TABLE OF CONTENTS HPTLC uses the same type of silica gel 60 layers, as in traditional TLC, with a thickness of 0.20 0.25 mm. However, the particle size is much smaller, typically ranging from 4 8 ?m, with an optimum of 5 6 ?m. Recently a new spherical HPTLC silica gel 60 of optimum particle size 4 ?m has become available (see comparison in Figure 40). The smaller particle gives a somewhat softer, smoother layer. However, commercial pre-coated HPTLC plates with polymeric binders, are sufficiently hard so as not to be easily damaged by the capillary tubes used for sample application.
The smaller particles, similar in size and quality to HPLC packing materials, give a lower theoretical plate height (H) and hence higher efficiency. However, this is only fully utilised if the plates are not overloaded with too much sample, the spot size is kept small (about 1.0mm), and the plate is developed only to the extent necessary for complete resolution (often only 5 cm and rarely more than 8 cm). A direct comparison of theoretical plates in HPTLC with HPLC serves little purpose as the number found is only valid for the spot used for calculation. The basic problem is that all analytes do not travel the same migration distance and are not measured in retention time as in column chromatography. In planar chromatography, separations are in distance, spots and bands...
