How to Monitor by TLC

For pictorial how-tos:

  1. Figure out which solvent system puts your starting reagent at an Rf of 0.3-0.4. Good Starting Points.
  2. Prepare a chamber with about 1/2 cm of your solvent mixture in it
  3. Get a TLC plate (1.5-2 cm wide and ~5 cm high is a good size) and put three pencil dots in a horizontal row at the bottom, ABOVE the 1/2 cm mark. When you put the plate in the chamber, these dots should not be submerged in the solvent.
  4. With a capillary pipette, put a sample of your starting material (diluted to a concentration similar to the reaction mixture) at the leftmost dot, and at the middle dot. Your sample spot should be small enough that it does not spread to the other pencil dots.
  5. With a capillary pipette, put a sample of your reaction mixture at the middle and rightmost dot. The middle dot is a "cospot" (See below)
  6. Run the TLC until the solvent front has almost (but not quite) reached the top of the plate.
  7. Look under a UV lamp and circle the UV-active spots with a pencil
  8. Dry the plate on either a hot plate or in the air, until the solvent is gone
  9. Dip into an appropriate TLC stain, and heat on a hot plate. You may have to try a few before you find the best stain for your compound
Commercially Available TLC Plates

Silica gel, alumina, reverse-phase silica gel, large plates for preparatory scale.


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.