Offline Liquid Phase Sample Application

The preferred method of placing a sample on a preparative layer is to apply it as a narrow streak across the plate. It is highly desirable to have the streak as straight and narrow as possible. With practice it is possible to streak a plate correctly by hand, using a syringe; the use of a Teflon tip on the end of the syringe has the advantage that no mechanical disturbance of the layer occurs. On applying a large amount of sample, the streak can be focused simultaneously and concentrated to a relatively thin line by the sequential technique, especially with nonpolar samples.

Application of a continuous streak can be automated. Most available applications can give a sample zone < 3-4 mm wide and up to 200 mm long for preparative separations. When the layer is not overloaded with sample it is generally accepted that the streak should be applied across the plate 2 cm from both edges. These areas are left free, partly because of the edge effect, which can cause migration of the mobile phase to be faster or slower at the edge than on the centre of the plate. For separations of extremely large amounts, the sample can be applied over the whole width of the plate. Otherwise migration of the mobile phase would be faster in the sample-free areas, resulting in poorer resolution.

When using pre-coated preparative plates with concentrating zones, the quality of streaking is not very important because the sample is applied to a practically inert zone. Pre-coated preparative layers with concentrating zone can be successfully used for PPC and for linear OPLC.

For preparative rotational planar chromatography (RPC), a concentrating zone can be self-prepared. The layer is removed by scraping to furnish a larger inner circle; a slurry with inert material is then cast in this circle. After final drying and scraping, the layer has a preadsorbent zone of approximately 2 cm. For C-RPC separations, after filling the planar column with the selected stationary phase, the last 1 cm can be filled with a deactivated sorbent.

Offline liquid-phase sample application can be used for practically all micropreparative and preparative PPC methods. The only exception is C-RPC, where the solvent for dissolution of the sample is not identical with the mobile phase. The planar column prevents elimination of the sample solvent. The separation may consequently be distorted, especially if the solvent has a higher solvent strength than the mobile phase.

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