Department of Chemistry /College of Sciences/ University of Baghdad Subject: Analytical Chemistry 4 Second stage 2nd semester Dr. Ashraf Saad Rsaheed 2017-2018

1. Lecture four
ChromatographyChromatography
Analytical Chemistry 4
Second stage
Dr.Ashraf Saad Rasheed

2. The dynamic theory (van Deemter equation)
1. In all of the above discussions in the theoretical plate model, the
solute diffusion and the velocity of the mobile phase in the column
were not taken into consideration.were not taken into consideration.
2. Consequently, the velocity must have an impact on the progress
of the solutes in the column outlet.
3 There is no real equilibrium created between the analytes in the3. There is no real equilibrium created between the analytes in the
mobile and stationary phases, due to the always-flowing mobile
phase in a chromatographic column.
4 The peak broadening happens because of several effects4. The peak broadening happens because of several effects
occurring in the chromatographic column. The first approach that
deals with band broadening in chromatography was proposed by
Van Deemter in 1956.a ee te 956
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3. • The Van Deemter equation describes the factors affecting band
broadening in a chromatographic separation. The van Deemterbroadening in a chromatographic separation. The van Deemter
equation is:
1 T A d ib dd diff i l k th ki1. Term A describes eddy diffusion, also known as the packing
factor. Non-retained analytes will not leave directly from the
column inlet to the column outlet. The solute also faced particles
of the stationary phase and it must move around themof the stationary phase and it must move around them.
Consequently, non-retained solutes may follow to a multi-pathway
in their travel via the column.
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4. Cu
HETP
HETP
H = A + B / u + Cu
A
B/u
H
Hmin.
Eddy diffusionTerm A:
Mobile phase linear velocity u
Uopt.
Term B: Longitudinal diffusion
Analyte
Resistance to mass transferTerm C:
1 1
2 2
Particles
of the stationary phase
DiffusionFlow direction
Term B: Longitudinal diffusion Resistance to mass transferTerm C:
2 2
3 3
Pore
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5. 1. The term A (eddy diffusion) in the van Deemter equation is:
2 I E i 1 12 d i h i l di d λ i2. In Equation 1.12, dp is the average particle diameter and λ is an
experimental packing factor (Coefficient describing the quality of
the packing). The more homogenous particles size in the column
(uniform particles) the closer the λ to one therefore it is an(uniform particles), the closer the λ to one, therefore, it is an
indication for the packing quality of the column.
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6. • Term B describes longitudinal diffusion.
• As a band of solute molecules travels in the mobile phase, it will
tend to diffuse in all directions, attributed to the concentration
gradient in the column.
• Thus, analyte diffusion along the travel direction of the mobile phase
in the chromatographic column will lead to peak broadening.
• To reduce the longitudinal diffusion the mobile phase velocity will set
to a reasonable value.
• The term B (longitudinal diffusion) in the van Deemter equation is
described by:
• In Equation 1.13, Dm is the diffusion coefficient of the analyte in the
mobile phase and δ is an obstruction factor, which describes themobile phase and δ is an obstruction factor, which describes the
obstruction of the free longitudinal diffusion due to collisions with
particles of the stationary phase.
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7. Cu
HETP
HETP
H = A + B / u + Cu
A
B/u
H
Hmin.
Eddy diffusionTerm A:
Mobile phase linear velocity u
Uopt.
Term B: Longitudinal diffusion
Analyte
Resistance to mass transferTerm C:
1 1
2 2
Particles
of the stationary phase
DiffusionFlow direction
Term B: Longitudinal diffusion Resistance to mass transferTerm C:
2 2
3 3
Pore
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8. • Term C is related to the resistance to mass transfer.
• The analyte molecules should be able to partition between the• The analyte molecules should be able to partition between the
stationary and mobile phases in order for an analyte to be retained.
• Accordingly, this indicate two processes.
• First resistance to mass transfer in the mobile phase C : The• First, resistance to mass transfer in the mobile phase Cm: The
analyte molecules are diffusing continuously from the mobile phase
to the stationary phase and back again during their travel through
the column This transfer process is not immediate; a limited time isthe column. This transfer process is not immediate; a limited time is
required for solutes to diffuse through the mobile phase in order to
access the interface and enter the stationary phase. This term is
given by:g y
• In Equation 2 14 f (k´) is a constant which represents a function of• In Equation 2.14, f (k ) is a constant which represents a function of
the retention factor, and r is the column radius. Term
Cm emanates from mass transfer in the mobile phase, which is the
first part of the C term in the van Deemter equationfirst part of the C term in the van Deemter equation.
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9. Cu
TP
HETP
H = A + B / u + Cu
A
B/u
HET
Hmin.
Eddy diffusionTerm A:
Mobile phase linear velocity u
Uopt.
1 1Particles
of the stationary phase
Term B: Longitudinal diffusion
Analyte
Resistance to mass transferTerm C:
2 2
3 3
of the stationary phase
Pore
DiffusionFlow direction
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10. • Second, resistance to mass transfer in the stationary phase CS:
• Once more, the analytes are in contact with the stationary phase, y y p
and may leave and reenter the mobile phase by diffusion.
• Before reentering the mobile phase, the analytes have a more or
less dispersed way through the stationary phase and, therefore,p y g y p , ,
varying distances for back diffusion to the surface on the stationary
phase. The term Cs is given by:
• Here d is the thickness of the film of stationary coated on the• Here, df is the thickness of the film of stationary coated on the
support and Ds is the diffusion coefficient of the analyte in the
stationary phase. The total resistance to mass transfer C is:
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11. • This phenomenon of eddy diffusion longitudinal diffusion and• This phenomenon of eddy diffusion, longitudinal diffusion, and
resistance to mass transfer are pictured in Figure 1.3. A typical
graphic of the plate high H versus the average linear velocity of
the mobile phase u in the column is shown in Figure 1.3.the mobile phase u in the column is shown in Figure 1.3.
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