Thin Layer Chromatography - Sample Application 4
Band applicators operate in an entirely different way 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 14.
The atomizer is assembled 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 of the sweep, and the number of sweeps, are usually controlled by a computer and the speed is adjusted such that the solvent evaporates 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. Devices such as those for controlling mobile phase flow, the application of samples, sample removal, sample detection and for quantitative spot measurement. 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. An example of a commercially available automatic sampler is shown in figure 15.
The great advantage of TLC is its low cost together with its rapid and relatively high separating capability. If the performance required to separate a sample is at the limit or beyond the capability of the technique, there is no point in trying to stretch the technique merely for the sake of the technique itself. 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 that is more appropriate.