Special techniques in TLC

The various special techniques applicable in TLC are enumerated below briefly with specific examples wherever possible, namely  

Example Methyl esters of mixed fatty acid may be separated on loose-layer of alumina using suitable solvent-system. 

Note Amount of constituents ranging between 0.1 and 100 g can be separated without any loss easily. 

Example Separation of mixture of fatty acids, cholesterols and their esters lecithins and polar 

Solvent System Position of Plate Separation Caused 

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Explain the following Special Techniques in TLC with suitable examples ... 

After some years, two of these development methods are still used, but new techniques of performing them have come along (see Chapter 11 Special Methods in TLC ). Ascending development is referred to in the present book as the classical method, as in the author s opinion it offers a wider spectrum of possibilities than horizontal development. Lively discussions continue in TLC expert circles on the subject of which of these two methods of development is the better . But what does better mean Every user must find this out for himself or herself at his or her workplace, often doing this afresh for each task. 

For the reader interested in staying up to date with TLC, the Journal of Planar Chromatography, first published in 1988, is a good source of information. Two hooks are also suggested for the reader who wishes to study planar chromatography in greater depth. The first one discusses planar chromatography, from the basics to instrumentation and computer-aided methods [15]. The second book concentrates on TLC and describes sample preparation, sample application, phases selection, visualisation, quantitation, special techniques and combined methods such as TLC-GC, TLC-IR and TLC-MS. It also contains a useful list of suppliers for TLC instrumentation and equipment [16]. 

At this point, the general technique of adsorption chromatography should be apparent to the reader. However, many samples require special handling, or certain specialized techniques may greatly facilitate their separation and/or analysis. Manipulative skills which fall in this category are beyond the scope of this book but are covered in detail in other books on chromatography and TLC. In this section we shall examine a few conceptual techniques which are useful in certain situations. 

A quantitative TLC technique has also been used to determine the functionality of polymers. This technique, thin layer chromatography-flame ionization detection (TLC-FID), is a specialized technique that requires more advanced equipment than standard TLC. In order to perform TLC-FID analysis of a polymer mixture, specially designed thin quartz rods coated with Si02 are used. Chromatographic separation of the mixture is performed on the rod analogous to traditional TLC. After separation, the rod is scanned with sufficient heat to ionize each component of the separated spots. The ionized spots are then quantitatively detected by a flame ionization detector. It is reported that this method has an error limit of about 2%. ... 

A technique that can achieve the maximum attainable resolution in TLC on a given separation distance without forced flow is automated multiple development (AMD). This step-gradient technique was developed by Burger. With respect to peak capacity the technique can be compared to HPLC, but it still maintains all benefits of planar chromatography. The heart of the instrument is a specially designed vacuum-tight chamber. Following sample application... 

Special techniques are necessary in order to apply larger volumes uniformly and as a narrow start band. A detailed description of the various methods is warranted since this is an important problem in preparative TLC. 

Once a sample has been transferred to a TLC plate, all the components of that sample will be located somewhere on the layer. If the analyte is colored, localization is simple. Most organic compounds are not colored, however, so special techniques are required to change the materials in such a way that some optical contrast is established in the visible or the ultraviolet region. Reagents useful in color formation have been listed in [1]. 

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