6.3.1 (a) TLC interpretation

(a) interpretation of one-way TLC chromatograms in terms of Rf values

In years 7 and 8 Chromatography usually involves felt-tip pens and ink spots moving up a piece of filter paper in a 250 ml beaker.

In practice, it's difficult to get the inks to separate fully because they have similar solubilities in water and there is a limit to how high water will climb through filter paper.

If the water moved indefinitely up an infinite piece of paper even inks with almost the same solubility would separate.

In theory, at least we can calculate the Rf value (Retardation Factor, hilarious if you're 11) for each ink

This question is a favourite for GCSE examiners.

TLC (Thin Layer Chromatography) plates look rather like paper from a distance

And they're usually cut from a sheet much like cutting filter paper.

The plate is generally Aluminium coated with a thin layer of an inert white solid like Aluminium Oxide.

The only advantage of a TLC plate is that it is stiff and so can stand in the solvent without collapsing and that it is rather easier to manufacture particles of powder of a fairly uniform size than it is to manufacture paper of equally uniform consistency.

The calculation is still the same, however.

TLC is a quick method of analysing a substance but it isn't infallible.

• • Lots of similar substances will have similar Rf values so can't be distinguished easily.

  • Unknown substances have unknown Rf values and therefore can't be identified this way.

However, it's not true to say that it only works for colourful compounds.

You can separate Amino Acids this way, you just can't measure their Rf values until you spray them with a substance that makes them colourful - in this case Ninhydrin