Figure 4.55
Methods of sample injection for
capillary electrophoresis.Modes of HPCE
Different separation mechanisms, which determine selectivity, can be exploited in HPCE by appropriate
choice of operating conditions. There are four principal modes of operation (Table 4.22) and it should
be noted that in only one, micellar electrokinetic capillary chromatography (MEKC), is it possible to
separate neutral species from one another.
Capillary zone electrophoresis (CZE), sometimes known as free-solution capillary electrophoresis
(FSCE), is the simplest and currently the most widely used mode. The capillary is filled with a
homogeneous buffer solution through which the various species migrate in discrete zones and at
different velocities according to their electrophoretic mobilities. However, although species with either
the same charge or opposite charges can be separated from one another, all neutral species have zero
mobility and move as a single zone at the same velocity as the EOF (Figure 4.53). Selectivity is
controlled and separations optimized mainly through the choice of buffer composition, pH and ionic
strength. Generally, low ionic strength and high pH favour fast migration and therefore the highest
efficiencies and shortest analysis times. Effective buffering is essential because the EOF is a function of
pH as is solute charge. Some commonly used