A forensic scientist uses paper chromatography to analyse an ink sample taken from a document. The ink produces four separate spots on the paper. The scientist then tests a pure reference compound and it produces a single spot. Explain how the scientist can use the Rf value of each spot to identify the compounds present in the ink sample.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Paper chromatography separates substances according to how they distribute between the mobile phase and the stationary phase. The Rf value of a substance is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.
Model answer (5 marks)
The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent.
1. The forensic scientist measures the distance travelled by each of the four spots on the ink sample and divides by the solvent front distance to give four Rf values.
2. The scientist also runs a pure reference compound under the same conditions and records its Rf value.
3. Each ink spot’s Rf value is compared with the Rf values of known reference compounds that have been run in the same solvent system.
4. When an ink spot has an Rf value that matches that of a reference compound, the ink spot is identified as that compound.
5. Because four distinct Rf values are obtained, the ink is a mixture of four different compounds rather than a single pure substance.
1. The forensic scientist measures the distance travelled by each of the four spots on the ink sample and divides by the solvent front distance to give four Rf values.
2. The scientist also runs a pure reference compound under the same conditions and records its Rf value.
3. Each ink spot’s Rf value is compared with the Rf values of known reference compounds that have been run in the same solvent system.
4. When an ink spot has an Rf value that matches that of a reference compound, the ink spot is identified as that compound.
5. Because four distinct Rf values are obtained, the ink is a mixture of four different compounds rather than a single pure substance.
Examiner tips
- State the Rf calculation formula first. Show the four Rf values for the ink spots. Explain the comparison with reference compounds. Mention that a match gives the identity and that four spots mean a mixture.
Common mistakes
- Mixing up the order of the numerator and denominator in the Rf formula. Assuming the ink contains only one compound because it is a single sample. Failing to note that the reference must be run in the same solvent system.
Mark scheme (5 marks)
- The Rf value is calculated as the distance travelled by the dissolved substance divided by the distance travelled by the solvent
- An Rf value is calculated for each of the four spots in the ink sample
- The Rf value of each spot is compared to the Rf value of known/reference compounds
- A compound is identified when its Rf value matches that of a known reference compound (in the same solvent)
- Because the ink produces four spots, it is a mixture of four different compounds (not a pure substance)
Key terms in this question
Rf value · paper chromatography
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