Sometimes a developed TLC plate may show streaks, crescents or strangely shaped spots, and these are all indicative of different problems.
There are several reasons why you may see a streak up the plate. Firstly, you may have overloaded the plate, and if you reduce the amound of material spotted on the baseline, you will increase the resolution on your plate.
If your sample contains a complex mixture of products, then you may see them as a long streak on the plate, and you may need to consider different solvent systems and lower loading to see if you can resolve the different components.
In the absense of overloading the plate, some pure compounds can streak on the plate, due to posessing strongly acidic or basic groups, which cling to the active sites of the adsorbant. One way to remove the streaking is to add a few drops of ammonia (for amines) or formic acid (for carboxylic acids) to the eluting solvent.
Strangely shaped TLC spots
Sometimes spots appear as a downward pointing crescent, which often indicates that spotting caused some scoring of the plate, and some adsorbant layer became detached at the point of application. If double spotting is observed, this is often due to applying the sample to the baseline in a polar solvent, which gives a ring on the baseline, rather than a tight spot.
Examples of TLC problems
Can you determine what is the problem in each of the TLC plates sketched above?
How could you fix the problem shown?
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