I have purchased linear isoprene farnesyl pyrophosphate (as ammonium salt, dissolved in ammonia: methanol) and a bicyclic non-aromatic alcohol geosmin (dissolved in methanol). I am trying to find out the common solvent system for their determination by TLC. Both the compounds are non-polar. I have tried to use high polar solvent system in varying composition like Methanol:DCM (5 - 95%), Ethyl acetate: Hexane (5 - 95%), 100 % methanol, 10 to 85% methanol in water. In 10% methanol : DCM geosmin rises on the TLC sheet but FPP didn't move. In ethylacetate : Hexane, none of them rises and in Methanol : water, only FPP rises. In Methanol and water, only one spot of FPP is visible and its Rf value was found to be 0.77 but no spot is visible in Geosmin lane, not even at the spot point. I have stained them with basic permanganate stain and also with p-anisaldehyde stain.
On heating after staining with p-anisaldehyde, a bluish to brown spot is visible in FPP lane but nowhere in geosmin lane.
My queries are:
1. Can we determine volatile compounds on TLC sheet? ( in case of p-anisaldehyde staining, heating is required for the development of the color).
2. If we can determine VOCs on TLC sheet, then what should be the suitable staining method?
Kindly suggest me how can I proceed with the TLC determination of VOCs.
Thanks to all in advance.
Dear Pranav, it was the most unusual idea I've ever seen to analyse non-polar geosmin and fairly polar FPP using one TLC plate ( silica or RP?) .. actually it's possible but to do it you should first use highly polar system (Dioxane:water:25% NH3 = 6:3:1) but stop the elution at 50% of the plates length and then (after effective cold drying) use DCM:MeOH till the end of plate. I'd recommend you to use iodine vapours to visualize TLC it doesn't need heating the plate.
P.S. I wouldn't say geosmin and FPP are volatile compounds
Dear Dmitry,
I put a spot of geosmin on Si-TLC and tried to visualize it in Iodine fumes, but got nothing but a brown ring which may be of the methanol in which it is dissolved.
Secondly, FPP and Geosmin, both are non-polar (logP value is >3). That is why I am trying to analyze both on one TLC plate.
It may be the case that geosmin being very volatile, could not get adsorbed on TLC sheet and that is why I am unable to visualize its spot on TLC. Because, if it is not moving up in the solvent system, then it should be on the spotted place, but this is not in my case.
I suggest a TLC solvent of 1:1 acetic acid:acetonitrile; air drying the glass backed plate and spraying the plate with 6% phosphomolybdic acid (PMA) in ethanol, then developing the chromatogram by heating on a hot plate. The PMA solution should be filtered and a clear yellow color before use. Do not dip the plate in the PMA solution, spray the reagent on the silica gel surface to help keep the geosmin from dissolving away.
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