High performance capillary electrophoresis - T.E.A.M.

Modes
From equation (25), resolution can be improved by optimizing
efficiency, selectivity, and/or the capacity factor. With
regard to the capacity factor, this can be most easily
ad justed by varying the concentration of the surfactant.
Generally, the capacity factor increases linearly with concentration.
A potential problem with the use of ionic surfactants,
especially at high concentrations, is the increase in
generated current. Power generation exceeding 5 to 10 W/m
at moderate field strengths can develop. Even with narrowbore
capillaries (25 to 50 mm) the use of extremely high
electric fields is often avoided and efficient capillary
thermostating is necessary.
Resolution is improved by extending the elution range or
time window. In the separation of neutral solutes, all solutes
elute between t 0
and t m
(figure 36). Hydrophilic solutes that
do not interact with the micelle elute with the EOF and
those that are totally retained by the micelles (Sudan III, for
example) elute with the micelles. While the time window is
often fairly small, the peak capacity can be very high due to
the high efficiency. It is therefore desirable to employ
conditions that open the time window, that is, moderate
EOF and micelles exhibiting high mobility.
Time window
EOF
Solutes
Micelle
Figure 36
Eluton time window for neutral solutes
in MEKC
0 t 0
t R t 1
R t t
2 R
3
m
Time
The selectivity can easily be manipulated in MEKC. Varying
the physical nature (that is, size, charge, geometry) of the
micelle by using different surfactants can yield dramatic
changes in selectivity, similar to those obtained by changing
65