Development 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. 

Application of anticircular development to preparative planar chromatography is not popular in spite of the possibility of obtaining a good resolution of mixture components, especially of higher Rp values. Delivery of the mobile phase to the... 

The anticircular development chamber contains many of the features. found in the D-chamber. The plate is developed horizontally, the chamber volume is small, and the vapor phase can be controlled by external means [111]. An outer circle is cut fron... 

A slit shaped beam brings light on to the chromatoplate. The plate lies on a table which moves with an adjustable constant velocity linearly under the slit. This procedure is also called linear scan in the literature. For circular or anticircular developed... 

Optimization of the solvent strength by varying the selectivity points is carried out until the required separation is obtained. If no adequate separation is obtained then a different layer or additional solvents must be selected and the new system optimized by the previous procedure. Nearly adequate separations can be improved in the third part of the Prisma model by selecting a different development mode. If an increase in efficiency is required to improve the overall separation then forced flow methods should be used. If the separation problem exists in the upper Rp range then anticircular development may be the best choice, if in the lower Rp range, then circular development is favored. 

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). 

Circular and Anticircular Development. Circular and anticircular development have been accomplished in a number of ways. Because of the need for solvent-delivery control in such systems, chromatography is best carried out in equipment specially designed for the purpose. It is also important that the chamber be kept level. Camag, Analtech, and Anspec all offer chambers for circular development. Samples are applied at the center, and mobile phase is wicked to the surface. Development causes the analyte mixture to separate into a series of concentric rings. Figure 3 illustrates these devices. 

Figure 7.2 Circular development with the point of solvent entry at the plate center (A). Anticircular development from the outer circle toward the center (B). [Reprinted with permission of the American Chemical Society from Feni-more and Davis (1981).]...

The anticircular development mode is the exact opposite of conventional circular development. Mobile phase is applied to the layer along a precise outer circle, from where it flows over the initial zones inward toward the center. Migration in linear and anticircular TLC are related by = 1 - (1 - (Kaiser,... 

Anticircular development is widely accepted in analytical TLC for increasing resolution in the higher / y range. A special device is necessary for preparative separations an example was presented by Studer and Traitler (37)... 

The separation distance depends on the dimensions of the plate, the development mode, and particle size and size distribution. The last property cannot be influenced by the user of precoated plates. Because capillary action is only effective for plates up to 20 cm in length, the maximum separation distance is 18 cm. For anticircular development, the separation distance is 9 cm using the circular mode for special separation problems, this distance is 8-9 cm. Despite the short separation distance, the correct selection of mobile phase and development mode may give high resolution. 

On a 20 X 20 cm chromatoplate in circular development mode, the maximum separation distance is 10 cm if the separation is started at the center of the plate and the sample is applied exactly at the center of the layer. With a specially prepared 20 x 40 cm plate, a 36 cm separation distance can be achieved (see Figure 10b,c) in the on-line operation in the circular and anticircular development modes. 

The circular development mode is used in preparative N- RPC, M-RPC, and U-RPC. In S-RPC, the circular and anticircular development modes can be combined as often as is required by the separation problem. Although C-RPC appears to be a circular development mode, it is in effect a linear development mode because the volume of the stationary phase is constant along the radius. 

In the anticircular development mode the solvent system enters the layer at a circular line and flows towards the center. Since the solvent flow velocity decreases with the square of the distance, but the area wetted also decreases with the square of the distance traveled, the speed of solvent system migration is practically constant. Therefore this developing mode is the fastest with respect to separation distance. Anticircular development is a widely accepted approach in analytical TLC, if the resolution must be increased in the higher / y-range (38). 

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