TABLE OF CONTENTS Before the advent of commercial pre-coated TLC plates, layers of silica gel and of other sorbents (e.g. cellulose or aluminium oxide) had to be prepared in advance of separation procedures. A typical method was to thoroughly mix 30 g of sorbent with 60 ml of water by shaking in a glass flask. The slurry produced was transferred to a spreader and applied evenly over the plate surface in one spreading operation. The applied slurry was allowed to set and dry for about 30 minutes. Final activation was completed in an oven for 30 minutes at a maximum temperature of 105 C. To improve the binding of the silica gel to the inert backing a gypsum binder was added; Stahl recommended 13% of calcium sulphate. Although the layer was soft and easily damaged, it remained stuck to the glass backing when the mobile phase migrated across the plate. This silica gel plate was given the designation "silica gel G". Organic binders have also been used including carboxymethylcellulose, starch (1 2%), and polyvinyl alcohol (1 5%). Although these give a stronger binding for the most part at a lower concentration of binder, they often suffer from solubilisation in aqueous based solvents and charring after treatment with strong sulphuric acid solutions and heating.
Commercially manufactured pre-coated TLC and HPTLC plates use organic polymeric binders at a concentration of about 1 2%. These binders are much more resistant...
