1. 1
   A forensic laboratory uses paper chromatography to separate the coloured dyes in ink recovered from a ransom note, comparing it against ink samples from several suspect pens.

   (a)

   Which statement correctly describes how paper chromatography separates a mixture of soluble coloured substances?

   [1 mark]

   • ADifferent substances in the mixture travel at different rates up the paper, carried by the solvent
   • BAll substances in the mixture travel at exactly the same rate up the paper
   • CThe mixture is separated by boiling point differences alone
   • DThe mixture reacts chemically with the paper to form new coloured compounds

   (b)

   The forensic scientist finds that the ink from the ransom note produces the same number of spots, at the same distances up the paper, as the ink from one particular suspect pen. What can be concluded from this observation alone?

   [1 mark]

   • AThe two inks have different solubilities in the solvent used
   • BThe two inks cannot possibly be the same, since chromatography always gives different results for different samples
   • CThe ink from the note must be a pure single substance
   • DThe two inks are likely to contain the same substances

   (c)

   State the equation used to calculate the Rf value of a substance in paper chromatography, and explain what this value tells the forensic scientist about a spot on the chromatogram.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A food safety inspector uses paper chromatography to check whether a soft drink contains a banned colourless artificial sweetener, which cannot be seen directly on the paper.

   (a)

   Which additional step is needed to detect this colourless substance?

   [1 mark]

   • AAdding limewater to the paper
   • BIncreasing the temperature of the solvent
   • CSpraying the paper with a locating agent
   • DUsing aqueous silver nitrate on the paper

   (b)

   After spraying the paper with the locating agent, the inspector calculates the Rf value of a newly visible spot from the soft drink sample and compares it with the known Rf value of the banned sweetener, measured under the same conditions. Which condition must be kept the same for this Rf comparison to be valid?

   [1 mark]

   • AThe paper used for each chromatogram must be a different type
   • BThe same solvent must be used for both chromatograms
   • CThe locating agent must be different each time
   • DThe starting line must be drawn in a different position each time

   (c)

   The inspector also runs a chromatogram of a genuine sample of pure banned sweetener as a reference. Explain why running this pure reference sample alongside the soft drink sample increases the reliability of the inspector's conclusion.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   An art conservator investigating a historic painting wants to identify the coloured pigments used in the paint without damaging the artwork significantly, and takes a tiny paint sample to analyse by paper chromatography. The conservator wants to compare the pigment spots with reference pigments known to have been available at the time the painting was made.

   (a)

   Describe how the conservator should set up the chromatography experiment, including how the sample should be applied to the paper.

   [3 marks]

   (b)

   Explain how the conservator could use this chromatogram, together with reference pigments known to have been available when the painting was made, to help determine whether the painting is a genuine historic work or a modern forgery.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A sports anti-doping laboratory wants to use paper chromatography to help screen athletes' urine samples for the presence of a banned, colourless performance-enhancing substance, alongside several known metabolites that are naturally coloured.

   (a)

   Explain, in full, how the laboratory should design and carry out this chromatography experiment to reliably identify the banned substance, including how both the coloured and colourless components would be visualised and compared.

   [6 marks]

   (b)

   A laboratory technician suggests that instead of comparing Rf values, the laboratory could simply check whether the athlete's chromatogram produces the same number of spots as a known clean reference sample, without calculating any Rf values at all. Evaluate whether this simplified approach would be a reliable way to detect the banned substance, and explain the advantages of calculating and comparing Rf values instead.

   [6 marks]

   Total for question 4: 12 marks

End of questions