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

The atomizer is mounted on a mechanical arm that can sweep from side to side while directing the atomized sample on to the surface of a TLC plate. The range and the number of sweeps, are usually under computer control and the speed of movement is adjusted such that the solvent is able to evaporate from a given area of sample before it receives the subsequent dose. After dosing, the plate is developed in the normal way. Some exceedingly novel and clever devices have been developed for TLC. These devices indeed improve the performance of the TLC analysis but are also expensive and in many cases tend to make the TLC system more like a liquid chromatograph. The great advantage of TLC is its low cost and its relatively high separating capability. If the required performance required is at the limit or beyond the capability of the technique, there is no point in trying to stretch it. The rational solution for the chemist or analyst would be to change to an alternative procedure such as liquid chromatography or to some other technique if more appropriate.