Can anyone suggest a silica gel (name a brand) exclusively for doing preparative TLC? I am facing additional spots after scraping-off the compound interest from the plates. Possibly due to fluorescence indicators present in the silica gel. Suggestions are most appreciated.
The fluorescence indicators are a zinc silicate compound that absorb light @ 254 nm and emit in the visible range. They work because a large number of compounds have some absorbance at 254 nm, although it may be weak, and this absorbance reduces the amount of light hitting the indicator, which reduces the flourescence.
If the extra spots you are seeing are dark, it probably isn't any of the F254 indicator but more likely impurities of some kind, possibly decomposition products. Depending on how much solvent used to pick up the compounds to separate them from the silica, possibly some residue from there although you'd need probably 100 mL (or more) of ACS grade solvent to concentrate the impurities to the point that you might notice.
We have used Silica gel G-coated TLC plates impregnated with silver nitrate with good success to separate terpenes (material came from Sigma-Aldrich and Merck). Alternatively, allumina is a good matrix for preparative separation. It very much depends on the compounds you want to enrich/separate.
I think there should be no problems with normal silica gel G used for TLC. You can use that. For a brand, I will advice Merck and Sigma.
The fluorescence indicators are a zinc silicate compound that absorb light @ 254 nm and emit in the visible range. They work because a large number of compounds have some absorbance at 254 nm, although it may be weak, and this absorbance reduces the amount of light hitting the indicator, which reduces the flourescence.
If the extra spots you are seeing are dark, it probably isn't any of the F254 indicator but more likely impurities of some kind, possibly decomposition products. Depending on how much solvent used to pick up the compounds to separate them from the silica, possibly some residue from there although you'd need probably 100 mL (or more) of ACS grade solvent to concentrate the impurities to the point that you might notice.
I think there may be two reasons either you scrapped the layer from the preparative plate and put it in methanol which may take some impurities from silica like phthalates or polyesters
or you may have left the layer soaked in the solvent somewhile that make decomposition of your compound
also let me know in which solvent you soaked your layer
Dear Dr. Silver if the amount of impurities from solvent is high how to get rid of them from the compound?
@Dr. Malash;
Assuming it is solvent impurities, I'll run another column, often "orthoganol" to the purification column. For example, if I run silica, I may run C18 or CHP-20. It doesn't have to be good chromatography because the compound is mostly pure.
Another thing I might do if the mystery spots from the solvent elute before your compound (high Rf on silica TLC), I would run the solvent that elutes the spots, then do a step gradient to get my compound off all at once, minimizing the solvent and potential contaminants. If the unwanted spots show a lower Rf than my compound (elute slower), I would use a solvent system that flushes my compound off quickly but not the spots. This is essentially "filtration chromatography", so load the column heavily since I'm assuming the impurities can be well resolved from the compound.
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