1. Visual detection

Intended for qualitative analysis, visual detection is widely used in the analysis of narcotics, for example. In all cases, the plate is illuminated by a suitable light source:

• white light is used to identify coloured substances;

• ultraviolet light enables fluorescent substances to be seen when illuminated at these wavelengths.

To facilitate detection, TLC plate manufacturers mix a fluorescent indicator into the adsorbent layer; this may be an inorganic salt that gives the plate a yellow (uranyl acetate) or yellow-green (zinc silicate) color. The most widely used is a mixed sulfide of zinc and cadmium, fluorescing at 254 nm, hence the name F 254.

Organic salts generally fluoresce at 366 nm; the most commonly used are 3-hydroxypyrene 5,8,10-trisulfonate...