I've got a TLC method which checks for impurities in a particular cyanine dye. This dye is symmetrical, which should reduce the number of possible impurities! It is reasonably soluble in methanol.
The method works very well, but it uses as eluent 1 + 1 chloroform + methanol. I'd like to avoid the use of chloroform and the like if possible: replacing it by any other halogenated solvent is therefore not an option. I've been unable to find any suggestions for how to go about choosing an eluent, the books I've got just say that solvent X is good for certain separations (and those don't usually include dyes of any sort!) The plates I'm using are silica gel F or G with UV fluorescer: we need the method rather quickly, so I don't want to have to buy any new plates because of the lead time on delivery.
I've tried several possibilities: methanol (alone) works but the spots are much less compact than with chloroform/methanol. 1 + 1 methanol + ethyl acetate gives significant tailing of the spots. 1 + 1 methanol + water gives dreadful tailing! With ethyl acetate on its own, the dye doesn't move. Can anybody suggest an approach which might lead to a successful eluent?
Dear David Beveridge,
This is one of an important question that needs a comprehensive response that gets to the heart of the matter.
Actually, (For column chromatography the correct solvent system should give an Rf between 0.2 and 0.3.).Then, here's a list of some standard solvents and their polarity (from LLP) which can pick it:
Very polar additives:
Methanol > Ethanol > Isopropanol
Moderately polar additives:
Acetonitrile > Ethyl Acetate > Chloroform >
Dichloromethane > Diethyl Ether > Toluene
Non-polar additives:
Cyclohexane, Petroleum Ether, Hexane, Pentane.
So, based on a description of your case, we can say that dye is nonpolar but impurities are moderately polar to very polar. On the other hand, the Chloroform solvent was mentioned as moderately polar, Therefore you can try to utilizing another Moderately polar solvent such as Acetonitrile or Toluene.
Best regards
Dear David Beveridge,
This is one of an important question that needs a comprehensive response that gets to the heart of the matter.
Actually, (For column chromatography the correct solvent system should give an Rf between 0.2 and 0.3.).Then, here's a list of some standard solvents and their polarity (from LLP) which can pick it:
Very polar additives:
Methanol > Ethanol > Isopropanol
Moderately polar additives:
Acetonitrile > Ethyl Acetate > Chloroform >
Dichloromethane > Diethyl Ether > Toluene
Non-polar additives:
Cyclohexane, Petroleum Ether, Hexane, Pentane.
So, based on a description of your case, we can say that dye is nonpolar but impurities are moderately polar to very polar. On the other hand, the Chloroform solvent was mentioned as moderately polar, Therefore you can try to utilizing another Moderately polar solvent such as Acetonitrile or Toluene.
Best regards
Hi!
Another choice. I think that the system n.hexane: Ethyl acetate (8:2) is good for your case.
Regards
As you are seeing the increasing polarity increases the trailing, (methanol + ethyl acetate gives significant tailing of the spots. 1 + 1 methanol + water gives dreadful tailing!). Why don't you try with some low polar solvent (like DEE)+ methanol?
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