Resolution, Efficiency and the van Deemter Curve · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Splitting peaks at the best speed

Science · Chemistry · ages 19-20
Name ______________________   Date ____________

  1. Which basic van Deemter contribution increases when mobile-phase speed u becomes very low?

    • Longitudinal diffusion, B/u
    • Mass-transfer resistance, Cu
    • Packed-path contribution, A

  2. For similar, roughly Gaussian peaks, which Rs value is a common near-baseline separation target?

    • Rs about 0.5
    • Rs about 1.0
    • Rs about 1.5

  3. In the basic packed-column van Deemter model, A represents different flow paths and is treated as independent of speed.

    Circle one:   True   False

  4. Why does plate height rise at high speed in the basic van Deemter model?

    • A increases linearly with u
    • Cu increases linearly with u
    • B/u increases linearly with u

  5. Two peaks have retention times of 40 and 46 seconds and baseline widths of 4 and 4 seconds. Calculate Rs.

    Answer: ______________

  6. Measured H values are 8, 6 and 7 micrometres at u = 1, 2 and 4 mm/s respectively. Which tested speed gives the best efficiency?

    • 2 mm/s
    • 1 mm/s
    • 4 mm/s

  7. At fixed plate height and mobile-phase speed, making a column longer must make its peaks narrower in seconds.

    Circle one:   True   False

  8. Two analytes have nearly identical retention factors despite adequate retention and efficiency. Which trial most directly targets selectivity?

    • Increase the length of the same column
    • Change to a different stationary phase
    • Raise the speed far above the optimum
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Splitting peaks at the best speed W1-mt_RUjdSb5Rgw-s1

  1. Longitudinal diffusion, B/u · Reducing u increases B/u. Slow passage allows more longitudinal diffusion.
  2. Rs about 1.5 · Rs about 1.5 is a common target under these assumptions, not an exact boundary for every peak shape.
  3. True · A is the constant packed-path term in this model; it is not a claim about every possible column.
  4. Cu increases linearly with u · Cu increases with speed. It represents resistance to mass transfer between phases.
  5. 1.5 · Rs = 2(46 - 40)/(4 + 4) = 12/8 = 1.5.
  6. 2 mm/s · The lowest measured H is 6 micrometres at 2 mm/s. This need not be the exact curve minimum.
  7. False · Length increases plate number and retention time. Resolution can improve without narrower absolute time widths.
  8. Change to a different stationary phase · A phase change may alter relative retention. More length mainly changes efficiency.
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