A-Level Biology October/November 2024 Q7(a): Paper chromatography is a technique that can be used to separate and identify different…

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  To identify the chloroplast pigments, the retardation factor ($R_f$) is calculated for each separated pigment spot on the chromatogram.

  The $R_f$ value is calculated using the formula:

  $R_f = \frac{\text{distance moved by pigment from origin}}{\text{distance moved by solvent front from origin}}$

  These calculated $R_f$ values are then compared with known, standard $R_f$ values for different chloroplast pigments, which are found in a reference table or database. A match between the calculated value and a known value identifies the pigment.

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  How the marks are awarded

  • B1 — The answer correctly states that the Rf value must be calculated for each pigment.
  • B1 — The correct formula for the Rf value is provided, defining it as the distance moved by the pigment divided by the distance moved by the solvent.
  • B1 — The answer explains that the calculated Rf values are then compared with known or standard values to achieve identification.

  Common mistakes

  • Inverting the Rf formula, i.e., dividing the solvent distance by the pigment distance.
  • Only mentioning the colours of the spots as the method of identification, without any reference to calculating Rf values.
  • Calculating the Rf value but failing to mention the crucial final step of comparing it to known/standard values.
  • Describing how to set up the chromatography experiment instead of how to interpret the results.

  Examiner tip: For any technique that generates quantitative results, ensure you can describe both how to calculate the key value (e.g., Rf, magnification) and how to use that value for interpretation or identification.

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