A student uses paper chromatography to identify the amino acids present in a sample of protein hydrolysate. After running the chromatogram, the paper is sprayed with ninhydrin solution to reveal the spots. Explain how the student can use the finished chromatogram to determine whether any of the amino acids in the sample are the same as those in a known reference mixture.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Amino acids are colourless and must be located using a locating agent such as ninhydrin. A reference mixture containing five known amino acids is run alongside the protein hydrolysate sample on the same chromatogram.
Model answer (4 marks)
1. Measure the distance from the baseline to each spot in the sample and in the reference mixture.
2. Measure the distance from the baseline to the solvent front.
3. Calculate Rf for each spot: Rf = (distance of spot)/(distance of solvent front).
4. Compare the Rf values of the sample spots with those of the reference spots.
5. Where a sample spot has the same Rf as a reference spot (under identical conditions), that amino acid is identified as the same.
2. Measure the distance from the baseline to the solvent front.
3. Calculate Rf for each spot: Rf = (distance of spot)/(distance of solvent front).
4. Compare the Rf values of the sample spots with those of the reference spots.
5. Where a sample spot has the same Rf as a reference spot (under identical conditions), that amino acid is identified as the same.
Examiner tips
- Show the Rf calculation for at least one spot. Use the same units for all distances. State that identical Rf values indicate the same amino acid.
Common mistakes
- Using different solvent fronts for sample and reference. Confusing distance travelled by solvent front with distance from baseline. Assuming spots match without comparing Rf values.
Mark scheme (4 marks)
- Calculate / measure the Rf value for each spot in both the sample and the reference mixture
- Rf is the distance travelled by the spot divided by the distance travelled by the solvent front
- Compare the Rf value(s) of spots in the sample with the Rf value(s) of spots in the reference mixture
- If the Rf value of a spot in the sample matches that of a spot in the reference mixture (under the same conditions), the amino acid is identified as the same
Key terms in this question
chromatogram · reference mixture
Related
- All Cambridge International IGCSE Chemistry (0620) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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