Chemical Forums
Chemistry Forums for Students => Organic Chemistry Forum => Topic started by: Yossarian on October 28, 2005, 12:54:14 PM
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I've been having trouble answering this question on column chromatography, so I hope I can find some help here. Thank you in advance.
Once the chromatographic column has been prepared, why is it important to allow the level of the liquid in the column to drop to the level of the alumina before applying the solution of the compound to be separated?
I don't feel like I really know this, but I think it is because if you apply the solution with the compound to the column before the liquid drops to the level of the alumina, then there is a substantial likelihood some of the compound will be carried out of the column prematurely with the liquid remaining in the columning that was still above the level of the alumina, once it is drained out, and you will end up losing some of the compound that was meant to be separated and purified.
How is my answer? If I'm wrong, can somebody explain to me the correct response to this question? Again, thank you, and your help is greatly appreciated.
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For optimum separation efficiency, you want as narrow a band of sample in the plane of the fluid flow. An example can be observed using thin layer chromatography (which you should do anyway during the column chromatography). Drop a real fat drop next to a thin drop and allow the material to elute down the TLC plate. The thin drop usually resolves better.
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Hello, Dude,
Thank you for your reply to my question. I feel like I understand the main point you were making regarding the separation efficiency of a small sample versus a large one. However, I'm still not sure how that relates to my initial question. How does the size of the material to be eluted down the chromatography column relate to why it is important to allow the level of the liquid in the chromatographic column once it has already been prepared to drop to the level of the alumina before applying the solution of the compound to be separated?
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Think about it this way: if you add your sample in small amount of solvent directly onto "dry" surface of alumina, it will enter the solid phase almost at the same time. If you add it to the solvent above the alumina, it will take some time to sink into the solid phase. In the first case you have a small, but concentrated "drop", in the second - it is a large "drop" with blurry borders.
That's a common reasoning at work, hopefully I am right ;)
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Borek,
Thank your very much for your help in answering my question - I understand this problem much better now.
Once again, thank you!
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The amount of solvent used to disslove your sample should be as small as possible. The solvent above Silica gel or Aluminium oxide could dilute your sample solution. The aftermath is your separation result will be compromised. It is a general rule in column chromatogrophy.
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