AQA A-Level› Organic Chemistry› 3.3.16 Chromatography› Flashcards
3.3.16

Flashcards

Interactive study deck for TLC, column chromatography, gas chromatography, and GC-MS.

Chromatography: read all 18 questions and answers
  1. Define chromatography.
    An analytical technique used to separate and identify components in a mixture based on their relative affinities for a stationary phase and a mobile phase.
  2. What is the stationary phase in thin-layer chromatography (TLC)?
    A thin layer of solid silica gel or alumina coated onto a glass or plastic plate.
  3. What is the mobile phase in thin-layer chromatography (TLC)?
    A liquid solvent (or mixture of solvents) that moves up the plate.
  4. Explain the mechanism by which components separate on a silica TLC plate.
    Adsorption. Polar components adsorb strongly to the polar silica, moving more slowly, whereas less polar components dissolve better in the solvent and move faster.
  5. Write the equation used to calculate Rf value in TLC.
    Rf = distance moved by substance / distance moved by solvent front.
  6. What are the units for Rf values and retention times?
    Rf values are dimensionless (no units); retention times are measured in seconds or minutes.
  7. Why must the solvent level in the TLC beaker be below the line where the sample spots are applied?
    To prevent the sample spots from dissolving directly into the solvent pool in the bottom of the beaker.
  8. Why is a lid placed on the TLC tank during development?
    To prevent evaporation of the mobile phase solvent, ensuring the tank atmosphere is saturated with solvent vapor.
  9. Name two methods used to locate colorless spots on a TLC plate.
    Spraying with ninhydrin (which turns purple upon reacting with amines/amino acids) or viewing the plate under ultraviolet (UV) light.
  10. What is the stationary phase in column chromatography?
    A solid adsorbent (usually silica gel or alumina powder) packed into a vertical glass column.
  11. How does column chromatography separate and collect components of a mixture?
    A solvent (eluent) is added to the top. Components move down the column at different rates and are collected in separate flasks (fractions) as they wash out (elute) at the bottom.
  12. What is the mobile phase in gas chromatography (GC)?
    An inert carrier gas, such as helium, neon, or nitrogen.
  13. What is the stationary phase in gas chromatography (GC)?
    A high-boiling non-volatile liquid coated on a solid support (or capillary column wall) inside a coiled column.
  14. Define retention time in gas chromatography.
    The time taken for a component to travel from the injection port to the detector.
  15. What two factors determine the retention time of a compound in gas chromatography?
    The compound's volatility (boiling point) and its relative solubility in the liquid stationary phase.
  16. How is the concentration of a component determined from a gas chromatogram?
    By measuring the area under its peak (peak area is directly proportional to concentration).
  17. What is GC-MS and why is it useful?
    Gas chromatography coupled with mass spectrometry. GC separates the components, and MS immediately determines their molecular masses and fragmentation patterns to identify them.
  18. How does increasing the polarity of the mobile phase affect Rf values in TLC?
    The mobile phase will compete more effectively for the polar sites on the stationary phase, causing all components to move further and increasing their Rf values.
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