Principle of Long Distance Oplc Ldoplc

Long-distance OPLC is a multi-layer development technique with specially prepared plates. Similar to the preparation of layers for linear OPLC development, all four edges of the chromatographic plates must be impregnated with a polymer suspension. The movement of the eluent with a linear solvent front can be ensured by placing a narrow plastic sheet on the layer or scraping a narrow channel in the sorbent for the solvent inlet. Several plates are placed on top of each other to ensure the long running distance. A slit is cut at the end of the first (upper) chromato-

graphic plate to enable the mobile phase to travel to a second layer. Here the migration continues until the opposite end of the second layer, where solvent flow can be continued to the next adjacent chromatographic plate, or the eluent is led away (Figure 4A) if migration is complete. Clearly, on this basis a very long separation distance can be achieved by connecting one plate to another.

Figure 4B shows a typical combination of the same type of chromatographic plate (homoplates). In the arrangement presented, the upper plate has an eluent inlet channel on one side and a slit on the other side for conducting the mobile phase to the next plate. The slit (width approximately 0.1 mm) can be produced by cutting the layer; this enables ready passage of the mobile phase and individual samples without mixing. The cushion of the OPLC instrument is applied to the uppermost layer only, and each plate presses the sorbent layer below. As a consequence of this, glass-backed plates can be used in the lowest position only. The illustrated fully offline separation is complete when the 'a' front (the front of the first solvent in an eluent solvent mixture) of the mobile phase reaches the end of the lowest plate.

The potential of the connected layers can be increased by use of different (hetero) stationary phases during a single development; this is shown in Figure 4C, in which the different sorbents are marked with various shades of gray. The eluate can, furthermore, be led from the lower plate, similarly to the way in which it was led in. This gives the possibility of online detection. For this fully online operating mode all layers placed between the highest and lowest plates must have 1 cm cut from the

Figure 3 Schematic diagram of multi-layer OPLC (ML-OPLC). (Bj (A) Linear one-directional development; (B) linear two-directional development; (C) circular development.

where circular development gives 4-5 times greater resolution. The separating power of circular development is better exploited if the samples are spotted near the centre. As the distance between the mobilephase inlet and sample application increases, the resolution begins to approach that of linear development. No preparation of the plate is necessary for offline circular OPLC (Figure 5C); for online circular OPLC (Figure 5D) a segment-shaped region must be isolated by removing the surrounding adsorbent and impregnating its edges.

Figure 3 Schematic diagram of multi-layer OPLC (ML-OPLC). (Bj (A) Linear one-directional development; (B) linear two-directional development; (C) circular development.

length of the plate, to leave a space for mobile phase outlet.

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