Frans_M_Everaerts_Isotachophoresis_378342.pdf

ADDITIVES TO THE ELECTROLYTES 171
6.6. ADDITIVES TO THE ELECTROLYTES
6.6.1. Introduction
As soon as the high-resolution UV detector became avdlable and the results could be
compared with those of the conductivity detector, with comparable resolution, the
initially non-reproducible results of both the conductivity detector and the UV detector
could be studied more intensively. The UV detector mainly does not disturb the isotachophoretic
pattern by its presence, except for compounds that may be destroyed by the UV
light applied or if the material of whch the narrow-bore tube is constructed is eventually
affected by the UV light. The conductivity detector, however, may disturb the electrophoretic
pattern as a result of the polarization initiated by the driving current or due to
a leak current, or due to excessive heat produced by the measuring current. Because in
our systems the last mentioned current is small compared with the driving current, the
heat produced by the driving current can be neglected.
The aim of making additions to the electrolytes may vary. The addition of surfactants,
for instance, not only sharpens the zone boundaries by depressing electroendosmosis,
especially visible if the combination of a UV and a conductivity detector can be applied,
but also influences the overpotential against electrode reactions on the micro-sensing
electrodes of the conductivity detector.
Additives can be classified into three categories: additives that affect the electro-
endosmotic flow; additives that influence various electrode reactions; and additives that
show both of these effects.
A study was undertaken in order to elucidate these phenomena. Another purpose of
the study was to show the difficulties that might arise if the electrophoretic equipment
is not well constructed. Many of the problems that were initially present during the
development of the conductivity detector have been overcdme, but more useful informa-
tion can be gained from considering these problems than from presenting the final
solution only.
6.6.2. Effect of additives on the electroendosmotic flow
Electroendosmosis is the movement of a liquid with respect to a solid wall as the result
of an applied potential gradient. Although it is generally assumed that the electroendos-
motic flow can be neglected in a single narrow-bore tube, with high-resolution detectors
this is not so. In the beginning of isotachophoresis (displacement electrophoresis), the
viscosity of the electrolytes was increased in order to suppress the electroendosmotic
flow, to prevent hydrodynamic flow (semipermeable membranes were not used) and to
suppress convection. The viscosity was increased by the addition of hydroxyethylcellulose,
linear polyacrylamide, arrowroot, agar agar or methylcellulose. These viscous liquids were
purified by shaking them with a mixed-bed ion exchanger. One of the disadvantages was
that between analyses considerable time was needed for rinsing the narrow-bore tube.
In the early days, a precise classification could not be made. Most electrokinetic
phenomena have to be explained in terms of the interaction between a flow of liquid in
the double layer, but the exact structure of the double layer may generally be left out