Question

Why is it better to use a volatile solvent for the mobile phase of Thin Layer Chromatography

Answer 1

The TLC plates (aluminum/ plastic backed) are dipped in the solvent system so as it reaches top of the plate by capillary action. It takes along with them the substances those are being analyzed. The distance up to which the analytes travels is specific to each solvent system.

After the plate is run by a mobile phase, they are to be observed so that the analytes are visualzed. Although there are several ways of visualization, the major ones are-

i) To observe under a UV cabinet: UV cabinet visualization is the method when fluorescent TLC plates are used. But when a solvent is adhered to the silica particles, they also fluoresce and the spots cannot be seen until the the particles are solvent free.

ii) To dip in an Iodine-saturated chamber: Since the solvents used are also organic, the presence of unsaturated functionalities in them cause the addition of iodine to them and cause ambiguity (although this is negligible, it happens- solvent front is often seen as a dark line).

iii) To spray or dip in specialized reagents so that the analytes get colored due to a reaction.

For anyone of the above to be choosen, it is always better that the solvent on the TLC is evaporated so that they can be visualized error free. In cases of some very polar substances like amino acids, glycosides or anthocyanins where water is used as a part of solvent system, the TLC plates are dried completely before they are visualized.