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QLDChemistryQuick questions
Unit 4: Structure, synthesis and design
Quick questions on Chromatography techniques: TLC, GC and HPLC (QCE Chemistry Unit 4)
15short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is choosing the right technique?Show answer
A typical IA3 design or EA short response asks why a particular technique was chosen for a particular analyte. The decision tree:
What is apparatus?Show answer
A thin layer of silica (or alumina) on a glass or aluminium plate, dipped vertically into a shallow pool of mobile phase. Sample is spotted near the bottom (above the solvent line); the solvent rises by capillary action.
What is measurement: Rf value?Show answer
When the solvent front has risen close to the top of the plate, the plate is removed and the position of each spot is measured.
What is visualisation?Show answer
Coloured compounds are seen directly. Colourless compounds are visualised under UV light (silica plates often contain a UV indicator that fluoresces, leaving dark spots) or by staining with iodine, ninhydrin, or other reagents.
What is identification?Show answer
Compare Rf with known standards run on the same plate. Co-spotting (mixing the sample with a standard at the spotting line) confirms identity if the combined spot appears as a single spot rather than two.
What are applications?Show answer
Quick monitoring of reactions (is starting material consumed?); identification of food colourings, pigments, amino acids in protein hydrolysates; purity check (a single spot vs multiple spots).
What are limitations?Show answer
Qualitative only (rough Rf comparisons; not suitable for concentration measurement). Limited resolution. Cannot handle volatile samples.
What is measurement: retention time?Show answer
The time taken for a compound to travel from the injector to the detector. Each compound has a characteristic retention time under fixed conditions (column, oven program, flow rate).
What is detector?Show answer
Flame ionisation detector (FID) is most common; gives peak area proportional to the amount of carbon. Mass spectrometer (GC-MS) is the gold standard combination, identifying each peak by its MS fragmentation.
What is measurement: retention time and peak area?Show answer
Retention time identifies the compound (by comparison with a standard). Peak area is proportional to amount.
What is quantification?Show answer
Run standards of known concentration; plot peak area vs concentration; the slope is the calibration curve. Read the sample concentration off the curve.
What is strengths over GC?Show answer
Handles non-volatile and thermally sensitive samples. Wide range of polarities. Mr range up to several thousand.
What is q1?Show answer
Define in thin-layer chromatography. State two ways to reduce the of a non-polar analyte. [3 marks]
What is q2?Show answer
A HPLC analyte has retention time at flow rate . The void time is . Calculate the retention factor .
What is q3?Show answer
Compare HPLC and gas chromatography. (a) State one similarity. (b) State two differences.