TABLE OF CONTENTS Relative humidity is defined as the amount of water vapour saturation in air at a given temperature. It is usually expressed as a percentage and in most instances will vary from 40 60% for comfortable conditions. In the laboratory, TLC sorbents will adjust to an equilibrium water vapour concentration depending on the relative humidity. The adsorption is reversible so enforced drying can be used to lower the relative humidity of the sorbent layer. Often TLC plates and sheets are heated at 120 C for at least half an hour to try to achieve a more constant activity. Really there is little point to this procedure as once the TLC plate is exposed to ambient conditions it takes only a few minutes at the most to readjust its water content to the surrounding environment. For the same reason it is usually a meaningless exercise to try to rejuvenate old TLC layers by heating.
Many manufacturers pack TLC plates at a standard relative humidity ( e.g. 40% RH) and once exposed to the atmosphere the percentage water uptake will normally rise to ambient. Under normal conditions this will cause only minor variation in the activity but high relative humidity in excess of 60% causes a rapid change. In these instances heating the plates will drive off this excess moisture. To maintain good reproducibility it is therefore important to keep packs of plates well closed when not in use, and to maintain the laboratory environment within reasonable...
