9701 · 37.1
Thin-layer chromatography flashcards
Revision flashcards for Cambridge 9701 Thin-layer chromatography (syllabus 37.1). Flip, recall, then mark a real past-paper question.
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What is the stationary phase in thin-layer chromatography?
A solid adsorbent material, typically silica ($SiO_2$) or alumina ($Al_2O_3$), coated thinly onto a flat, inert support like a glass or plastic plate.
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What is the mobile phase in thin-layer chromatography?
A liquid solvent or mixture of solvents (the eluent) that moves up the plate by capillary action, carrying the components of the mixture with it.
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Define the Retention Factor ($R_f$).
The ratio of the distance travelled by a separated component to the distance travelled by the solvent front. It is a characteristic value for a specific compound under specific conditions.
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Why must the baseline on a TLC plate be drawn in pencil?
Ink from a pen contains a mixture of dyes which would also be separated by the chromatogram, interfering with the results. Pencil graphite is inert and will not move with the solvent.
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Why must the solvent level in the developing tank be below the baseline?
If the solvent level is above the baseline, the spotted sample will dissolve directly into the bulk solvent at the bottom of the tank instead of being separated as the solvent front moves up the plate.
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What is a locating agent in TLC?
A substance used to visualise colourless spots on a chromatogram. Examples include iodine vapour, UV light (for plates with a fluorescent indicator), and ninhydrin spray (for amino acids).
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How does UV light help visualise spots on a TLC plate?
The TLC plate is often coated with a fluorescent substance. When viewed under UV light, the whole plate glows. The spots of the compound quench this fluorescence and appear as dark patches.
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How are amino acids typically visualised on a TLC plate?
By spraying the plate with a ninhydrin solution and then gently heating it. The amino acids react with ninhydrin to produce purple-coloured spots (except for proline, which gives a yellow spot).
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A compound has a high $R_f$ value. What does this suggest about its properties?
It suggests the compound has a low affinity for the polar stationary phase (e.g., it is less polar) and a high solubility in the mobile phase. It has travelled further up the plate.
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Why is it important to place a lid on the developing tank during a TLC experiment?
To create a saturated atmosphere of solvent vapour inside the tank. This prevents the solvent from evaporating from the surface of the plate as it moves up, ensuring the solvent front moves evenly and the $R_f$ values are reproducible.
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How can TLC be used to assess the purity of a sample?
A pure sample should ideally produce a single spot on the chromatogram. The presence of multiple spots indicates that the sample is impure and contains several components.