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Lecture 8

Lecture 8. Van Deemter Equation!. (. ). k’. 1. 1+k’. 4. efficiency. selectivity. retention. Resolution. Describes how well 2 compounds are separated. Rs = . N 1/2 (  -1). t R -t M. 1 < k’ < 10. k’ = . t M. (. ). k’. 1. 1+k’. 4. Resolution.

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Lecture 8

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  1. Lecture 8 Van Deemter Equation!

  2. ( ) k’ 1 1+k’ 4 efficiency selectivity retention Resolution Describes how well 2 compounds are separated Rs = N1/2 (-1) tR-tM 1 < k’ < 10 k’ = tM

  3. ( ) k’ 1 1+k’ 4 Resolution Describes how well 2 compounds are separated Rs = N1/2 (-1) L Maximize N N = H L H

  4. Resolution • L - length of column • Cannot increase indefinitely • Limited by: • Long runs times • Back pressure (LC) • H - height equivalent of a theoretical plate • Measure of Efficiency • Always want to minimize H • Getting the best performance from system • H depends on: • column parameters • mobile phase • flow rate Described by Van Deemter

  5. B  ∞ Van Deemter Equation A + H + C  is flow rate

  6. Van Deemter Equation B A + ∞ H + C  C H H min A B (flow rate)

  7. Van Deemter Equation A term ‘Multipath Effect’

  8. Van Deemter Equation A term ‘Multipath Effect’ Ce = particle shape dp = diameter of particle A Ce dp ∞ • A term • Entirely dependent on column • Only important in LC

  9. Van Deemter Equation A term ‘Multipath Effect’ A ∞ H H A (flow rate)

  10. Van Deemter Equation B term ‘Longitudinal diffusion’

  11. Van Deemter Equation B term ‘Longitudinal diffusion’ DMP ∞ DMP = diffusivity of mobile phase B  • B term • Inversely proportional to flow rate (fast) • Only important in GC (DMP of a gas) • Typical LC flow rate 0.2-0.5 mL/min • Typical GC flow rate 1-2 mL/min

  12. Van Deemter Equation B term ‘Longitudinal diffusion’ B ∞ H  H B (flow rate)

  13. Van Deemter Equation C term ‘Mass transfer’ dt2 dt = diameter of tube DMP = diffusivity of MP ∞ GC C m DMP dp2 dp = diameter of particles DMP = diffusivity of MP  = tortuosity ∞ LC C m DMP

  14. Van Deemter Equation C term ‘Mass transfer’ dt2 ∞ GC C m DMP dp2 ∞ LC C m DMP

  15. Van Deemter Equation C term ‘Mass transfer’ dt2 ∞ GC C m DMP dp2 ∞ LC C m DMP

  16. Van Deemter Equation C term ‘Mass transfer’ dt2 ∞ GC C m DMP dp2 ∞ LC C m DMP

  17. Van Deemter Equation C term ‘Mass transfer’ ∞ H C C H (flow rate)

  18. Van Deemter Equation GC B X A + ∞ H + C  C H H min A B (flow rate)

  19. Van Deemter Equation GC B ∞ H + C  C H H min B (flow rate)

  20. Van Deemter Equation GC   DMP dt2 ∞ + H m DMP   C H H min B (flow rate)

  21. Van Deemter Equation GC • Ideal Column (open tubular): • Small internal diameter (dt) • Use length to increase N (N=L/H) • Ideal Mobile Phase: • High diffusivity to C term and allow higher flow rates

  22. Van Deemter Equation LC B X A + ∞ H + C  C H H min A B (flow rate)

  23. Van Deemter Equation LC A + ∞ H C C H A (flow rate)

  24. Van Deemter Equation LC    dp2  + Ce dp ∞ H  DMP  C H A (flow rate)

  25. Van Deemter Equation LC • Ideal Column (packed): • Small particles (dp) • Uniform particles (Ce and ) • Cannot use length to increase N • Ideal Mobile Phase: • High diffusivity (DMP) to C term and allow higher flow rates

  26. Van Deemter Equation LC dp2 + Ce dp ∞ H  DMP Dong, M. Today’s Chemist at Work. 2000, 9(2), 46-48.

  27. Van Deemter Equation LC dp2 + Ce dp ∞ H  DMP Ascentis Express, Supelco, technical information

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