Frans_M_Everaerts_Isotachophoresis_378342.pdf

150 DETECTION SYSTEMS
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primary coil
Fig.6.19. Construction of the transformer for a good galvanic separation of the high potential at the
micro-sensing electrodes from the low potential of the conductivity measuring circuit. A good
insulation is necessary because a leak current towards earth often causes the formation of a coating
that may obscure the recording. All dimensions are given in millimetres.
In our equipment, this transformer, which separates galvanically the sensing electrodes
from the circuitry at low potential was mounted on the electrophoretic equipment itself
about 3 cm away from the conductivity probe. Even if a well insulated cable is used, a
length of about 1 m is enough to influence the recording by the parasitic capacitance
resulting from it.
This, as already mentioned, results in the formation of a coating deposited on the
micro-sensing electrodes, built up by the electrolytes present inside the narrow-bore tube
during the analysis. This effect is particularly large if pyridine is used as a buffering
counter-ion, but even with moderate buffers such as histidine the effect is far from
negligible in the long term. More attention is paid to these undesirable effects in this
chapter, where the coating of electrodes is dealt with. Uncontrolled coating of the micro-
sensing electrodes decreases the resolution of the detection of the isotachophoretically
moving zones and, in particular, the passage of a zone boundary is recorded more slowly.
The detector must be cleaned either with aqua regia while it remains mounted in the
electrophoretic equipment or with a suitable metal polish while it is dismounted. For
optimal detection, the electrodes, made of Pt or Pt-Ir, must be passivated, which can be
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