A forensic scientist uses thin-layer chromatography (TLC) to separate compounds found in an unknown ink sample. Four spots are visible on the TLC plate after development. The scientist calculates the Rf value of each spot. Explain what Rf values are, how they are calculated, and how the scientist could use Rf values to identify the compounds present in the unknown ink.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Thin-layer chromatography (TLC) is a technique used to separate and identify mixtures. Each compound travels a certain distance up the plate, and its Rf value can be compared to known reference values.
Model answer (5 marks)
Rf value is the ratio of the distance travelled by the compound to the distance travelled by the solvent front.
Rf = (distance travelled by spot)/(distance travelled by solvent front).
Rf values are always between 0 and 1 and have no units.
Each pure compound has a characteristic Rf value in a given solvent system.
The scientist compares the Rf values of the four spots on the unknown ink to the Rf values of reference compounds run under the same conditions.
If an unknown spot’s Rf matches that of a reference compound, the compound can be identified.
Rf = (distance travelled by spot)/(distance travelled by solvent front).
Rf values are always between 0 and 1 and have no units.
Each pure compound has a characteristic Rf value in a given solvent system.
The scientist compares the Rf values of the four spots on the unknown ink to the Rf values of reference compounds run under the same conditions.
If an unknown spot’s Rf matches that of a reference compound, the compound can be identified.
Examiner tips
- Use the exact definition of Rf and the formula; include the 0–1 range and no units. Show the comparison step and state that matching Rf values identify the compound. Mention that the comparison must be under identical conditions.
- Avoid giving extra information such as the mechanism of TLC or the nature of the solvent front.
Mark scheme (5 marks)
- Rf value is the ratio of the distance travelled by the compound (substance/spot) to the distance travelled by the solvent (front)
- Rf values are always between 0 and 1 (have no units)
- Each pure compound has a characteristic/unique Rf value (in a given solvent)
- The Rf values of the unknown spots are compared to Rf values of known/reference compounds (under the same conditions/same solvent)
- A match between the Rf value of an unknown spot and a reference value (in the same solvent) allows the compound to be identified
Key terms in this question
Rf value · thin-layer chromatography
Related
- All WJEC A-Level Chemistry (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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