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

Modes
EOF [mm/min]
50
40
30
20
10
These covalent modifications are intended to be permanent
and to require little or no maintenance. Since the capillaries
are usually washed after use (adsorption may occur even
with the coating), they must be stable to washing solutions
and to hydrodynamic flow. Unfortunately, the stability of
most coatings is limited. It is anticipated that numerous
types of stable coatings will soon be purchasable, similar to
LC and GC columns.
0
-10
-20
0 2 4 6 8 10
pH
Figure 26
Reversible electroosmotic flow in a
a-lactalbumin-coated capillary 12
3.1.2.2 Dynamic deactivation
Addition of modifiers to the running buffer is an alternative
to the bonded or adhered phases. An advantage of dynamic
coatings is stability. Since the modifier is in the buffer, the
coating is continuously regenerated and permanent stability
is not required.
As with covalent coatings, additives can interact with the
wall and alter charge and hydrophobicity. These modifiers
are both easier to implement and optimize since they are
prepared by simple dissolution of the modifier in the buffer.
Several dynamic deactivation methods are listed in table 13
and an illustration of the use of cationic surfactants to
reverse the EOF was shown in figure 22.
A potential disadvantage of the dynamic modification
approach is that solutes as well as the capillary surface are
affected. Biological-type conditions will be sacrificed by the
use of pH extremes and addition of surfactants. Another
limitation can be the equilibration time needed to obtain a
reproducible surface and constant EOF. Furthermore, postcolumn
analyses such as mass spectrometry and enzymatic
assays are sensitive to additives, especially those in high
concentrations.
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