Flashcards · 9620 · A Level Topic 34

Chromatography (A Level) flashcards

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What decides how fast a component moves in chromatography?: The balance between its solubility in the moving phase and its retention by the stationary phase.
Moving and stationary phases in TLC, column chromatography and GC: TLC: solvent rises up a thin solid layer on a plate. CC: solvent runs down a column packed with solid. GC: a gas carries the sample through a column of solid (which may be liquid-coated), at high temperature.
Retention time (GC): The time from injecting the sample into the column to a component reaching the detector.
How are components identified by TLC or GC?: Compare Rf values or retention times with standards run under the same conditions (same plate and solvent, or same column, temperature and gas flow).
GC-MS: The separated components leave the GC column one at a time and pass straight into a mass spectrometer, which gives a spectrum of each: the molecular ion gives Mr, and a precise m/z fixes the formula.
TLC technique: why a pencil baseline, solvent below the baseline and a lid?: Pencil does not dissolve (ink would run). Solvent below the baseline so spots are carried up, not dissolved away. The lid keeps the air saturated with solvent vapour so the plate does not dry.
Spot centre 3.8 cm and solvent front 5.0 cm above the baseline. Rf?: Rf = 3.8 ÷ 5.0 = 0.76 (no units; always between 0 and 1).