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

4.1.2 Electrokinetic injection
Electrokinetic, or electromigration, injection is performed
by replacing the injection-end reservoir with the sample
vial and applying the voltage (figure 51d). Usually a field
strength 3 to 5 times lower than that used for separation
is applied. In electroldnetic injection, analyte enters the
capillary by both migration and by the pumping action of
the EOF. A unique property of electrokinetic injection is that
the quantity loaded is dependent on the electrophoretic
mobility of the individual solutes. Discrimination occurs for
ionic species since the more mobile ions are loaded to a
greater extent than those that are less mobile.
The quantity injected, Q (g or moles), can be calculated by
Instrumentation/Operation
(µ
Q = e
+ µ EOF
) Vpr 2
Ct
(30)
L
80
60
40
Electrokinetic
injection
K +
Li + K +
Li +
Hydrostatic
injection
where µ = electrophoretic mobility of the analyte
µ EOF
= EOF mobility
V = voltage
r = capillary radius
C = analyte concentration
t = time
L = capillary total length
20
0
4 6 12 16
Resistance (k )
Figure 52
Quantity of sample loaded as a function
of sample resisteance for hydrodynamic
and electrokinetic injection 28
As described by equation (30), sample loading is dependent
on the EOF, sample concentration, and sample mobility.
Variations in conductivity, which can be due to matrix
effects such as a large quantity of an undetected ion such
as sodium or chloride, result in differences in voltage drop
and quantity loaded (figure 52). Due to these phenomena
electroldnetic injection is generally not as reproducible as
its hydrodynamic counterpart.
87