A pharmaceutical company is developing a new painkiller. Quality control chemists use thin-layer chromatography (TLC) to check the purity of a synthesised compound. The synthesised compound is spotted onto a TLC plate alongside a reference standard of the pure compound and the plate is developed in a suitable solvent. The synthesised compound produces two spots on the developed TLC plate, whereas the reference standard produces only one spot. The chemists then use nuclear magnetic resonance (NMR) spectroscopy to investigate the impurity further. Explain what the TLC results indicate about the purity of the synthesised compound, and explain how NMR spectroscopy could be used to help identify the impurity.

Edexcel A-Level Chemistry (9CH0) — 19.1 NMR and chromatography · Explain · 5 marks · View as Markdown

Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).

Model answer (5 marks)

The two spots mean the sample contains two different substances. Each spot has a different Rf value because the two substances interact differently with the stationary phase and therefore travel different distances.
The Rf of one spot matches that of the reference standard, so that spot is the desired painkiller.
The other spot is the impurity.
NMR gives a spectrum with signals at different chemical shifts, each signal representing protons in a distinct chemical environment. By comparing the number, position and integration of the peaks with reference data or databases, the structure of the impurity can be deduced.

Examiner tips

  • State that two spots = two components; mention Rf differences; match one spot to standard; explain NMR gives chemical shifts; compare peaks to reference data.
  • Use correct terminology: Rf, stationary phase, chemical shift, integration.
  • Keep answer concise and directly address both parts of the question.

Common mistakes

  • Confusing Rf values with retention times; not recognising that one spot matches the standard; giving generic NMR explanation without linking to impurity identification.

Mark scheme (5 marks)

  1. The synthesised compound produces two spots, showing it contains two different substances / components
  2. Each spot has a different Rf value because each substance travels a different distance relative to the solvent front, depending on its interaction with the stationary phase
  3. The Rf value of one spot from the synthesised compound matches the reference standard, confirming which spot corresponds to the desired compound
  4. NMR spectroscopy produces a spectrum showing peaks / signals at different chemical shifts, each corresponding to hydrogen atoms (protons) in different chemical environments
  5. The number and position of peaks in the NMR spectrum can be compared to reference data / databases to identify the structure of the impurity

Key terms in this question

thin-layer chromatography (TLC) · nuclear magnetic resonance (NMR) spectroscopy

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