Principles and Practice of Chromatography - Chromatography Applications > Page 69
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.
Gas chromatography has an entirely different field of applications to that of liquid chromatography. In general, gas chromatography is used for the separation of volatile materials and liquid chromatography for the separation of involatile liquids and solids. There are certain compounds, however, that can be separated with either techniques, and more importantly, many involatile substances such as amino acids, steroids and high molecular eight fatty acids can be derivatized to form volatile substances that can be separated by GC. The derivatization must be highly reproducible and usually proceed to completion in order to maintain adequate accuracy. The capillary columns in GC can have much higher efficiencies than their LC counterpart and thus GC can more easily handle multicomponent mixtures such as essential oils. On the other hand, only LC can separate the peptides, polypeptides, proteins and other large biopolymers that are important in biotechnology.