Development techniques anticircular

Figure 8.19 illustrates another example of the versatility of multidimensional OPLC, namely the use of different stationary phases and multiple development ("D) modes in combination with circular and anticircular development and both off-line and on-line detection (37). Two different stationary phases are used in this configuration. The lower plate is square (e.g. 20 cm X 20 cm), while the upper plate (grey in Figure 8.19) is circular with a diameter of, e.g. 10 cm. The sample must be applied on-line to the middle of the upper plate. In the OPLC chamber the plates are covered with a Teflon sheet and pressed together under an overpressure of 5 MPa. As the mobile phase transporting a particular compound reaches the edge of the first plate it must-because of the forced-flow technique-flow over to the second (lower) stationary phase, which is of lower polarity. 

Single linear developments are mostly employed in the vertical mode. The apph-cabihty of the horizontal mode is discussed in Chapter 6. For circular and anticircular developments, the movement of the mobile phase is two-dimensional however, from the standpoint of sample separation it is a one-dimensional technique. Circular developments result in higher hRp values compared to linear ones imder the same conditions, and compoimds are better resolved in the lower-AR range. The same effect is noticed on plates with a layer thickness gradient (see Section 5.2.1). On the other hand, using antieircular developments, compounds are bettCT resolved in the upper-M range. 

When comparing the above two methods to linear development, anticircular apparently is superior in terms of sensitivity, number of samples per plate, speed of analysis, and solvent consumption. Conventional linear TLC ranked second to the anticircular techniques [46], Also refer to the Handbook of Thin-Layer Chromatography [27] for additional details. 

Development in thin-layer chromatography is the process by which the mobile phase moves through the layer, thereby inducing differential migration of the sample components. The principal techniques are linear, circular and anticircular with the mobile phase velocity controlled by capillary forces or external pressure. The application of any of these techniques can be extended using continuous or multiple development. 

Other special devices, which are of historical interest, include a special chamber for short bed continuous development. In this technique, the plate extends though a slit out of the actual chamber allowing the developing solvent to evaporate. Separation of very similar compounds can thus be achieved at low R values. The U-chamber according to Kaiser was a special device for circular and anticircular HPTLC. 

Circular and anticircular development methods. This technique produces a radial chromatogram which requires special scanners and is generally not used for lipid separation. The principles of these development methods and the names of companies manufacturing chambers for use in such methods are reviewed by Cserhati and Forgacs (1996). 

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