One of the primary reasons for employing TLC methods for analysis is the ease of applying selective chemical reactions for the quantification of substances with poor UV/visible absorption properties or to obtain additional selectivity in group-type and family type detection. These reactions may take place before separation in a vial or on the plate by applying the reagent to the layer above the sample application zone, referred to as prechromatographic derivatization. In a few cases, reaction may take place during the separation by adding the reagent to the mobile phase.In most cases, however, the reaction takes place after completion of the separation directly on the dry layer, and is referred to as post chromatographic derivatization. Separated substances in TLC are also easily detected by biomonitoring using suitable or ganismsor enzymes to determine toxicity or other physiological properties directly.