Stains for developing TLC plates, the stain list

     STAINS FOR DEVELOPING TLC PLATES

     Once a TLC has been developed, it is frequently necessary to aid in the visualization of the components of a reaction mixture.  This is true primarily because most organic compounds are colorless.  Frequently, the organic compounds of interest contain a chromophore which may be visualized by employing either a short or a long wave UV lamp.

  

     Typical examples of functional groups which may be visualized through this method are aromatic groups, 𝜶,𝜷-unsaturated carbonyls, and any other compounds containing extensively 𝞹-conjugated systems.  While exposing these TLC plates to UV light, you will notice that the silica gel will fluoresce, while any organic molecule which absorbs UV light will appear as a dark blue spot.  Circling these spots gently with a dull pencil will permit an initial method for visualization.  

     Fortunately, there are a number of permanent or semi-permanent methods for visualization which will not only allow one to see these compounds but also provide a method for determining what functional groups are  contained within the molecule.

    THE STAIN LIST

Iodine:

     The staining of a TLC plate with iodine vapor is among the oldest methods for the visualization of organic compounds.  It is based upon the observation that iodine has a high affinity for both unsaturated and aromatic compounds.

ULTRAVIOLET LIGHT:

     Good for visualizing any compounds which are UV-active particularly those with extended conjugation, aromatic rings, etc.  Spot must be lightly traced with a pencil while visible, since when the UV light is removed, the spots disappear.

Ninhydrin:

     Excellent for amino acids.

Dinitrophenylhydrazine (DNP):

     Developed mainly for aldehydes and ketones; forms the corresponding hydra zones, which are usually yellow to orange and thus easily visualized.

Potassium Permanganate:

     This particular stain is excellent for functional groups which are sensitive to oxidation.  Alkenes and alkynes will appear readily on a TLC plate following immersion into the stain and will appear as a bright yellow spot on a bright purple background.  Alcohols, amines, sulfides, mercantilism and other oxidization functional groups may also be visualized, however it will be necessary to gently heat the TLC plate following immersion into the stain.