A student carries out thin-layer chromatography (TLC) to analyse a mixture of organic compounds. After running the chromatogram, the student measures the distances travelled by the solvent front and by each spot. Explain how the student uses these measurements to identify whether one of the compounds in the mixture is the same as a known reference compound, and describe one limitation of using TLC alone to confirm the identity of an unknown organic compound.

1. Calculate the Rf value of each spot by dividing the distance travelled by the spot by the distance travelled by the solvent front.
2. Calculate the Rf value of the reference compound under the same conditions.
3. Compare the two Rf values – if they are the same (to the precision of the measurement) the student concludes that the unknown spot is likely the same compound as the reference.
4. Note that different compounds can give identical Rf values in a particular solvent system, so a match does not prove identity.
5. To strengthen the evidence, the student should repeat the test with a different solvent (or a mixture of solvents); if the Rf values still agree, the likelihood of identity increases.
6. Finally, the student should confirm the identity with an additional technique such as mass spectrometry or infrared spectroscopy, because TLC alone cannot provide definitive confirmation.