Impregnation Techniques: Thin- Layer (planar) Chromatography

The use of impregnated phases in thin-layer chromatography (TLC) has a long history, beginning in the very earliest days of the technique. As a consequence, much of the key literature in this Reld dates back to the 1960s and 1970s. The literature also contains a considerable number of examples of impregnation reagents where subsequently the methods have received little attention except in reviews of the subject. The impregnation of the stationary phase in thinlayer, or planar, chromatography can be used to achieve a number of ends. These include changing the mode or selectivity of the chromatographic system, improving chromatographic performance and enhancing detectability. An example of the use of impregnation to change the type of chromatography would be the use of a nonpolar, water-immiscible material such as parafRn oil which can be used to enable reversed-phase separations to be performed on silica-gel TLC plates. Alternatively, plates impregnated with silver nitrate can be used to improve the separation of compounds containing double bonds (argentation chromatography). Similarly, boric acids or boronates are useful for compounds such as carbohydrates and sugars containing vicinal diols, and ion pair reagents provide a means for successfully separating polar ionic species. Although the use of impregnation techniques for enhancing detectability will not be considered in depth here, examples would include the incorporation of ammonium acetate followed by heating, which can result in the formation of intensely Suorescent derivatives, whilst parafRn oil impregnation can stabilize or enhance Suorescence and the radiolabelled substances can be detected using impregnation with a scintillant. There are a variety of methods for obtaining impregnated layers, including the preparation of the TLC plate with slurries containing both the stationary phase and the impregnating reagent, preor post-chromatographic dipping, preor postchromatographic development of the plate in a solvent containing the impregnating agent or spraying, and these are described below. Impregnation Techniques