ChromatographyEdexcel A-Level Chemistry: Subtopic test

1. 1
   A forensic analyst separates the dyes in a sample of black ink using thin-layer chromatography (TLC). The plate is coated with a thin layer of silica gel, and a solvent rises up the plate. When the plate is removed, the solvent front has travelled 8.0 cm from the baseline and the centre of one dye spot is 3.2 cm from the baseline.

   (a)

   What is the Rf value of the dye?

   [1 mark]

   • A0.40
   • B0.60
   • C2.5
   • D4.8

   (b)

   Which substance is the stationary phase in this experiment?

   [1 mark]

   • AThe solvent
   • BThe dye molecules
   • CThe silica gel on the plate
   • DThe solvent front

   (c)

   Explain why the dyes in the ink separate on the plate.

   [2 marks]

   Total for question 1: 4 marks

2. 2
   A laboratory tests urine samples from athletes for banned drugs using gas chromatography coupled with mass spectrometry (GC–MS). The sample is vaporised and carried through a long column by a gas.

   (a)

   What is the mobile phase in gas chromatography?

   [1 mark]

   • AA liquid solvent pumped at high pressure
   • BAn inert gas such as helium
   • CA solid packed into the column
   • DA liquid coated on the surface of the column

   (b)

   What is meant by the retention time of a component in this analysis?

   [1 mark]

   • AThe distance moved by the component divided by the distance moved by the solvent front
   • BThe time taken to vaporise the sample
   • CThe total time taken for the whole analysis
   • DThe time between injection of the sample and detection of the component

   (c)

   Explain why the gas chromatograph is coupled to a mass spectrometer.

   [2 marks]

   Total for question 2: 4 marks

3. 3
   A student separates a mixture of two amino acids by one-way thin-layer chromatography. After development, the solvent front is 12.5 cm from the baseline. Spot 1 has its centre 4.5 cm from the baseline and spot 2 has its centre 9.0 cm from the baseline. Under identical conditions, a reference sample of leucine has an Rf value of 0.72.

   (a)

   Calculate the Rf value of each spot and deduce which spot is leucine.

   [3 marks]

   (b)

   Explain, in terms of the mobile and stationary phases, why spot 2 has moved further than spot 1, and why the Rf value of leucine can only be used for identification under identical conditions.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   An analytical laboratory handles two types of sample. The first is urine from athletes, to be tested for banned drugs, some of which are large, polar molecules that decompose when heated. The second is debris from fire scenes, to be tested for traces of volatile accelerants such as petrol.

   (a)

   Describe how high-performance liquid chromatography (HPLC) coupled with mass spectrometry can be used to identify a banned drug in an athlete's urine sample, and explain why the components of the sample separate.

   [6 marks]

   (b)

   Evaluate the suitability of gas chromatography (GC) and of HPLC for analysing each of the two types of sample.

   [6 marks]

   Total for question 4: 12 marks

End of questions