Frans_M_Everaerts_Isotachophoresis_378342.pdf

EQUIPMENT 229
mounted equiplanar, we found it advantageous to have this capacitor between the
measuring electrodes; possibly an exact equiplanar construction is not always possible.
Owing to the high potentials applied, some leak current will decrease the resolution
after a small series of experiments. The signals derived from the transformer are handled
electronically and result in a trace on the potentiometric recorder. This trace has a
continuous stepwise character if the isotachophoretic zones pass the detector. If the
trace does not have a continuous stepwise character, e.g, if a drift is obtained or dips
and/or overshoots are recorded, something is wrong. A drift of the base line is obtained
if, for instance, impurities are present that are more mobile than the leading ion, the
buffer capacity of the counter ion is not sufficient or some electrode reaction occurs at
the micro-sensing electrodes. Dips (or negative steps) can be expected if the buffer
capacity of the counter ion is not sufficient, if an enforced isotachophoretic system is
obtained or if the electrodes are coated with a polymer as a result of an electrode
reaction or the physical adsorption of any material. Overshoots can be expected if the
buffer capacity of the counter ion is not sufficient, if the temperatures of two adjacent
zones are too different or if an electrode reaction occurs.
A modified Brandenburg (Thornton Heath, Great Britain) power supply of the
alpha-series is used. It is modified in such a way that it not only can be applied as a
constant-voltage source (+30 kV), but for the isotachophoretic experiments can also be
applied as a current-stabilized power supply (lt30 kV). Various current-stabilized power
supplies, however, are commercially available nowadays, even up to 60 kV.
Between the injection block, shown in detail in Figs.7.5 and 7.6, a six-way tap
(Figs.7.2-7.4), whch can easily be removed without changing the narrow-bore tube if
necessary, is mounted. In the equipment shown, an injection can also be made with a
normal commercially available micro-syringe, as the sample can be introduced via the
tap. Thus this instrument combines all the advantages of taps and syringes. Also, instead
of the cylindrical counter electrode compartment (Figs.7.8 and 7.1 5), a counter electrode
compartment with a flat membrane (Figs.7.9 and 7.10) can be used. These electrode
compartments can easily be changed if necessary without any problems or the need to
fit a new narrow-bore tube.
All components of the isotachophoretic equipment shown in Fig.7.15 are replaceable
because no adhesive is applied, rubber O-rings and screw-threads being used for clamping.
We found that it is sometimes necessary to replace the narrow-bore tube as their life was
found to vary from several years to only a few months. A narrow-bore tube needs to be
changed if a decreased resolution cannot be improved.
Owing to differences in the behaviour of the various operational systems, or compounds
of the sample, even the ‘inert’ PTFE can become coated with material that is not easy
to remove, and the resolution may decrease. Impurities, possibly building up over a long
period, that are adsorbed on the walls of the injection system or the counter electrode
compartment have less influence on the detection or the separation itself, because the
bores in these parts of the instruments are considerably greater. The conductivity probe,
if washing with a non-ionic detergent gives no improvement, can easily be cleaned with
some metal polish and a cotton thread.