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

Modes
3.1.1 Selectivity and the use of additives
Selectivity, the relative order of solute migration, is determined
by the mechanism that effects the separation. The
capability to control selectivity can improve resolution and
also yield complementary information that is useful for
validation of a separation. In essence, the different modes
of CE (that is, CZE, MEKC, CGE, and CIEF) yield different
selectivities due to different separation mechanisms. While
the focus in this section is on selectivity within CZE, mechanisms
of separation in CZE performed with additives sometimes
overlap with MEKC.
In CZE, selectivity can most readily be altered through
changes in running buffer pH or by use of buffer additives
such as surfactants or chiral selectors. A more complete list
is given in table 9. Note that while these and other methods
also result in changes in EOF, the EOF is itself not responsible
for the changes in selectivity, only in migration time
and resolution.
3.1.1.1 Buffer selection
The running buffer selection is extremely important to the
success of any CE separation. The sensitivity of EOF to pH
requires the use of buffers that maintain constant pH.
Effective buffer systems have a range of approximately two
pH units centered around the pK a
value. Polybasic buffers
such as phosphate and citrate have more than one useful
pK a
and thus can be used in more than one pH range. A
buffer for use in CE should possess the following properties:
• good buffering capacity in the pH range of choice;
• low absorbance at the wavelength of detection;
• low mobility (that is, large, minimally-charged ions)
to minimize current generation.
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