ABSTRACT
The general advantages of (HP)TLC as a method for the separation and analysis of low molecular weight, low volatility compounds are well known; it is a simple, convenient, economical, high-throughput method, which in its basic format, at least, needs only portable, low-cost equipment. In addition, the whole polarity spectrum is present between the origin and the solvent front. There are a wide range of detection methods for in situ localization, partial identification, and quantification of compounds, which are not subject to interference from the mobile phase, since this is generally allowed to evaporate off before detection. The detection methods are generally sensitive (nanogram to microgram range) and provide good linearity of response, even if only over a limited range. All these advantages apply to the TLC of sterols and steroids and, in former times, TLC was extensively applied to the analysis of these classes. However, it has now been almost completely supplanted by HPLC as the method of choice, because of the greater possible loading, higher preparative recoveries, greater resolution, and the widespread availability of HPLC equipment nowadays. Consequently, there have been few systematic studies of the chromatographic behavior on TLC of the different classes of phytosteroids, in spite of their major significance in medicine and chemical ecology. Rather, where TLC is used at all today, it is used to solve a specific problem, and not for comparison of a wide range of analogues of a particular phytosteroid class. A notable exception here is the (phyto)ecdysteroids, for which a significant body of TLC data exists, comparing not only many analogues, but also different TLC systems and approaches, including various one-and two-dimensional systems, forced-flow systems, and coupled systems (e.g., TLC-MS).
