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Chromatography: Flashcards

Revise stationary and mobile phases, Rf values, TLC technique, polarity, retention times and peak areas.

10 cardsIntermediate

10 cards

  1. Question
    What are the two phases in chromatography?
    Answer

    The stationary phase (stays in place: paper, silica, column coating) and the mobile phase (moves: solvent or gas).

  2. Question
    Why do substances separate?
    Answer

    They have different attractions to the two phases; those held more strongly by the stationary phase move more slowly.

  3. Question
    Define Rf.
    Answer

    Distance moved by the spot ÷ distance moved by the solvent front, both from the baseline. No unit; between 0 and 1.

  4. Question
    Why is the baseline drawn in pencil?
    Answer

    Pencil (graphite) does not dissolve and move; ink contains dyes that would separate too.

  5. Question
    Why must the solvent be below the baseline?
    Answer

    Otherwise the spots dissolve into the solvent instead of travelling up the plate.

  6. Question
    On a polar silica plate, which has the higher Rf: a polar or a non-polar compound?
    Answer

    The non-polar compound: polar compounds are held strongly by the silica and move less.

  7. Question
    How is an unknown spot identified?
    Answer

    By matching its Rf with standards run on the same plate (same solvent, stationary phase and temperature).

  8. Question
    What is retention time in GC?
    Answer

    The time from injection to the peak for a substance; under fixed conditions it identifies the substance.

  9. Question
    What does the area of a GC peak show?
    Answer

    The amount of that substance; calibration with standards turns area into concentration.

  10. Question
    Does one spot prove a sample is pure?
    Answer

    Not on its own: two substances can share an Rf. Use a second solvent or method to confirm.