After removing a TLC plate from the developing chamber, what is the first thing to do with the TLC plate?

The solvent front line should be drawn.

Explanation:

TLC is thin layer chromatography, a techniques used for the separation of substances based on their mobility on their solvents. The silver or thin metal films are used for the absorption.

The TLC plate must be removed from the chamber. After its removal the solvent front line should be drawn to known the absorbance of the material. This technique is quiet useful for the distinction of molecules.