Before the TLC Development Process

In this Chapter we describe methods of handling precoated layers and also how to apply and dry samples before development. 

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Linking TLC with a tandem instrument differs from combining GC or LC with an appropriate spectrometer. Hyphenation of planar chromatographic techniques represents a niche application compared to HPLC-based methods. Due to the nature of the development process in TLC, the combination is often considered as an off-line in situ procedure rather than a truly hyphenated system. True in-line TLC tandem systems are not actually possible, as the TLC separation must be developed before the spots can be monitored. It follows that all TLC tandem instruments operate as either fraction collectors or off-line monitoring devices. Various elaborate plate extraction procedures have been developed. In all cases, TLC serves as a cleanup method. 

Multiple unidimensional development (MUD) is the simplest approach for enhancement of the separation capacity in TLC [2]. In this approach, the TLC plate is developed for a selected distance, then the plate is withdrawn from the developing chamber and the adsorbed solvent is evaporated before repeating the development process. MUD is a very versatile strategy for the separation of complex mixtures. The main feature of MUD is that it leads to an increase in the spot reconcentration mechanism. There is an optimum number of developments that provide maximum separation. 

Schematic representation of the exchange processes between the gas space, the solvent system reservoir and the TLC plate before and during development.

The residual activity of the racemic mixture and the most insecticidally active stereoisomers [S,S] were investigated on soil samples. Before application the isomer separation of the C-fenvaler-ate was carried out by six consecutive developments in hexane-ether (20 1). The [S,S] and [R,R] isomers were located at R/0.46 and the [R,S] and [S,R] stereoisomers at R/0.49 (Table 9). The isonter pairs were finally resolved in HPLC using a chiral stationary phase. Soil extracts were analyzed in order to investigate the degradation processes and persistence of C-fenvalerate (Table 9) (115). The parent compound and its degradation products were separated by two-dimensional TLC (Table 9)... 

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