It can be used during the reaction to monitor its progress and at the end of the reaction to access the purity of the product.
The heart of the technique is the chromatography plate which comprises special silica gel coated unto a glass, aluminum or plastic plate.
Small plate with thin coatings about a quarter of a millimeter thick is used for analytical work.
And these are usually kept in an oven or a desiccators to maintain their activity.
The choice of solvent is crucial to a successful TLC separation.
Non polar compounds require non polar solvents.
Polar compounds require polar solvents.
Frequently, a mixture of solvents is used which is tailored to the compound under investigation.
Take a prepared glass or aluminum plate or cut a piece of the appropriate size from a plastic plate.
Mark on it a line in pencil for the start line or origin.
This should be at about 1cm from the bottom edge. Do not use a ball point.
Place of small spots of liquid or solution of solids onto the start line using a drawn capillary tube.
Do not use a dropping pipette or melting temperature tube because these give spots much larger than the 2-3mm diameter required.
Use a fresh capillary for each solution to be spotted unto the plate.
Allow the spots to dry using air blower, compressed air or waive the plate about to dry it.
But do not breadth on the plate to dry them.
If the liquid samples is very dilute several spots may be applied
Wait for the solvent to evaporate before adding another spot.
Note the location of each sample as soon as it’s being applied.
Take a small screw cap bottle or a larger tank and cut a piece of filter paper to go into it.
Pour some of the eluting solvent over the filter paper.
Ensure that the filter paper dip into the solvent but the depth of the solvent is not so deep as to cover the spot on the plate.
Because the solvent used for TLC are often volatile and flammable, a TLC tank should be used in a fume cupboard.
Put the plate into the bottle or tank with a pair of tweezers taken care that the edges of the plate do not touch the sides of tank or the filter paper and that the solvent does not cover the spot on the plate.
Put the lid on to allow a saturated atmosphere of solvents to develop in the tank.
Over a period of time, the spot move up to the plate by capillary action as evident by the dark shadow of the solvent front advancing up the plate.
When the solvent front has almost reached the top of the plate, carefully remove it from the tank with tweezers and mark the edge of the plate the place that the solvent front has reached.
Allow the plate to dry in air or blow it dry with an air blower.
Some spots may be seen with the naked eye but others are revealed under ultraviolet light or by chemical staining for example using an iodine jar.
By dipping or spraying which is often followed by heating to develop the colors?
The utmost situation occurs when the spots travel about a halfway up the plate.
If the solvent is too polar, the spots end up close to the solvent front at the top of the plate.
In this case, a fresh plate should be eluted using a less polar solvent.
Very little travel means the eluting solvent was insufficiently polar. This time, the same plate can be used again with a more polar solvent.
Such reuse takes advantage of the non destructive nature of TLC
A problem often encountered in TLC is the presence of over spots.
This is called TAILING
Tailing indicates that the plate has been overloaded and can be overcome by spotting fewer compounds.
When monitoring a reaction, add the reference spot of the starting material alongside each spot of reaction mixture.
The initial plate shows that the reaction is yet to commence as the sample spot on the right is the same as the starting material reference spot on the left.
Later, two new spots indicate the presence of two reaction products.
Later still, reduction of the starting material spot shows that the reaction is well advanced.
And finally, its removal shows the reaction is complete.
The movement of the spot B relative to the solvent can be used to give information about substance B.
Carefully measure the distances X and Y to give a value X/Y which is the Rf value of B for this particular solvent.
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