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Offline Rutherford

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Thin layer chromatography
« on: July 21, 2012, 07:24:52 AM »
First time I was studying about chromatography, and I used this text: http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html#top , but I didn't understand the concept so I have the following questions:

1.According to the picture, there is a vertically placed plate in a glass, and then it is said "A pencil line is drawn near the bottom of the plate and a small drop of a solution of the dye mixture is placed on it", but how to place a small drop of the dye mixture onto the pen line, wouldn't the drop fall down because of gravity?

2.I am not sure if I  understood the following thing-then the solvent is added so that it is under the drawn line, and the dye mixture components will move upwards because of the solvent vapors. Did I understand this correctly?

Offline discodermolide

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Re: Thin layer chromatography
« Reply #1 on: July 21, 2012, 07:39:23 AM »
First time I was studying about chromatography, and I used this text: http://www.chemguide.co.uk/analysis/chromatography/thinlayer.html#top , but I didn't understand the concept so I have the following questions:

1.According to the picture, there is a vertically placed plate in a glass, and then it is said "A pencil line is drawn near the bottom of the plate and a small drop of a solution of the dye mixture is placed on it", but how to place a small drop of the dye mixture onto the pen line, wouldn't the drop fall down because of gravity?

2.I am not sure if I  understood the following thing-then the solvent is added so that it is under the drawn line, and the dye mixture components will move upwards because of the solvent vapors. Did I understand this correctly?

You put the plate flat and spot your compound. Blow it dry, then put it in the tank.
When the plate is placed in the tank the solvent will migrate up the plate under capillary action thus causing a separation of the components.
The tank empty space will also be saturated with the solvent vapors. 
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Offline Rutherford

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Re: Thin layer chromatography
« Reply #2 on: July 21, 2012, 07:50:43 AM »
Thanks for the quick answer.
So, I put the plate horizontaly, mark one end with a pencil, add a drop of the mixture in the center of the drawn line, dry it so it will be adsorbed on the surface of the plate, I place  the plate in a tank filled with water (now vertically), and then I close the tank. Is this correct?

And about this: "When the spot of mixture is dry, the plate is stood in a shallow layer of solvent in a covered beaker. It is important that the solvent level is below the line with the spot on it."
How the capillary action occurs when the beaker with the solvent that is in the tank is covered, and how the vapors escape the covered beaker?

Offline Dan

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Re: Thin layer chromatography
« Reply #3 on: July 21, 2012, 08:07:02 AM »
It might help you to search for a video on youtube, there are many.
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Offline Rutherford

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Re: Thin layer chromatography
« Reply #4 on: July 21, 2012, 08:26:57 AM »
Thanks for the suggestion, found this great video http://www.youtube.com/watch?v=rbp_Qc4jMAc.

Offline Arkcon

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Re: Thin layer chromatography
« Reply #5 on: July 21, 2012, 08:46:59 AM »
Thanks for the quick answer.
So, I put the plate horizontaly, mark one end with a pencil, add a drop of the mixture in the center of the drawn line, dry it so it will be adsorbed on the surface of the plate, I place  the plate in a tank filled with water (now vertically), and then I close the tank. Is this correct?

Good.

Quote
And about this: "When the spot of mixture is dry, the plate is stood in a shallow layer of solvent in a covered beaker. It is important that the solvent level is below the line with the spot on it."

Raderford:, at over 300 posts on this board, you should be able to do some things for yourself.  This one is not a chemistry question.  Can you give us a reason why the dried spot shouldn't be submerged in solvent? 

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How the capillary action occurs when the beaker with the solvent that is in the tank is covered,

This way: http://en.wikipedia.org/wiki/Capillary_action  The TLC method is no different from narrow tubes, or cloth as it pertains to the physics of fluid travel.

 
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and how the vapors escape the covered beaker?

They don't and that's important.  If the solvent traveling up the plate dries out, you'll get poor results.

When you're presented with a block of text with many questions, you should split them up, on separate lines, and look at the answer to each one.  The ones that you can answer by yourself, you should leave out of posting to these forums.  You will need to do this, rapidly, on an examination, so you don't miss questions.
Hey, I'm not judging.  I just like to shoot straight.  I'm a man of science.

Offline Rutherford

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Re: Thin layer chromatography
« Reply #6 on: July 21, 2012, 10:47:41 AM »
"When the spot of mixture is dry, the plate is stood in a shallow layer of solvent in a covered beaker. It is important that the solvent level is below the line with the spot on it." -the wording confused me here a lot. Now I think I understand it. Thanks to everyone who helped me on this topic.

Offline discodermolide

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Re: Thin layer chromatography
« Reply #7 on: July 21, 2012, 11:26:42 AM »
"When the spot of mixture is dry, the plate is stood in a shallow layer of solvent in a covered beaker. It is important that the solvent level is below the line with the spot on it." -the wording confused me here a lot. Now I think I understand it. Thanks to everyone who helped me on this topic.

Not being rude, are you sure you understood it?
Why is this "It is important that the solvent level is below the line with the spot on it" statement so important?
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Offline Rutherford

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Re: Thin layer chromatography
« Reply #8 on: July 21, 2012, 12:30:00 PM »
The sentence before that one was confusing me the most. After I watched few times the videos on youtube, I think that I got everything.

Offline discodermolide

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Re: Thin layer chromatography
« Reply #9 on: July 21, 2012, 12:37:44 PM »
The sentence before that one was confusing me the most. After I watched few times the videos on youtube, I think that I got everything.

Ok, I was only making sure it was clear. ;D
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Offline Rutherford

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Re: Thin layer chromatography
« Reply #10 on: July 21, 2012, 03:55:35 PM »
Thanks for doing that ;).

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