Principles and Practice of Chromatography - Thin Layer Chromatography Apparatus > Sample Application > Page 67

The device consists of a plate with a small indentation, at the center of which is an aperture that can be connected either to a vacuum or to a source of gas pressure. A polymer film is placed over the plate surface and a vacuum applied to suck the film into the indentation. A small quantity of sample solution is placed in the indentation and the solvent evaporated. This procedure is repeated until sufficient sample is present on the film for a satisfactory TLC separation. It is important that the sample is not evaporated to dryness as the transfer of solid materials to the thin layer plate can be very inefficient. When sufficient sample has been accumulated and the droplet is still liquid, the thin layer plate is placed over the film and a positive pressure now applied to the aperture at the center of the indentation. The film extends to the surface of the TLC plate and the sample is transferred by contact, as a result of surface tension forces and the adsorptivity of the TLC plate surface. The plate is then conveyed to the solvent chamber and the separation developed in the usual way.

If larger samples are required for semi preparative work, sample bands can be applied to the plate as opposed to sample spots. Sample bands can be applied either by using TLC plates with concentrating zones or alternatively by using band applicators. A diagram of a TLC plate that includes a concentrating zone is shown in figure 34.

Figure 34 TLC Plate with Sample Concentrating Zone