Edexcel A-Level Chemistry Paper 3, June 2023: Question 6

7 marks · Medium difficulty · Practical Techniques and Data Analysis

Analyze amino acids using two-way chromatography, identify reagents for visualization, and state the technique used alongside gas chromatography in forensics.

Practise this question

Question

A multipart chemistry exam question about chromatography of amino acids. Part (a) asks how chromatography separates components. Part (b) includes a diagram of a developed chromatogram, a table of Rf values in two different solvents for alanine, glycine, and valine, a grid for completing a two-way chromatogram, and a question asking for a locating reagent. Part (c) asks for the technique used in conjunction with gas chromatography in forensic testing.
Question text

6 Amino acids can be separated using chromatography.

(a) State how chromatography separates the components of a mixture.

(1)

(b) A sample of a tripeptide was hydrolysed and then placed on an ‘X’ at the

bottom right-hand corner of a piece of chromatography paper.

A simplified diagram of a developed chromatogram is shown.

solvent front 1

baseline 1

(i) Give a possible reason for the presence of only two spots for the tripeptide

other than two amino acids have almost identical Rf values.

(1)

(ii) Some amino acid mixtures cannot be effectively separated in one

chromatography ‘run’.

The chromatography paper from the first run is dried but not developed.

The chromatography paper is then rotated clockwise by 90° and placed in a

different solvent.

Complete the simplified diagram of the developed chromatography paper

after a second ‘run’ for a tripeptide of alanine, glycine and valine by adding

labelled spots for each amino acid.

Amino acid Rf in solvent 1 Rf in solvent 2

14 alanine 0.38 0.43

*P71914A01432*glycine0.330.26

valine 0.39 0.58

(3)

solvent front 2

baseline 1

solvent front 1

baseline 2

(iii) Name a reagent that locates colourless amino acids by producing a

coloured compound.

(1)

(c) State the technique that is used in conjunction with gas chromatography (GC)

when carrying out forensic testing.

*P71914A01532* (1)

(Total for Question 6 = 7 marks)

Mark scheme

Show the mark scheme The mark scheme provides expected answers for parts 6(a) through 6(c), detailing reference points regarding mobile and stationary phases, identical Rf values or duplicate amino acids, correct plotting and labeling on the two-way chromatography grid based on Rf values, ninhydrin as the locating reagent, and mass spectrometry for the gas chromatography coupling.

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

6(a)

• each component in the mixture is attracted to mobile and Allow affinity for ‘attracted to’

stationary phases but more strongly to one than to the other Allow different for ‘more strongly’

Allow solvent for mobile phase

Do not allow reference to reacting with either of the

phases

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

6(b)(i)

• two of the amino acids present are the same Ignore one amino acid has not moved from the original

pencil line

Ignore one amino acid is still ‘seen’

Ignore one amino acid is still present on the original

pencil line

Ignore one amino acid is insoluble

Question

Answer Additional Guidance Mark

Number

Allow a tolerance of ±¼ 0.25 of a square within each large square (3)

6(b)(ii) • 3 spots moved in correct (horizontal) position for solvent 1 (1) Any alterations to the grid dimensions scores (0)

• 3 spots moved in correct (vertical) position for solvent 2 (1)

• labelling of alanine, glycine and valine provided in correct Rf

position for M1 or M2 (1)

Example of suitable diagram

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

6(b)(iii)

• ninhydrin Allow phosphomolybdic acid (commonly referred to as PMA) or p-

anisaldehyde or cerium molybdate (Hanessian’s stain) or

bromocresol green

Question

Answer Additional Guidance Mark

Number

An answer that makes reference to the following point: (1)

6(c)

• mass spectrometry Allow mass spectroscopy

Allow just ‘MS’

Allow mass spec

Allow infrared spectroscopy, UV spectroscopy, visible

spectroscopy, fluorescence spectroscopy

Allow NMR

(Total Question 6 = 7 marks)

How to answer it

Edexcel A-Level Chemistry: Chromatography & Amino Acids Study Guide

What this question tests

This question assesses your understanding of thin-layer/paper chromatography principles, two-way chromatography data processing and plotting (Rf values), visualising agents for colourless compounds, and analytical coupling techniques used in forensics (GC-MS).

Question 6 (a)

Principles of Chromatography

State how chromatography separates the components of a mixture.

✅ Correct Answer

Each component in the mixture has a different affinity/attraction for the mobile phase (solvent) and the stationary phase, interacting more strongly with one than the other.

💡 Key Knowledge

Separation relies on differential partitioning between a stationary phase (e.g., solid paper/silica) and a mobile phase (solvent). Components that dissolve better in the mobile phase travel further.

❌ Common Errors

Do not state that components chemically "react" with the stationary or mobile phases. The separation is physical, based on intermolecular forces and relative solubility/adsorption.

Mark: 1 mark
Question 6 (b)(i)

Interpreting Chromatograms

Give a possible reason for the presence of only two spots for the tripeptide other than two amino acids have almost identical Rf values.

✅ Correct Answer

Two of the amino acids present in the tripeptide are the same (e.g., alanine-glycine-alanine).

🧠 Exam Technique

Read the stem carefully: a tripeptide contains 3 amino acid residues after complete hydrolysis. If only 2 spots appear and their Rf values are distinct, it mathematically means a repeating residue must be present.

Mark: 1 mark
Question 6 (b)(ii)

Two-Way Chromatography Plotting

Complete the simplified diagram of the developed chromatography paper after a second 'run' for a tripeptide of alanine, glycine and valine.

✅ Correct Answer

Three distinct spots plotted accurately using the provided Rf values for both solvent 1 (horizontal axis) and solvent 2 (vertical axis), with correct labels for alanine , glycine , and valine .

📐 Calculation & Plotting Guide

  1. Glycine: Solvent 1 = 0.33, Solvent 2 = 0.26
  2. Alanine: Solvent 1 = 0.38, Solvent 2 = 0.43
  3. Valine: Solvent 1 = 0.39, Solvent 2 = 0.58
  4. Measure distance from baseline 1 / baseline 2 proportionally to the total distance to the respective solvent fronts. Ensure proper labelling.

❌ Common Errors

Altering the grid dimensions or misinterpreting which baseline corresponds to which solvent run will result in zero marks. Ensure spots sit within the grid coordinate boxes corresponding to the given Rf values.

Marks: 3 marks
Question 6 (b)(iii)

Locating Agents

Name a reagent that locates colourless amino acids by producing a coloured compound.

✅ Correct Answer

Ninhydrin (Also accepted: phosphomolybdic acid / PMA, p-anisaldehyde, cerium molybdate, or bromocresol green).

💡 Key Knowledge

Ninhydrin reacts with primary amines (amino acids) to form a purple/blue complex ( Ruhemann's purple), making invisible spots visible.

Mark: 1 mark
Question 6 (c)

Forensic Analytical Techniques

State the technique that is used in conjunction with gas chromatography (GC) when carrying out forensic testing.

✅ Correct Answer

Mass spectrometry (Acceptable abbreviations: MS , mass spec ).

💡 Key Knowledge

Coupling Gas Chromatography with Mass Spectrometry ( GC-MS ) allows the separation of complex mixtures via GC, followed by unambiguous structural identification and mass fragmentation data from MS.

Mark: 1 mark

Topics

Organic Chemistry · Core Practicals · Topic 19: Modern Analytical Techniques II · Topic 18: Organic Chemistry III

Question and mark scheme from the Edexcel A-Level Chemistry examination, Paper 3, June 2023. QuestionVault is an independent revision resource; questions remain the copyright of the awarding body.