Analytical OPLC Separations

In OPLC, the most frequent modes of development are linear one- and two-directional (Figure 5A,B). Linear OPLC, however, requires a special chromato-graphic plate sealed along the edge, by impregnation, to prevent the solvent from flowing off the layer.

The advantage of circular development, in which the mobile phase migrates radially from the centre of the plate to the periphery, is well known for the separation of compounds in the lower RF range,

Figure 4 Schematic diagram of long-distance OPLC (LD-OPLC). (A) Principle of the method; (B) fully offline LD-OPLC using homolayers; (C) fully online LD-OPLC using heterolayers.

Figure 5 Development modes in analytical OPLC using 20 cm x 20 cm HPTLC chromatographic plates. (A) Linear unidirectional; (B) linear two-directional; (C) circular with 8 cm development distance; (D) circular with 18 cm development distance, or online circular; (E) anticircular; (F) anticircular with 18 cm development distance, or online anticircular.

Figure 5 Development modes in analytical OPLC using 20 cm x 20 cm HPTLC chromatographic plates. (A) Linear unidirectional; (B) linear two-directional; (C) circular with 8 cm development distance; (D) circular with 18 cm development distance, or online circular; (E) anticircular; (F) anticircular with 18 cm development distance, or online anticircular.

Conventional offline anticircular separation (Figure 5E) is rather difficult to perform because of the large perimeter of the mobile-phase inlet (ca. 60 cm for a 20 cm x 20 cm plate). Fully offline and online anticircular separations can, however, be performed over a separation distance of 18 cm, after suitable preparation of the plate by isolating a segment of the layer (by scraping) and sealing the isolated segment with polymer suspension (Figure 5F).

In linear OPLC the maximum separation distance is 18 cm for 20 cmx 20 cm chromatographic plates. In offline circular OPLC the maximum separation distance is 10 cm, and only one sample can be analysed. If the distance between the mobile phase inlet and the point of sample application is 2 cm, a separation distance of 8 cm can be achieved; this enables application of more samples.

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