A forensic scientist uses paper chromatography to separate the inks from two pens found at a crime scene. Ink A produces three spots with Rf values of 0.23, 0.51 and 0.78. Ink B produces three spots with Rf values of 0.51, 0.65 and 0.78. Explain what Rf values are and what the results tell the scientist about the two inks.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Rf value = distance travelled by substance ÷ distance travelled by solvent
Model answer (5 marks)
An Rf value is the ratio of the distance travelled by a spot to the distance travelled by the solvent front.
Rf values are fixed for a given substance under the same conditions, so they can be used to identify substances.
Ink A and Ink B share two components, as they both give spots with Rf 0.51 and 0.78.
Ink A has an additional component with Rf 0.23 that Ink B lacks, and Ink B has an additional component with Rf 0.65 that Ink A lacks.
Therefore the two inks are not identical – they are different inks and did not come from the same pen.
Rf values are fixed for a given substance under the same conditions, so they can be used to identify substances.
Ink A and Ink B share two components, as they both give spots with Rf 0.51 and 0.78.
Ink A has an additional component with Rf 0.23 that Ink B lacks, and Ink B has an additional component with Rf 0.65 that Ink A lacks.
Therefore the two inks are not identical – they are different inks and did not come from the same pen.
Examiner tips
- Define Rf and explain its constancy; link shared Rf values to common components; point out the unique Rf values to show difference; conclude inks are different.
Common mistakes
- Confusing Rf with the distance travelled; not recognising that identical Rf values indicate common components; failing to state that the inks are different because of unique Rf values.
Mark scheme (5 marks)
- An Rf value is the ratio of the distance travelled by a substance (spot/dye/component) to the distance travelled by the solvent front
- Rf values are fixed/characteristic for a given substance under the same conditions, so they can be used to identify substances
- The two inks share two components in common (Rf values 0.51 and 0.78 appear in both)
- The two inks are not identical / they are different inks because Ink A has a component with Rf 0.23 that Ink B does not have, and Ink B has a component with Rf 0.65 that Ink A does not have
- The two pens / inks did not come from the same source / are not from the same pen, because their compositions are different
Key terms in this question
Rf value · paper chromatography
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