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### Lecture 8

Van Deemter Equation!

)

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

- 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

A term

‘Multipath Effect’

Ce = particle shape

dp = diameter of particle

A

Ce dp

∞

- A term
- Entirely dependent on column
- Only important in LC

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

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

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

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

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