Thin Layer Chromatography - The Iodine Reagent

An example of a commercially available Reagent Spray is shown in figure 17.

Figure 17. A commercial TLC Plate Spray

There are a number of different reagents that can be used and some of the more common are described as follows. However, only a limited number of these are suitable for quantitative assessment.

The Iodine Reagent

The majority of compounds adsorb iodine and become visible if exposed to iodine vapor. Although reaction may take place with some unsaturated compounds, for the vast majority of compounds, the iodine appears to be physically bound to the material and does not chemically react. The after carrying out the elution process the plate is dried and placed in an enclosure containing iodine crystals and is preferably heated to about 50oC. Brown spots appear where the solutes are situated. When the plate is removed from the enclosure, the spots rapidly disappear and so this detection technique appears to be essentially non-destructive.

This detection procedure can be improved by first exposing the plate to a very high concentration of iodine vapor, the plate is the removed and allowed to stand for a few minutes to allow the excess iodine to evaporate. The plate is then sprayed with a 1% starch solution. As a result the solute bands appear as blue spots on the plate. These methods will detect most organic substances and in particular unsaturated compounds.