Thin Layer Chromatography - Coating the Thin Layer Plate 1
Coating the Thin Layer Plate
The production of an efficient TLC plate demands an even coating of the adsorbant and such a coating is essential for its effective use. The greatest obstacle to the successful development of thin layer chromatography in the early days was the lack of an efficient adsorbent "spreader". In addition, due to the lack of precise screening techniques, the particles originally used for making the plates were relatively large and included a fairly wide range of particle diameters. For example, early thin layer plates were coated with particles ranging in size from 4 mm to 40 mm with a mean diameter of 20 mm. Furthermore, there was poor quality control of the silica that and, thus, the retentive retentive capacity of the thin layer plates that were produced would differ widely from batch to batch. However, with the advent of improved manufacturing techniques and better quality control the quality of the plates was vastly improved. In addition, with the introduction of air jet grinders and rotary classifiers, and the consequent improved grading precision, the mean particle size of the silica employed in the production of TLC plates was reduced and the range of particle diameters within a specific grade greatly diminished. Today the mean particle size used for high quality thin layer plates is about 5 mm and 90% of the particles will have diameters lying between 4 mm and 7 mm.
Since both the multipath dispersion and also that due to resistance to mass transfer in the mobile phase is a function of particle diameter, thin layer plates having smaller diameter particles will give smaller spot diameters due to the consequent reduction in band spreading. Such plates would logically be called high efficiency thin layer plates but such terminology appears to have been considered insufficiently grandiose. Separations carried out on such plates have been given the rather pompous term High Performance Thin Layer Chromatography (HPTLC). Today TLC plates are mechanically coated with the stationary phase employing a modified form of the technique that was first introduced by Stahl (6). A form of the apparatus used is depicted in figure 1.