Lecture 8

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# Lecture 8 - PowerPoint PPT Presentation

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 &lt; k’ &lt; 10. k’ = . t M. (. ). k’. 1. 1+k’. 4. Resolution.

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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

(

)

k’

1

1+k’

4

Resolution

Describes how well 2 compounds are separated

Rs =

N1/2 (-1)

L

Maximize N

N =

H

L

H

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

B

Van Deemter Equation

A +

H

+ C

 is flow rate

Van Deemter Equation

B

A +

H

+ C

C

H

H min

A

B

(flow rate)

Van Deemter Equation

A term

‘Multipath Effect’

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

Van Deemter Equation

A term

‘Multipath Effect’

A

H

H

A

(flow rate)

Van Deemter Equation

B term

‘Longitudinal diffusion’

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

Van Deemter Equation

B term

‘Longitudinal diffusion’

B

H

H

B

(flow rate)

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

Van Deemter Equation

C term

‘Mass transfer’

dt2

GC

C

m

DMP

dp2

LC

C

m

DMP

Van Deemter Equation

C term

‘Mass transfer’

dt2

GC

C

m

DMP

dp2

LC

C

m

DMP

Van Deemter Equation

C term

‘Mass transfer’

dt2

GC

C

m

DMP

dp2

LC

C

m

DMP

Van Deemter Equation

C term

‘Mass transfer’

H

C

C

H

(flow rate)

Van Deemter Equation

GC

B

X

A +

H

+ C

C

H

H min

A

B

(flow rate)

Van Deemter Equation

GC

B

H

+ C

C

H

H min

B

(flow rate)

Van Deemter Equation

GC

DMP

dt2

+

H

m

DMP

C

H

H min

B

(flow rate)

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

Van Deemter Equation

LC

B

X

A +

H

+ C

C

H

H min

A

B

(flow rate)

Van Deemter Equation

LC

A +

H

C

C

H

A

(flow rate)

Van Deemter Equation

LC

dp2

+

Ce dp

H

DMP

C

H

A

(flow rate)

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

Van Deemter Equation

LC

dp2

+

Ce dp

H

DMP

Dong, M. Today’s Chemist at Work. 2000, 9(2), 46-48.

Van Deemter Equation

LC

dp2

+

Ce dp

H

DMP

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