MEKC is most commonly performed

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Migration Time (Elution Window for Neutral
Solutes)
MEKC is most commonly performed with a robust
EOF directed towards the cathode. Generally, the
EOF of SDS micelle is less than the EOF So
micelle moves eventually to the cathode. Because
micelles are actually moving during the
separation process, they are called
pseudo- stationary phase (PSP) and migration
window is generated.
Det (-)
Inj ()
For neutral solutes, the migration window is
defined by the EOF time (to) and the micelle
migration time (tm) (aka. migration window (tm/t0)
A neutral solute, which spend no time with the
micellar phase will migrate at t0.
A neutral solute, which spend all of its time
with micellar phase will migrate at tm.
A neutral analyte, which partition between the
two phases will migrate at an intermediate time.
How to optimize migration window? Moderate
EOF, high mobility micelles, increase
surfactant concentrations
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Elution range ---? finite in MEKC Elution range
---? infinite in GC, HPLC
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Peak Capacity
--As tmc/t0 increases, elution range increases,
peak capacity increases ---As N improves peak
capacity increaes, but it has square root dependcy
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MEKC (Resolution)
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10
2
1
15
4
4
7
1
15
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Surfactant Type in MEKC
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INFLUENCE OF SAMPLE MATRIX IN MEKC
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The Resolution Equation in MEKC
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2. Effect of Intermicellar Diffusion
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Electrostatic attractive/repulsive,
hydrogen-bonding, hydrophobic
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40 mM LiPFOS
1 benzene, 2 benzonitrile, 3 nitrotoluene,
4 acetophenone, 5 anisole
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k nmc/naq ------------------(1)
k K Vmc/Vaq ------------------------------
---(3)
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-? The ability to control partitioning
through the phase ratio makes method
development easier in MEKC compared to HPLC
-? Above set of electropherograms shows
the theoretical influence of increasing the
phase ratio (by increasing surfactant
concentration) on the separation set of
compounds. It is clearly seen that there is an
optimum value of this phase ratio for a given
peak pair, (i.e., there is an optimum value) for
the capacity factor
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When the net mobility of the micelle and
bulk aqueous phase is in the same direction,
increasing the value of tmc/t0 increases
Rs. ultimate case is when micelles is
highly charged and have net movement towards
the detector. In such cases, window can be
increased by increasing the mobility of micelle
or decreasing the EOF.
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Impact of pH on the EOF (veo) and the
electro- phoretic velocity of the SDS micelle
(Vep) and the net direction of micellar migration
(vmc)
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(b)
(a)