Tlc how does it work

Because a mobile phase must be able to transport an analyte, inorganic solvents only work with inorganic materials, and organic solvents can work with either organic or inorganic materials. Confinement of a TLC plate in a chamber which has its headspace the air in the chamber saturated with solvent vapor allows for elution of a sample by capillary action.

The solvent simply rises up the slide and brings the analyte with it. Solvents are not strictly one and only one compound. Different solvents can be mixed in varying ratios to achieve a good separation.

Determining the optimum solvent mixture for your TLC experiment can be challenging, as there are no steadfast rules governing this procedure. It is almost entirely a matter of building experience through trial and error. However, understanding how chromatography works can make your guesswork a bit more educated. Allow the capillary to cool down, and then break it in the middle. Make sure that you break off the closed end on one of them. Do not use gloves when you pull capillaries.

You will have much better control without them! The thin end of the spotter is placed in the dilute solution; the solution will rise up in the capillary capillary forces. Touch the plate briefly at the start line.

Allow the solvent to evaporate and spot at the same place again. This way you will get a concentrated and small spot. The spots should be far enough away from the edges and from each other as well. If possible, you should spot the compound or mixture together with the starting materials and possible intermediates on the plate. They will serve as internal reference since every TLC plate is slightly different.

The solvent level has to be below the starting line of the TLC, otherwise the spots will dissolve away. The lower edge of the plate is then dipped in a solvent. Non-polar solvents will force non-polar compounds to the top of the plate, because the compounds dissolve well and do not interact with the polar stationary phase.

Take the plate out and mark the solvent front immediately. Do not allow the solvent to run over the edge of the plate. Next, let the solvent evaporate completely. Works well for. Unsaturated and aromatic compounds. Brown spots. Not permanent. Sulfuric acid. General stain. Brown or black spots. Chromic acid. The diagram shows a small part of the silica surface. The surface of the silica gel is very polar and, because of the -OH groups, can form hydrogen bonds with suitable compounds around it as well as van der Waals dispersion forces and dipole-dipole attractions.

The other commonly used stationary phase is alumina - aluminium oxide. The aluminium atoms on the surface of this also have -OH groups attached. Anything we say about silica gel therefore applies equally to alumina. As the solvent begins to soak up the plate, it first dissolves the compounds in the spot that you have put on the base line. The compounds present will then tend to get carried up the chromatography plate as the solvent continues to move upwards. How fast the compounds get carried up the plate depends on two things:.

Suppose the original spot contained two compounds - one of which can form hydrogen bonds, and one of which can only take part in weaker van der Waals interactions.

The one which can hydrogen bond will stick to the surface of the silica gel more firmly than the other one. We say that one is adsorbed more strongly than the other. Adsorption is the name given to one substance forming some sort of bonds to the surface of another one. Adsorption isn't permanent - there is a constant movement of a molecule between being adsorbed onto the silica gel surface and going back into solution in the solvent.

Obviously the compound can only travel up the plate during the time that it is dissolved in the solvent. While it is adsorbed on the silica gel, it is temporarily stopped - the solvent is moving on without it. That means that the more strongly a compound is adsorbed, the less distance it can travel up the plate.

In the example we started with, the compound which can hydrogen bond will adsorb more strongly than the one dependent on van der Waals interactions, and so won't travel so far up the plate. It is very unlikely that both will hydrogen bond to exactly the same extent, and be soluble in the solvent to exactly the same extent. It isn't just the attraction of the compound for the silica gel which matters.

Attractions between the compound and the solvent are also important - they will affect how easily the compound is pulled back into solution away from the surface of the silica. However, it may be that the compounds don't separate out very well when you make the chromatogram. In that case, changing the solvent may well help - including perhaps changing the pH of the solvent.

This is to some extent just a matter of trial and error - if one solvent or solvent mixture doesn't work very well, you try another one. Or, more likely, given the level you are probably working at, someone else has already done all the hard work for you, and you just use the solvent mixture you are given and everything will work perfectly!

Jim Clark Chemguide. Measuring R f values If all you wanted to know is how many different dyes made up the mixture, you could just stop there. What if the substances you are interested in are colorless? There are two simple ways of getting around this problem. Using fluorescence You may remember that I mentioned that the stationary phase on a thin layer plate often has a substance added to it which will fluoresce when exposed to UV light.

Showing the spots up chemically In some cases, it may be possible to make the spots visible by reacting them with something which produces a coloured product. Using thin layer chromatography to identify compounds Suppose you had a mixture of amino acids and wanted to find out which particular amino acids the mixture contained.

How does thin layer chromatography work? The stationary phase - silica gel Silica gel is a form of silicon dioxide silica. What separates the compounds as a chromatogram develops?