Flashcards · 7405 · A Level Topic 34

Chromatography (A Level) flashcards

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What does separation in chromatography depend on?: The balance between solubility in the moving phase and retention by the stationary phase.
Rf value: Distance moved by the spot ÷ distance moved by the solvent front, both measured from the baseline. No units; between 0 and 1.
Retention time (GC): The time from injection into the column until the component reaches the detector.
Stationary and moving phases in TLC, column chromatography and GC: TLC: solid on a plate; liquid solvent moving up. Column: solid packed in a column; solvent moving down. GC: solid, or liquid-coated solid, in a column; gas under pressure.
Spot centre 3.15 cm and solvent front 7.00 cm from the baseline. Rf?: 3.15 ÷ 7.00 = 0.45
TLC: why draw the baseline in pencil and keep the solvent below it?: Pencil graphite does not dissolve or separate, unlike ink. If the solvent were above the baseline, the samples would dissolve into the solvent pool.
What does GC-MS do?: Gas chromatography separates the components; each then passes into a mass spectrometer, which identifies it from its molecular ion (and precise m/z).
Why cover the TLC tank with a lid?: It saturates the air with solvent vapour, stopping solvent evaporating from the plate so the front rises evenly.