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

Modes
pH 2.6 pH 8.2
32 115
98 104
t m
39 32
104 39, 98
115 16
Table 11
CZE elution order of HPLC-collected
peptides
Number corresponds to reversed phase LC
retention time
to alter solute charge, the EOF may be too high so that
solutes elute before resolution is achieved. In this case it
would be necessary either to increase the effective length
of the capillary or reduce the EOF by one of the methods
described in table 1 of the previous chapter.
An example of selectivity differences as a result of pH
change is illustrated for peptide mapping (table 11). Here
peptides were separated and collected by reversed phase LC
and then analyzed by CZE. The elution orders were different
not only for LC and CE, but within CZE itself upon change in
pH from 2.6 to 8.2.
3.1.1.3 Surfactants
Surfactants are among the most widely used buffer additives
in CE. Numerous types of surfactants can be used in
CZE (that is, anionic, cationic, zwitterionic, or non-ionic).
At concentrations below the critical micelle concentration
(CMC) monomer ionic surfactant molecules can act as
solubilizing agents for hydrophobic solutes, as ion-pairing
reagents, or as wall modifiers. The interaction of the
monomer surfactant with the solute can occur via two
mechanisms; ionic interactions with the charged end of the
surfactant and/or through hydrophobic interactions between
the alkyl chain and hydrophobic moieties of the
solute.
In addition to interacting with the solute, many surfactants
adsorb to the capillary wall, modifying EOF and also limiting
potential solute adsorption. Depending on surfactant
charge, EOF can be increased, reduced, or reversed. EOF
reversal, for example, can be obtained by addition of
cationic surfactants such as CTAB to the buffer. As depicted
in figure 22, CTAB monomers adhere to the wall through
ionic interactions. The positive charge results from hydrophobic
interaction of free CTAB molecules with those bound
to the wall.
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