CAIE A-Level Chemistry 37.2 Gas–Liquid Chromatography
Practise defining phases and retention time, interpreting peaks and calculating percentage composition.
- Syllabus
- 2028–2030
- Course
- Chemistry 9701
- Level
- A2
Exam points
- identify an unreactive carrier gas as mobile phase and a high-boiling non-polar liquid as stationary phase
- define retention time from injection to detection and link longer times to stronger stationary-phase attraction
- calculate each component’s percentage as its peak area divided by total peak area, multiplied by 100
Covered syllabus points
37.2 Gas / liquid chromatography question 1
Lactic acid, , is the only monomer needed to form the polymer polylactic acid, PLA.
Question (a)
(a)
An impure sample of contains pentan-3-one as the only contaminant. The mixture is analysed using gas/liquid chromatography. The pentan-3-one is found to have a longer retention time than the lactic acid.
[ 3 ]
Question (i)
(i)
Explain what is meant by retention time.
[ 1 ]
Question (ii)
(ii)
Suggest suitable substances, or types of substances, that could be used as the mobile and stationary phases.
mobile phase
stationary phase
[ 1 ]
Question (iii)
(iii)
Describe how the percentage composition of the mixture can be determined from the gas/liquid chromatogram.
[ 1 ]
37.2 Gas / liquid chromatography question 2
Question (a)
(a)
A polypeptide formed from four amino acids, A, B, C and D, is completely hydrolysed and then analysed by gas-liquid chromatography.
The chromatogram produced is shown in Fig. 8.2.
Fig. 8.2
The number above each peak represents the area under the peak.
The area under each peak is proportional to the mass of the respective amino acid in the mixture.
[ 2 ]
Question (i)
(i)
Calculate the percentage by mass of amino acid A in the original mixture.
percentage by mass of amino acid A=
[ 1 ]
Question (ii)
(ii)
The retention time for amino acid D is the longest.
Explain why D has a longer retention time than the other amino acids A, B and C.
[ 1 ]
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