Frans_M_Everaerts_Isotachophoresis_378342.pdf

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DETECTION SYSTEMS
TPX was found to be very poor, even if surfactants in high concentrations were applied.
Experiments in which the TPX was 'coated' with a small layer of silicone oil showed
improved resolution, which means that the TPX itself makes a large contribution to the
electroendosmotic flow, which is difficult to suppress.
TPX also show a poorer performance in methanol compared with Perspex, although
TPX must be used because Perspex is affected in a long run. No additives have so far been
found that can be added to methanolic electrolytes to give improved resolution.
The inhibition by the surfactants of electrode reactions, if the thickness of the
electrodes is too great or the insulation towards earth is not sufficiently suppressed, is
poor. Therefore, additives still need to be added for this purpose, or the electrode
reactions must be prevented in another way: low current densities and thin electrodes of
hard material (Pt-Ir). A disadvantage of most of the additives that are suitable for this
purpose is that most of them show UV absorption, which can be neglected if they need
to be added only in trace amounts, but they cannot be used as counter ions. No observable
difference in the current-voltage curve could be found if trace amounts of surfactants
were applied (even high concentrations gave a smaller effect than expected, as shown in
Fig.6.45. This aspect, however, will be discussed in more detail in section 6.7.
Fig.6.45. Current-voltage curve measured with the equipment shown in Fig.6.40. (a) Bright platinum
electrodes with the addition of 2% of Mowiol (polyvinyl alcohol). (b) Bright platinum electrodes
with no addition of surfactant. The curves were measured in a 0.1 Mpotassium chloride solution.
They show that the surfactant must reduce the electroendosmotic profile (compare the results shown
in Fig.6.44); the effect on the electrode reactions is small.