Question

2. When illuminated by UV-Vis light, why do the organic compounds show up as a dark spot?

Answer 1

TLC plates made up of Silica gel on which inorganic fluorescent agent (<0.5%) impregnated into the adsorbent layer. When our compounds are illuminated with an ultraviolet (UV) lamp, the absorbent then glows the pale green or blue colored fluorescent light. When we use fluorescent plates, a number of compounds can be observed by illuminating the plate with short-wave UV. Quenching causes dark spots on the surface of the plate. Dark spots are appearing in the places where UV absorbing organic compounds are located on the TLC plate because they quench the fluorescence.