Principles and Practice of Chromatography - Thin Layer Chromatography Apparatus > Sample Application > Page 68

The concentrating zone is about 2 cm wide and consists of a coating made from relatively large particles of silica with a relatively low surface area and consequently low retentive capacity. The concentrating band is coated closely adjacent to the normal retentive coating which, consists of the usual particles 5-7 mm in diameter but with a much higher surface area and, thus, much greater retentive capacity. Several samples of a few microliters or more can be placed sequentially on to the concentrating zone and the solvent allowed to evaporate until there is an adequate quantity of sample on the plate. The sample is now spread along the concentration zone in a fairly broad band. When the plate is developed the solutes move rapidly through the concentration zone due to its low retentive character to the interface between the layers. At the interface the solutes are slowed down by the more retentive layer and are thus focused as a sharp band at the front of the plate. As development proceeds, the solutes separate in the normal high retentive layer in the usual manner. This procedure has other advantages. If the sample is contaminated with salts or biological polymers, these will be trapped in the concentration zone and, thus, will not pass onto the separation region of the plate and effect the quality of the separation.

Band applicators operate differently and are usually fully automated. The sample is atomized in a stream of air or nitrogen depending on the nature of the sample and its tendency to oxidation. A diagram of the type of atomizer used in band application is shown in figure 35.

Figure 34 TLC Plate with Sample Concentrating Zone