Thin-layer chromatography (TLC) is a most flexible and versatile chromatographic technique and therefore remains an indispensable tool for many applications. Applications range from monitoring of chemical reactions, comparison of fractions, and preparative separations to qualitative analyses such as identification of raw materials, tests for adulterations, and detection of biological activity to demanding quantitative determinations including assays, impurity profiling, and cleaning validation. In addition to traditional TLC plate development in an enclosed tank, there are also gradient and even forced flow techniques now available. The variety of stationary phases for planar separations is quite large, and it would be difficult to cover the subject comprehensively in this column. Therefore, in the present coverage, we will focus only on TLC plates used in the state-of-the-art qualitative and quantitative high-performance separations. For a more general approach, we recommend the corresponding chapters of the books edited by Sherma (1) and Nyiredy (2). Previous issues of LCGC have covered various aspects of modern TLC (3–7). This present contribution will serve to bring readers up to date on the latest developments in stationary phases.

Modern TLC is a powerful, reliable, and GLP–GMP (good laboratory practices–good manufacturing practices) compliant analytical technique that depends strongly on the commercial availability of high-quality, precoated plates. For many years, such plates have been on the market, featuring different formats, stationary phases, binders, and supports. Worldwide sales are increasing steadily, and users find it difficult to decide which of the many TLC products to select. This month's installment of "Column Watch" provides an overview of modern plates for TLC, their properties, applications, and performance from a theoretical and also a practical point of view. Although much lower in cost, self-made plates usually cannot compete with commercial plates with regards to stability, performance, homogeneity, and batch-to-batch consistency.

There are a number of official TLC regulatory methods, for example, in the international pharmacopoeias, the food laws, and in the collection of official methods of AOAC International. Many of those methods are rather old and their description of the TLC plate is often inadequate: "Use a plate coated with a suitable silica gel." Even though in official methods, a general suitability test can be given, it still remains difficult to select the proper plate because the expected results are not always precisely described. Therefore, a secondary focus of this article is on the important features of modern TLC plates, which affect the reproducibility of the chromatographic experiment.

For various reasons, plastic- or aluminum-backed plates (probably better called foils or sheets) also have become popular. These plates are usually less expensive than glass plates and can be cut conveniently into any size. When handled properly, their performance is comparable to that of glass plates, but TLC sheets require much more attention during sample application, chromatogram development, and particularly during derivatization. These precautions are necessary because of their inherent flexibility and somewhat limited tolerance to chemicals and heat.