Frans_M_Everaerts_Isotachophoresis_378342.pdf

COUNTER ELECTRODE COMPARTMENTS
Even if the narrow-bore tube is arranged in a horizontal position, this membrane is
needed. Moreover, gas will generally be produced at the electrodes as a result of the
electric current necessary for electrophoretic separations. These gas bubbles may also
introduce a hydrodynamic flow of electrolyte if the electrodes are not separated by a
semipermeable membrane from the narrow-bore tube in which the separation is performed.
It is of minor importance that the electroendosmotic profile is somewhat suppressed
by the semipermeable membrane, as discussed in Chapter 6. Moreover, mainly those
conditions such that electroendosmotic flow can be prevented must be sought. Although
semipermeable membranes need to be applied, one must bear in mind that their use
always causes a shift in pH on both sides of the membrane, due to the potential gradient
across the membrane and the difference in the ionic mobilities of the various ions through
the membrane. This shift in pH may disturb or minimally influence the analysis in a long
run, especially if a counter flow of electrolyte is applied.
7.3.2. Cylindrical counter electrode compartment
Fig.7.8 shows schematically a cylindrical counter electrode compartment. The main
parts of this electrode compartment should be made of material resistant to various
solvents; so far, Perspex, Kel-F, Arnite and Pyrex glass have been used. The membrane,
made of cellulose polyacetate, fits around the two cylinders that are provided with a
central bore. The membrane is fixed with Araldite, which in fact does not really stick
the membrane to the two central cylinders, but still prohibits any leakage from the
side on which the electrode is mounted towards the narrow-bore tube, or vice versa. The
cylindrical membrane is made by wrapping a sheet of cellulose polyacetate (0.1 mm)
around a rod with an external diameter equal to the external diameter of the cylinders
on which the membrane will finally be mounted. During the wrapping of the cellulose
polyacetate, acetone, in which the membrane is soluble, is applied. In order to remove
this acetone, the rod, with the wrapped sheet on it, is immersed in a stream of water. A
white, small-pore cylindrical membrane is the result, which is easy to remove from the rod
with aid of a sheet of abrasive paper. The mechanical stability of the membrane is very
high, the thickness being approximately 0.3 mm. The main advantage of a cylindrical
membrane is that during rinsing and re-filing of the instrument with leading electrolyte,
possible gas bubbles can easily be removed and do not stick to the wall. Also, the washing
of the entire system is very easily effected. If experiments with a counter flow of
electrolyte are performed, this procedure is normally carried out via the tap, a common
PTFE-lined Hamilton (1MM1) valve. The disadvantage is that the fresh electrolyte has to
pass the membrane, by which the existing pH jump is transported by the counter flow
quickly into the narrow-bore tube. Of course, the separation is influenced by this effect.
This has been partially overcome by the construction of a special connection for the
counter flow of electrolyte between the counter flow compartment and the narrow-bore
tube in which the separation is carried out. (In section 7.3.3, another counter flow
compartment is described that is much more suitable for experiments with a counter flow
of electrolyte.) The connection with the narrow-bore tube in which the separation is
carried out is similar to that described in section 7.2.3.
At the side on which the electrode is mounted, the counter electrode compartment is
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