surface area will be similar for the silica's used in both LC and TLC. The major difference will be the addition of dehydrated gypsum a binder to the silica used for TLC that helps stabilize the layer on the plate. The layer of particles on the glass plate must be thin, smooth, constant and durable in order to obtain efficient separations. In addition, the coated plate must be physically stable, both in the dry form (for satisfactory storage) and when wetted with solvent to ensure consistent chromatographic properties. In the early days of TLC, the stability of the layer of stationary phase was a major problem and was ultimately achieved by the use of a binder. Organic polymers (particularly the polymethacrylates) have also been employed as binders but, partially dehydrated gypsum is still used in the production of over 80% of all thin layer chromatography plates.
The gypsum will certainly exhibit some interactions with any solutes that are separated and thus, may contribute somewhat to retention. However, as it is only present at a level of 2-5% w/w and, in addition, has a very small surface area, its contribution to retention will be minimal. If a methacrylate polymer is used as the binder, interactions between the methacrylate and the solute are likely to be much greater in magnitude and, thus, may make a more significant contribution to solute retention. In contrast to the interactions between the solutes and gypsum (which are largely polar), interactions with the methacrylate will be significantly dispersive in nature.