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1.2.4Principles of Chemistry

Interpreting Chromatograms

5 min read 5 parts Lesson 1 of 50 Unit quiz

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Section 1 of 5Reading a Chromatogram

1.2.4 Interpreting Chromatograms

A chromatogram is the paper showing the separated components of a sample after a chromatography run.

Pure substance
  • Gives a single spot, because it contains only one component that moves at one rate in the solvent.
Mixture
  • Gives several spots, because each component has a different solubility in the solvent and so travels a different distance.
Reference spot
  • A known compound spotted alongside the sample, so that matching spots can be compared and an unknown can be identified.
Matching spots
  • Spots that line up at the same height as a known substance suggest that the two substances are the same.

Section 2 of 5Calculating Rf

Calculating Rf
Formula
  • The Rf value equals the distance moved by the substance divided by the distance moved by the solvent, both measured from the baseline.
Measurement
  • Measure from the baseline to the centre of the spot and from the baseline to the solvent front, using the same units for both.
Properties
  • Rf is a ratio with no units, and it is always less than 1, so values can be compared directly with reference data.
Dependence
  • The Rf value of a substance is fixed for a given solvent, but it changes if the solvent is changed.

Section 3 of 5Identifying an Unknown

1
Run the unknown alongside known reference substances on the same chromatogram, using the same solvent for every spot.
2
Measure the distance moved by the solvent front and by the centre of each spot from the baseline.
3
Calculate the Rf value for each spot and compare it with the values of the known substances from the same solvent.
4
Conclude that the unknown matches a reference substance only where both the spot position and the Rf value agree.

Section 4 of 5Worked Example

Question

  • On a chromatogram the solvent front moved 8.0 cm from the baseline and one spot moved 5.2 cm from the baseline.
  • Calculate the Rf value of the spot.

  1. Rf = distance moved by substance ÷ distance moved by solvent.
    What the symbols mean: Rf has no unit and is always between 0 and 1 · both distances are measured from the baseline
  2. Rf = 5.2 ÷ 8.0.
  3. Rf = 0.65.

Answer The Rf value is 0.65 (no units).

Section 5 of 5Exam Checklist & Tips

1.2.4 – Key Points for the Exam

  • Chromatogram The visible pattern of spots on the paper after a chromatography run.
  • Baseline The pencil line on which the samples are spotted before the run starts.
  • Solvent front The furthest point the solvent reaches on the paper, marked after the run.
  • Rf value The distance moved by a substance divided by the distance moved by the solvent.
  • Reference spot A spot made from a known substance, placed for comparison.

Common Mistakes

  • Measuring the spot distance to its edge rather than to its centre, which gives an incorrect Rf value.
  • Measuring from the bottom edge of the paper rather than from the baseline, which changes the distance travelled.
  • Stating that Rf values are the same for any solvent; changing the solvent changes the Rf value.
  • Calling a chromatogram with three spots a pure substance; more than one spot shows a mixture.
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