posted by ~christina~ on .
I have a question about TLC plate solvents. What solvents will leave stains on a plate with a TLC plate out of the ones below and why?)
(n-Octane, n-Hexane, Decane, Acetone, 1-Propanol, 2-Propanol, Ethanol, Toluene, Cyclohexane, Methanol, n-Heptane, n-Pentane, Butanol)
I saw 2 spots on the TLC plate I ran and was told that they could leave spots..so I was going to evaporate off the solvent and then use my own solvent to dissolve the compound but wasn't sure on which one would not leave a spot. (so I could know whether the second spot was a compound or not)
I don't know but it's easy enough to spot the plates with those solvents and see. I'm not an ace on TLC chromatography; I didn't know any of those compounds would leave a spot.
Actually thinking about it, it is not hard to spot the plate with the compound in the different solvents since I'm using the same developing solvent.
I do have one last question. Is it possible to use the same solvent for dissolving the compound and for developing the plate as well?
i.e. ethyl acetate for dissolving and running the plate.
Since you don't know the composition of the unknown and the solvent, you really won't know until you try it. Have you been given an unknown with no instructions? Any hints? When I took organic qual, I had a set of instructions that I followed. After doing the "simple" unknowns (and some were not simple--simple just meant they were single component compounds), we were given two mixtures that could contain anywhere from one to five compounds plus the solvent, We were told one was water soluble and one was not water soluble; we had different instructions for each. The scheme we followed actually separated the compounds so we ended up with single compounds which we then identified using the "simple" compound techniques. That included the solvent. From what you have written, it appears to me you have been left holding the bag. I realize that crime scenes don't give any hints (unless you keep a watchful eye out for them) but susrely you have a scheme to follow.
I did try to use the GC to see what solvents I had (out of the above)but there was significant trailing in some of them so this complicated the situation. I initially thought that the trailing peak at about 4 min on polar, was the unknown compound but since octane and decane trailed, I don't know what to think now.
The hints we have received are in the context of the extraction, and what to do if you have, "basic/acidic/neutral" drug." What's interesting, is that I was told that there would be no way to find out what type of compound the unknown was.(base/acid/neutral compound) IF this is so, this would be useless information, right?!
We can have up to two unknown compounds and the solvents above which could include, water as well. (14 components possible) Solubility was not told to us but it would obviously be seen from the liquid they gave us. (solid on bottom or just liquid layer like I have)
Unfortunately the only, "instructions" given was the flow chart telling us how to perform extraction. This just left me searching all over the internet for specific details.
Even after reading the thread I am not sure I understand the questions. The solvents you list will not themselves leave a stain on the plate. However, depending on the make of plate and how it has been manufactured some of the polar solvents do shown a slight change in the shades of white which remains after the solvent has evaporated.
Yes, you can use the same solvent to spot the plate as develop the plate.
You can find out if the compound is an acid or alkaline by seeing if it extracts into acid solution (it is a base) or extracts into alkaline solution (it is an acid).
You can also get alumina TLC plates as well as silica TLC plates. Acids are difficult to move on an alumina TLC plate whereas amines (bases) move easily. On a silica plate amines tend to have lower Rfs than acids.
There are also stain reagents that give colours with amines on TLC plates. These give different colours (blues and purples) with some amines so you can tell them apart. Iodoplatinic acid is one such reagent.
Thank you Dr.Russ.