TLC Staining solutions

General Staining:

Cerium-ammonium-molybdate, CAM

40g of ammonium pentamolybdate + 1.6g of cerium(IV) sulfate + 800mL of diluted sulfuric acid (1:9, with water, v/v). On heating, blue-black spots on light background. Slowly fades over several days. Quite universal, often very sensitive. Some amines, amides and oxidation-resistant aromatics do not detect well.

Basic KMnO4

40g of K2CO3 + 6g of KMnO4 in 600mL of water, then 5mL of 10% NaOH added. (KMnO4 takes some time to dissolve completely. Lazy people like me substitute it with NaMnO4 concentrated aq. solution from Aldrich). No heating. Brown spots on pink background. Often very sensitive but staining very disproportionate to quantity, depending on the compound. Fades within hours. Oxidizes anything with diol, C=C, reactive methylene, phenol, thiol, phosphine etc. Particularly useful for detection of tertiary amines.

Phosphomolybdic acid

30-40g of phosphomolybdic acid in 100mL of ethanol (preferably non-denaturated). Good grade of phosphomolybdic acid should provide clear, bright yellow solution. (If there is cloudiness, let it settle and decant.) Light sensitive. On heating, blue-black spots on yellow-green. Good for lipids. Do not overheat or the background goes dark. Usefull for spraying but expensive as dipping-jar solution. (I stopped using PMA, in favor of CAM. One advantage of PMA is that it is compatible with aluminium-backed TLC places whereas most other universal stains containing diluted sulfuric acid, like CAM or anisaldehyde, are not and must be used with glass TLC plates.

Anisaldehyde 

40mL of conc. H2SO4 is added (slowly!) into ethanol 800mL, followed by acetic acid 12mL and anisaldehyde 16mL. Light and oxidation sensitive. On heating, colorfull spots on pink background. Color varies on the compound. Good for all things with active methylene, and for distinguishing closely-spaced spots on TLC by their color difference.

Iodine

Iodine vapor chamber is made from a TLC jar with a good-sealing lid, by adding a dry mix of iodine crystals  (a small spoon) covered with silicagel for chromatography, about a half-inch layer. Put dry TLC in the chamber onto the silica layer face down and watch the brown spots developing. Works on variety compounds but often it is only moderately sensitive. Iodine stained TLC can be developed subsequently with other stains.

Functional group-selective stains

Ninhydrin

20g of ninhydrin in 600mL of ethanol. (Don’t spill ninhydrin onto your fingers – they would go blue.) Primary amines produce blue spots at R.T., very sensitive detection. Boc-protected primary amines produce spots on heating (as the Boc falls off). Secondary amines sometimes detect but the stain is weak.

Dinitrophenylhydrazine

3g of dinitrophenylhydrazine in 750mL of 2M HCl. (If htere is some insoluble portion, decant it off.) Aldehydes and ketones produce yellow-orange spots at R.T., quite selective.

Stain dip station: 4 or 6 wide-mouth jars covered with aluminum foil (secured by tape) to protect from light, placed within a tray (to guard against spill).

Spray station: Hand-operated rubber-baloon/glass top sprayers are wonderful but antiquity now – they are hard to get by nowadays. Compressed gas sprayers are much harder to control. Spray advantage: economical on stain solution, the TLC spots do not move as with dipping. Disadvantage: big mess in the hood, a receiver for spraying has to be built (i.e. from a cardboard box). I used to be very partial to spray-staining but was won over by the dip jar convenience.

Update: A procedure for improved Dragendorff stain that keeps well in refrigerator (+4C). This stain is particularly useful for detection of lipophilic amines, basic heterocycles like pyridines but also aryl phosphines, crown ethers and polyethylene glycol polymers. Brown spots on yellow background develop almost instantly at R.T.

A solution of L(+)-tartaric acid 20g in D.I. water 80 mL was added to BiO(NO3) 1.7g and the mixture was sonicated on ultrasonic bath for 15 minutes. A solution of KI 32g in D.I. water 80 mL was added into the mix. Finally, a solution of tartaric acid 175g in D.I. water 950mL was added. The resulting bright orange mixture was stirred for 15 minutes and then placed into a fridge overnight. The solution was decanted off from precipitated crystalline solids (probably K-tartarate), transferred into a wide-mouth dip jar and kept in fridge when not in use. (This Dragendorff reagent gradually darkens over time but the aged reagent still performs quite well even after several months in the fridge.)