Chromatography techniquesAQA A-Level Chemistry: Mind map

Exam questions on Chromatography techniques

1. A student separates the dyes in a black ink by thin-layer chromatography (TLC). A line is drawn near the bottom of a plate coated with silica, spots of the ink and of three known dyes are placed on the line, and the plate is stood in a shallow depth of solvent in a covered beaker.
   Explain why the different dyes in the ink travel different distances up the plate.2 marks
2. In a TLC experiment the solvent front moved 7.5 cm from the baseline. A pure dye P moved 4.5 cm from the baseline. A sample of an unknown ink, run on the same plate, gave two spots, at 4.5 cm and 1.8 cm from the baseline.
   Calculate the Rf value of the second spot in the ink and state, with a reason, which of the two substances in the ink is more strongly retained by the stationary phase.2 marks
3. A forensic chemist analyses a mixture of volatile organic compounds using gas chromatography linked to mass spectrometry (GC–MS). The column is packed with a solid coated with a liquid, and an inert gas is passed through it under pressure at high temperature. The mixture gives three peaks, with retention times of 2.1, 3.8 and 5.2 minutes. A standard of compound S, run under identical conditions, has a retention time of 3.8 minutes.
   Explain how gas chromatography separates the components of the mixture and how mass spectrometry is used after the separation.3 marks