The Cospot: During reaction monitoring, a typical TLC plate has three lanes: the reactant, the reaction mixture, and a "cospot" where reaction mixture was spotted directly on top of reactant. The cospot is important for reactions where reactant and product have similar Rfs. Also, in some cases the reaction mixture affects the appearance of the reactant during chromatography. The reactant can appear to be gone, but in actuality just looks different on TLC under the reaction conditions. This situation is easily identified by a cospot.
Preparatory TLC: Full-size plates with a thick layer of silica are used for preparatory separation by thin layer chromatography.
Up to 100 mg of compound is deposited in a horizontal thin line at the bottom of the plate, and the plate is run in the appropriate solvent system as usual.
The product is (ideally) located by UV visualization, and marked with a pencil.
A razor blade is used to scrape the silica containing the product off the plate.
The silica is placed in a fritted funnel and flushed with a polar solvent (like EtOAc). The pure product can be isolated from the filtrate.