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The Rate Theory of Chromatography

The document discusses the rate theory of chromatography. It explains that the rate of elution and different paths molecules take through the stationary phase affect band broadening and peak shape. It then presents the Van Deemter equation, which accounts for different factors that contribute to band broadening, including eddy diffusion, longitudinal diffusion, and resistance to mass transfer between phases. These factors are affected by the velocity of the mobile phase. Van Deemter plots of plate height versus flow rate are useful for determining the optimal flow conditions.

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594 views2 pages

The Rate Theory of Chromatography

The document discusses the rate theory of chromatography. It explains that the rate of elution and different paths molecules take through the stationary phase affect band broadening and peak shape. It then presents the Van Deemter equation, which accounts for different factors that contribute to band broadening, including eddy diffusion, longitudinal diffusion, and resistance to mass transfer between phases. These factors are affected by the velocity of the mobile phase. Van Deemter plots of plate height versus flow rate are useful for determining the optimal flow conditions.

Uploaded by

Sana Batool
Copyright
© © All Rights Reserved
We take content rights seriously. If you suspect this is your content, claim it here.
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Download as DOCX, PDF, TXT or read online on Scribd
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The Rate Theory of Chromatography

A more realistic description of the processes at work inside a column takes account
of the time taken for the solute to equilibrate between the stationary and mobile
phase (unlike the plate model, which assumes that equilibration is infinitely fast).
The resulting band shape of a chromatographic peak is therefore affected by the
rate of elution. It is also affected by the different paths available to solute molecules
as they travel between particles of stationary phase. If we consider the various
mechanisms which contribute to band broadening, we arrive at the Van Deemter
equation for plate height;
HETP = A + B / u + C u
Where u is the average velocity of the mobile phase. A, B, and C are factors which
contribute to band broadening.
A - Eddy diffusion
The mobile phase moves through the column which is packed with stationary phase.
Solute molecules will take different paths through the stationary phase at random.
This will cause broadening of the solute band, because different paths are of
different lengths.
B - Longitudinal diffusion
The concentration of analyte is less at the edges of the band than at the center.
Analyte diffuses out from the center to the edges. This causes band broadening. If
the velocity of the mobile phase is high then the analyte spends less time on the
column, which decreases the effects of longitudinal diffusion.
C - Resistance to mass transfer
The analyte takes a certain amount of time to equilibrate between the stationary
and mobile phase. If the velocity of the mobile phase is high, and the analyte has a
strong affinity for the stationary phase, then the analyte in the mobile phase will
move ahead of the analyte in the stationary phase. The band of analyte is
broadened. The higher the velocity of mobile phase, the worse the broadening
becomes.
Van Deemter plots
A plot of plate height vs. average linear velocity of mobile phase.

Such plots are of considerable use in determining the optimum mobile phase flow
rate.P

Gas Chromatography Diagram

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