JibbyDee
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Why do my TLC plates never work?
I'm doing half a semester of organic chem lab at the moment and everything is going well except for my TLCs. For some reason it never works for me.
Every time, I end up with a big streak that spans the whole width of the TLC plate. There is usually a few small spots that traveled different
distances but theres also a big streak that traveled almost as far as the solvent front. This streak is always up the top of the TLC plate and always
travels pretty much as far as the solvent front. What am I doing wrong? What causes a big streak that crosses every lane and spans the whole width of
the TLC plate to form? I considered that the mobile phase in the jar might be too deep and that when I drop the plate in, the spots get submerged in
the mobile phase but I tried spotting my compounds half way up the TLC plate to be 100% sure that this does not happen and it didn't change anything,
I still ended up with a streak. I also tried running a plate with a single spot to see what happened and again, I ended up with one spot that didn't
travel too far, and a streak that travelled almost as far as the solvent front. What is the problem here? Am I spotting too much compound onto the
plate or something?
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DJF90
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Maybe something in your solvent? Have you tried running a blank TLC plate? What compounds are you using? And your eluant?
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JibbyDee
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Didn't try running a blank TLC plate, no. I tried 2 different mobile phases: 100% dichloromethane and 80:20 ethyl acetate:n-hexane. The compounds I
spotted were anthracene, N-bromosuccinimide and 9-bromoanthrcene.
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JibbyDee
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I've been googling and found out that streaking is caused by spotting a solution that is too concentrated onto the plate. I had been dropping a small
amount of the substance into a small plastic vial and filling the vial with solvent. I see now that I only need to add a near microscopic amount of
substance into a vial this size.
A quick side question: Does it matter what side of the tilted TLC plate is facing the solvent when you put it in the jar (developing chamber)?
Obviously, if the TLC plate is tilted so that the silica side is facing the solvent, more solvent vapour will be hitting the TLC plate. With that in
mind, is it better to have the silica side facing upward toward the lid of the jar?
[Edited on 9-12-2011 by JibbyDee]
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fledarmus
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It's better to have a way to equilibrate the vapor across as much of the TLC chamber as possible. This is usually done by placing a piece of filter
paper against one wall of the chamber, so that it will draw solvent up against the wall and keep the vapors in equilibrium. Usually, I lean me TLC
plates against the opposite wall, with the silica gel facing the center of the chamber.
If you are getting a solid streak in the lane that you are spotting, yes, you are spotting too heavily. Take the vial that you made up, remove about
1/10 of the material into another vial and fill it back up with solvent, then take 1/10 of that sample and dilute it in a third vial. Spot all three
vials on one plate and see if there is any improvement. I usually get adequate results spotting my NMR samples, which are ~5mg/mL
If you are seeing thick bands across the entire solvent front, you might have impurities in your solvent or in your TLC chamber. The blank TLC plate
will show these as well. Wash your chamber out thoroughly and dry it, then refill it and try again. If you are still seeing the streaks, your solvents
have a UV active impurity.
One other possibility - certain high-boiling solvents can make a streaky mess of your TLC. Anything with DMSO or DMF particularly, you will have to
get rid of the solvent by extracting a small aliquot of your reaction mixture, before you will be able to get a good TLC.
[Edited on 9-12-2011 by fledarmus]
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Sedit
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Quote: Originally posted by JibbyDee  | | I've been googling and found out that streaking is caused by spotting a solution that is too concentrated onto the plate. |
I was going to tell you just this, to dilute your sample but it appears you already have your answer, It makes the world of difference when it comes
to resolution as you shall soon see. GL dude.
Knowledge is useless to useless people...
"I see a lot of patterns in our behavior as a nation that parallel a lot of other historical processes. The fall of Rome, the fall of Germany — the
fall of the ruling country, the people who think they can do whatever they want without anybody else's consent. I've seen this story
before."~Maynard James Keenan
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peach
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Grab yourself a cup of tea and enjoy the following nerd presentations;
<iframe sandbox width="640" height="480" src="http://www.youtube.com/embed/e99nsCAsJrw" frameborder="0" allowfullscreen></iframe>
<iframe sandbox width="640" height="480" src="http://www.youtube.com/embed/ml58GCq078o" frameborder="0" allowfullscreen></iframe>
{edit} I love these videos by MIT. They're really well made and free, and there are more of them. The only two things I can add are that, if you are
unsure about the solvents, you can always give them a rough test on filter papers (preferably fine ones) prior to going onto the plates, and that you
can use fine syringe blades with the tips ground flat (on a piece of sand paper) as spotters if you've not got any micropipettes. You can also buy
syringe blades that are flat to begin with, they're usually used for precisely dispensing glue or brazing compounds.
[Edited on 14-12-2011 by peach]
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