Frans_M_Everaerts_Isotachophoresis_378342.pdf

EQUIPMENT
the large bore, a more direct connection between the narrow-bore tube and the
electrode compartments exist, so that the potential gradient in the canal where the
syringe will be inserted is negligibly small. Of course if any gas were produced, it would
destroy the analysis. So far, no other electrode reactions have been observed. The
connection with the narrow-bore tube is as described in section 7.2.3,
The great advantage of this counter electrode compartment is that a counter flow of
electrolyte is permitted that does not pass the membrane. A considerable time is needed
for the pH jump at the membrane to enter the narrow-bore tube, where the analysis is
carried out, by the direct electric current, because again the bore, which forms a direct
connection between the PTFE narrow-bore tube and the membrane, is relatively large,
so that a small potential gradient exists in this bore. In addition, the surface area and
thickness of the membrane are small tie., the disturbance is relatively small) and,
moreover, the buffer capacity of the electrolyte present in the 2-mm bore is high, so
that any disturbance can be counterbalanced easily. A further advantage, of course, is
that no adhesive is used with the membrane, so that a membrane can be changed and
experiments in, e.g., methanol can be carried out more easily.
7.4. ECUIPMENT
7.4.1. Introduction
With the injection systems and the counter electrode compartments briefly discussed
in this chapter, and the detectors discussed separately in Chapter 6, many types of
instruments can be constructed. Moreover, the different components are connected in
such a way that no adhesive need be applied and therefore the different parts are
interchangeable. The means of thermostating can also be taken into consideration, e.g.,
the narrow-bore tube may be free-hanging in air that is thermostated with circulating
water, a thermostated aluminium block can be applied with the narrow-bore tube
mounted on it in a helix, or the narrow-bore tube may be thermostated directly with, e.g.,
circulating kerosene.
The development and combination of electrode compartments, injection systems and
auxiliary equipment has, of course, resulted in a continuous gradation of types of
instruments and modifications, and it is sometimes difficult to distinguish one type
from another. Therefore, in this section only three types of equipment will be discussed,
Fig.7.9. Counter electrode compartment with a flat semipermeable membrane and a septum for
experiments with a counter flow of electrolyte. 1, Perspex connection between the central bore of
the counter electrode compartment and the narrow-bore tube of the electrophoretic equipment,
provided with an O-ring; 2 = brass screw for clamping component (1); 3 = brass support for component
(1); 4, 14 = Perspex units for mounting the counter electrode compartment on a rail (see Fig.7.16) and
for clamping 8 and 11; 5, 15 = brass pen with screw-thread; 6, 16 = bolts; 7 = cap of the electrode
compartment, provided with a hole; 8 = electrode compartment; 9 = flat cellulose polyacetate
membrane; 10 = rubber O-ring; 11 = central housing with canals of 2 mm diameter that pass along the
flat membrane and the septum; 12 = septum; 13 = screw-head for clamping the septum in the central
housing; 17 = PTFE-lined Hamilton (1MMl) valve.
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