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Topic: TLC invisible spot  (Read 2557 times)

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Offline m17

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TLC invisible spot
« on: May 06, 2018, 11:35:30 AM »
Hello!

Thank you in advance for reading my question and for helping me to solve my organic-practicals problem.

Recently, I had to separate three unknown organic compounds with TLC with silica gel on an aluminium plate as stationary phase. I had my substances dissolved in ethyl acetate.
I was thaught in my theoretical classes that if a spot doesn't occur on TLC plate it is suggested to vary the polarity of the eluent or the visualization method.

I tried different eluents by mixing petroleum ether and ethyl acetate (while increasing concentration of ethyl acetate up to 1:1 ratio) and also dichloromethane, chloroform and toluene (and mixtures of those substances).
As visualization method I used an UV-Lamp, KMNO4 and Phosphomolybdic acid.

However, I could always see just two spots instead of three.
Did you ever make such an experience or have an idea what could have been gone wrong?

Thank You very much for reading this and for your suggestions.   :)
And sorry for my bad English, I am not native.

Offline Mitch

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Re: TLC invisible spot
« Reply #1 on: May 06, 2018, 11:39:18 AM »
Do you have access to the three unknowns individually?
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Offline m17

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Re: TLC invisible spot
« Reply #2 on: May 06, 2018, 12:04:43 PM »
Unfortunately not, I was supposed to mix the compounds for the purpose of my experiment. 

Offline OrganicDan96

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Re: TLC invisible spot
« Reply #3 on: May 06, 2018, 12:55:54 PM »
what about using a different visualisation method such as permanganate stain

Offline Babcock_Hall

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Re: TLC invisible spot
« Reply #4 on: May 06, 2018, 02:00:12 PM »
OrganicDan96, The OP mentioned potassium permanganate.

Offline Arkcon

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Re: TLC invisible spot
« Reply #5 on: May 06, 2018, 02:31:18 PM »
When nothing else works, misting the developed plate with conc. sulfuric acid and heating it will give you the charred spots.  You can run the plate with parallel spots, and treat only one, if you need the invisible spot later for some reason.  At any rate, you'll have your answer -- if one sport is not separating, or is degrading before you can visualize it.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Mitch

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Re: TLC invisible spot
« Reply #6 on: May 06, 2018, 09:59:16 PM »
I had to do this once. Run in one solvent that separates two, turn TLC plate 90 degrees and use different solvent.
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Offline wildfyr

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Re: TLC invisible spot
« Reply #7 on: May 06, 2018, 11:36:50 PM »
Its called 2D TLC

Offline wildfyr

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Re: TLC invisible spot
« Reply #8 on: May 06, 2018, 11:39:43 PM »
And to be frank, I think the most likely problem here is an incorrectly made sample being given to you, not an issue with visualization.

Offline m17

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Re: TLC invisible spot
« Reply #9 on: May 07, 2018, 12:13:35 AM »
Hey!

Thank You all for your replies.
The Idea of 2D TLC sounds reasonable, I will try it.

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