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Topic: what causes streaking of TLC samples?  (Read 37085 times)

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Offline asdf

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what causes streaking of TLC samples?
« on: June 08, 2006, 10:19:30 PM »
I'm trying to separate a sample by TLC and I'm getting a "streak".  There are dark spots visible under UV inside this streaked area, and I'm curious as to what is causing the streaking to occur.

I've used several different eluents and varied proportions of eluents.  It either comes out streaked or condensed(very little separation), and I've even had half moon looking spots.

I'm pretty new to TLC (when it comes to finding my own solvent system rather than following a prescribed experiment from a lab class.), so any help would be appreciated.

If it matters, I'm using silica, trying to separate out an indole.
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Offline Mitch

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Re: what causes streaking of TLC samples?
« Reply #1 on: June 08, 2006, 10:33:52 PM »
Make your spot really lite. You can also dilute the sample or spot lightly and quickly.
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Offline barcrphd

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Re: what causes streaking of TLC samples?
« Reply #2 on: June 09, 2006, 12:35:18 AM »
i totally agree with mitch and i would like to add one more reason for streaking. this may be due to improper drying of the TLC spot before developing. this problem generally happens when one is forced to use solvents like DMF or DMSO for spotting.

Offline bbrols

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Re: what causes streaking of TLC samples?
« Reply #3 on: June 09, 2006, 02:13:40 AM »
I agree with the above, TLC is tricky and even when you think you have cracked it a new compound will give you issues.

I have had problems with compounds that are ever so slightly acidic, I found that adding a couple of drops of acetic acid to the solvent sorted my problem right out. yours are indoles so they will be slightly basic, try 2-3 drops of triethylamine. This should will prevent the acidic nature of the silica from protonating your indole.


Offline movies

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Re: what causes streaking of TLC samples?
« Reply #4 on: June 09, 2006, 01:28:05 PM »
Streakiness can also be a result of decomposition on the silica gel.  You can check for this by doing a 2-D TLC.  To try this, cut a square TLC plate and spot your stuff near one corner.  Run the plate up one direction, then take it out, let the solvent evaporate from the plate and then turn it 90 degrees and run it up again.  If your stuff is stable on TLC, then all of your spots should appear on the diagonal of the plate.  If they are decomposing, then you will see off-diagonal spots.

The other thing that commonly causes streaks is some kind of isomerization of your compound.  For example, with beta-ketoesters it is common to see some separation between the keto and enol tautomers on TLC.  In some cases (where tautomerization is slow) you can completely separate these spots on TLC, but if the two isomerize on the plate, you often see a big streak.

Offline asdf

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Re: what causes streaking of TLC samples?
« Reply #5 on: June 09, 2006, 05:12:26 PM »
it was due to a too concentrated solution.

and more specifically it's a methoxyindole i'm trying to separate, running a prep plate on it as i type.

thanks for the help.
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Offline chiralic

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Re: what causes streaking of TLC samples?
« Reply #6 on: June 11, 2006, 08:39:27 PM »
Hi asdf:

Sometimes a substance will move along a TLC plate as a long streak, rather than as a single discrete spot. This is the result of spotting the plate with too much substance, more than the moving solvent can handle. The solvent moves as much substance as it can, but a substantial amount of substance is left behind. The substance is dragged along by the solvent leaving a trail of substance that may sometimes span the entire distance between the starting line and the solvent front.

Streaking can be eliminated by systematically diluting the spotting solution until development and visualization show the substances moving as single spots, rather than elongated streaks.
If this doesn't work, you will have to move to a different solvent system.

Also, If you have access to journals:

1) Validation of Standardized High-Performance Thin-Layer Chromatographic Methods for Quality
    Control and Stability Testing of Herbals
    Journal of AOAC INTERNATIONAL Volume: 86 | Issue: 5: September 2003 Page(s): 909-915

2) On Journal of Chemical Education you can find this article:
    Thin-Layer Chromatography Experiments That Illustrate General Problems in Chromatography
     October 1996 Vol. 73 No. 10 p. 974

Regards,

Chiralic

Offline asdf

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Re: what causes streaking of TLC samples?
« Reply #7 on: July 06, 2006, 10:55:29 AM »
much appreciated, i'll look into that.
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