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

Instrumentation/Operation
Despite quantitative limitations, electrokinetic injection is
very simple, requires no additional instrumentation, and is
advantageous when viscous media or gels are employed in
the capillary and when hydrodynamic injection is ineffective.
4.1.3 On-capillary sample concentration
Several techniques have been described to enhance sensitivity
by on-capillary sample concentration during or just
after sample injection. These methods are based on the
field strength differences between the sample zone and the
running buffer, and are called “stacking”. Generating an
isotachophoretic system is one method. As described in
section 3.5, in ITP the concentrations of each migrating
analyte will adopt the concentration of the leading electrolyte.
Theoretically, orders of magnitude concentration can
be obtained by ITP. Despite often not attaining true steadystate
ITP, the properties can be used to increase sample
concentration upon injection simply by proper choice of
running buffers.
Another method of stacking is obtained when the conductivity
of the sample is significantly lower than that of the
running buffer. Upon application of the voltage, a proportionally
greater field will develop across the sample zone
causing the ions to migrate faster. Once the ions reach the
running buffer boundary, the field decreases and they
migrate slower. This continues until all of the ions in the
sample zone reach the boundary and cause the sample to
become concentrated into a smaller zone. At this point, the
field becomes homogeneous in the zone and normal
electrophoresis begins.
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