The information provided above (including the procedure page) may give the impression that TLC is a relatively simple procedure. However, consider the experience of conducting a TLC for the first time, where one might observe spots scattered throughout and blurred, streaked areas. Like any technique, improvement comes with practice. Below are examples of typical issues faced during TLC:
The compound runs as a streak rather than a spot:
The sample was overloaded. Run the TLC again after diluting your sample. Or, your sample might just contain
many components, creating many spots which run together and appear as a streak. Perhaps, the experiment did not go as well as expected.
The sample runs as a smear or a upward crescent:
Compounds which possess strongly acidic or basic groups (amines or carboxylic acids) sometimes show up on a TLC plate with this behavior. Add a few drops of ammonium hydroxide (amines) or acetic acid (carboxylic acids) to the eluting solvent to obtain clearer plates.
The sample runs as a downward crescent:
Likely, the adsorbent was disturbed during the spotting, causing the crescent shape.
The plate solvent front runs crookedly:
Either the adsorbent has flaked off the sides of the plate or the sides of the plate are touching the sides of the container (or the paper used to saturate the container) as the plate develops. Crooked plates make it harder to measure Rf values accurately.
Many random spots are seen on the plate:
Make sure that you do not accidentally drop any organic compound on the plate. If get a TLC plate and leave it laying on your workbench as you do the experiment, you might drop or splash an organic compound on the plate.
You see a blur of blue spots on the plate as it develops:
Perhaps you used an ink pen instead of a pencil to mark the origin
No spots are seen on the plate:
You might not have spotted enough compound, perhaps because the solution of the compound is too dilute. Try concentrating the solution, or spot it several times in one place, allowing the solvent to dry between applications. Some compounds do not show up under UV light; try another method of visualizing the plate (such as staining or exposing to iodine vapor). Or, perhaps you do not have any compound because your experiment did not go as well as planned. If the solvent level in the developing jar is deeper than the origin (spotting line) of the TLC plate, the solvent will dissolve the compounds into the solvent reservoir instead of allowing them to move up the plate by capillary action. Thus, you will not see spots after the plate is developed. These photos show how the yellow compound is running into the solvent when lifted from the developing jar.