How to prepare silica gel slurry for TLC?

 

I have silica gel G with Calcium sulfate in my lab. What proportion of silica powder and water should I use to make the slurry? I have already tried it, but after activation the silica powder just slides out of plate. Any suggestion?

 

It is important that anyone making TLC plates are aware of the binders required for silica gel TLC plates. The fine silica will not stay on the plate after coating if some type of binder is not included. It would literally fall or blow from the plate. The classical binder used is gypsum (G),which is CaSO4•½ H2O, or Plaster of Paris. In most silica gels it is put in at a level of from 10-15% to give good binding to the glass plates. For coating five 20x20 plates, about 30g TLC silica gel to 60ml water is needed. Once made, then the time available to put in the applicator and to pull across the glass plates is limited, since the binder will hydrate and the slurry will no longer be a flowing liquid. The other formulation offered by Merck KGaA, Darmstadt, Germany is the H binder, which is a silicic acid. This also gives a soft layer plate but might be considered to be pure silica gel. Both the G and H layers have to be handled and used carefully so as not to disturb the layer. For a stronger layer, polyvinyl alcohol or polyvinyl pyrollidone can be added to a TLC grade silica gel (without G or H binders) in a level of 1-2% by weight as a polymeric binder. Merck KGaA, Darmstadt, Germany, does not sell these binders, however. Heat drying after air-drying makes cross-linking of this type of polymer binders. After coating the plates, the plate holder is lifted a couple of mm and dropped on the benchtop to help release air bubbles that might be caught in the slurry. Then the plates in the holder tray are allowed to remain flat while the gypsum (or other binder) sets up or the excess moisture evaporates. The gypsum on hydrating makes the layer turn from a translucent, shiny appearance to a flat, matte finish. Allow the plates to air-dry overnight. This is particularly true if thicker preparative layers are being made. Slower drying of these layers insures no cracking. The next day or before use, the plates can be activated if desired, to remove more of the adsorbed water. If not using immediately, then the water still remaining on an unactivated plate will actually protect the surface more by not allowing impurities to collect on the activated silanols. The plate can be stored with this water and activated for 30 minutes at 110°C just before use.