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These days I am working on a research project where I am supposed to run TLC for samples which are wax extracts from coconut butten nuts using chloroform as the solvent. All the samples have been concentrated to 10% from its original volume. I recently run a TLC for a sample using hexane(90%):diethyl ether(7.5%):acetic acid(1%) as the solvent system (this solvent system has been used to run similar wax extracts). But my TLC run resulted me very diffused streaks having some curves towards both the sides of the tlc plate. Can anyone tell me why this happens??enter image description hereenter image description hereenter image description here

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  • $\begingroup$ Welcome to Chem SE. Take a tour of our site. Visit the help center for more information on how SE works $\endgroup$ Commented Jun 8, 2017 at 6:29
  • $\begingroup$ Curvature: Maybe it is a defective TLC plate? Or you did not allow enough time for the solvent to saturate the TLC chamber. Band: If other TLC do not have the dark band: it is possible that there is an impurity in the solvent. $\endgroup$ Commented Jun 8, 2017 at 15:27
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    $\begingroup$ Since you said you spotted but got a band, the spot was always above the liquid level, right? Just asking to be absolutely sure. $\endgroup$ Commented Jun 9, 2017 at 8:18
  • $\begingroup$ Yeah, spot was few millimeters above the liquid level. Today I run phenolic extract (by methol) on a portion of the same TLC plate, it didn't make that problem. No streaky bands seen except the solvent front. Few spots seen. $\endgroup$
    – NT Prasad
    Commented Jun 9, 2017 at 12:30

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Several things that may help your situation, listed roughly in the order in which I'd try them:

1. Application of the sample to the TLC plate

TLC is most usually ran by applying a small spot of the sample to the plate, and allowing it to run up. Analytically, this gives the greatest resolution, and as such for things such as monitoring reactions or identifying components in column chromatography fractions, spots are the most widely used.

enter image description here

In some cases, the TLC plate is used to purify components of the sample, this is called preparative layer chromatography, or PLC for short. In PLC, bands are applied to the plate, to allow greater quantities of the sample to be applied and hence separated.

When applying bands to the PLC plate, it's crucial to ensure that the band doesn't get too close to the edges (as it has in the images you've uploaded), as a rule of thumb, at least 1/2 cm from each side should be left clean.

The reason for this is that the solvent doesn't run perfectly level up the plate, this is most evident at the edges of the plate. If you re-run your TLC plates with a shorter band, you should see a decrease in the curving effect that you're observing.

2. Quantity of sample loaded onto the TLC plate

Looking at the images you've provided, you may also have loaded too much sample onto the TLC plate, saturating the silica and reducing the separation.

You could try either increasing the size of the plate, increasing the thickness of the silica layer (special PLC plates are available with thicker layers of silica, allowing more material to be loaded), or simply just diluting your sample and loading less on.

3. Solvent system

Although you say that "this solvent system has been used to run similar wax extracts", its always worth trying different eluents with your chromatography, unless your sample is identical to the prep you're following. A different modifier (formic acid or trifluoroacetic acid often work well), or different solvent mixtures (methanol in dichloromethane is often a lifesaver) might help tighten up the bands.

Finally, and possibly most of all, make sure you completely dry the plate out before visualising it (leave it sat on the side for a few minutes or use a hot air gun).

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  • $\begingroup$ Thank you very much for your valuable comment. I just applied just a spot not a band. Eventhough I applied a spot finally I received some bandsas shown in pictures. As you mentioned the sample was a concentrated one. I will try to dilute it first. $\endgroup$
    – NT Prasad
    Commented Jun 9, 2017 at 0:45

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