QCE Chemistry - Unit 4 - Properties and structure of organic materials

Analytical Techniques | QCE Chemistry

Learn how spectroscopy and chromatography evidence helps identify organic compounds in QCE Chemistry.

Updated 2026-05-17 - 5 min read

QCAA official coverage - Chemistry 2025 v1.3

Exact syllabus points covered

  1. Explain how amino acids can be separated and identified by paper/TLC chromatography, including intermolecular forces/solubility in mobile and stationary phase and retention (RF) values.
  2. Explain how amino acids can be separated and analysed by electrophoresis, including pH of buffer, isoelectric points, and movement of charged ions.
  3. Analyse data, including paper/TLC chromatograms and electrophoresis, to determine the identity of amino acids and retention factors. (Formula: $R_f = \frac{\text{distance moved by the amino acid}}{\text{distance moved by the solvent}}$)
  4. Analyse data from spectra, including mass spectroscopy and infrared to determine the identity and structure of organic molecules.

Analytical techniques give evidence about what a substance is, how pure it is and which functional groups it may contain. In organic chemistry, one technique rarely proves everything on its own. The strongest conclusions combine several pieces of evidence.

Spectroscopy summary

Original Sylligence diagram for spectroscopy summary.

Spectroscopy summary

Chromatography

Chromatography separates substances because they distribute differently between two phases:

  • the stationary phase, which stays in place
  • the mobile phase, which moves through or across the stationary phase

A substance that interacts strongly with the stationary phase moves more slowly. A substance that interacts more strongly with the mobile phase moves faster.

Paper and thin-layer chromatography

In paper chromatography or thin-layer chromatography, results often appear as spots. The $R_f$ value can be calculated:

$ R_f = \frac{\text{distance moved by substance}}{\text{distance moved by solvent front}} $

An $R_f$ value can help compare substances run under the same conditions. A pure sample usually gives one spot, while a mixture gives more than one spot.

However, $R_f$ values depend on the stationary phase, solvent, temperature and method, so they are not universal constants.

Gas chromatography

Gas chromatography separates volatile substances and records peaks at different retention times. A pure substance should give one main peak under suitable conditions. A mixture gives multiple peaks.

Peak area can be used to estimate relative amount, especially when calibration data is available.

Infrared spectroscopy

Infrared spectroscopy, or IR spectroscopy, measures absorption of infrared radiation by bonds. Different bonds absorb radiation at characteristic wavenumbers, so IR is useful for identifying functional groups.

Important signals include:

| Feature | What it may suggest | | --- | --- | | broad $\mathrm{O-H}$ absorption | alcohol or carboxylic acid | | strong $\mathrm{C=O}$ absorption | aldehyde, ketone, carboxylic acid, ester or amide | | $\mathrm{N-H}$ absorption | amine or amide | | $\mathrm{C-H}$ absorptions | hydrocarbon framework |

IR is especially good for ruling functional groups in or out. If a molecule is proposed to be an alcohol but the IR spectrum has no suitable $\mathrm{O-H}$ absorption, the proposed structure is probably wrong.

Be careful with the carbonyl group. A strong $\mathrm{C=O}$ peak tells you a carbonyl is present, but it does not by itself distinguish an aldehyde from a ketone, ester, carboxylic acid or amide.

Mass spectrometry

Mass spectrometry ionises molecules and separates ions by mass-to-charge ratio, written as $m/z$.

Useful features include:

  • the molecular ion peak, which can indicate relative molecular mass
  • the base peak, which is the tallest peak and is assigned 100 percent relative intensity
  • fragment peaks, which give evidence about parts of the molecule

Mass spectrometry is powerful because it can support the molecular mass and possible fragments. It still needs careful interpretation because different structures can sometimes produce related fragments.

Combining evidence

A good analytical answer explains how each piece of data supports or weakens a proposed structure.

For example, suppose an unknown compound:

  • gives one chromatography peak
  • has a strong IR absorption consistent with $\mathrm{C=O}$
  • has a molecular ion peak matching $\mathrm{C_3H_6O}$

This evidence suggests a relatively pure carbonyl compound with formula $\mathrm{C_3H_6O}$. Possible structures include propanal and propanone. More evidence would be needed to distinguish them confidently.

Purity vs identity

Purity and identity are different claims.

A sample may be pure but not the compound you expected. A sample may also contain the target compound plus impurities.

Analytical evidence should match the claim:

  • purity claim: one spot or peak, sharp melting point, expected lack of impurities
  • identity claim: matching functional group evidence, molecular mass, retention time or comparison with a known standard

Quick check

Exam traps worth knowing

  • Claiming a compound is identified from one peak alone.
  • Treating chromatography $R_f$ values as universal constants.
  • Ignoring impurities when interpreting unexpected peaks.
  • Confusing the molecular ion peak with the base peak in mass spectrometry.
  • Forgetting that a missing expected signal can be just as important as a present signal.

Sources