Frans_M_Everaerts_Isotachophoresis_378342.pdf

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DETECTION SYSTEMS
Fig.6.8. Schematic diagram of the high-frequency conductivity detector. The four electrodes
(E, , E,, E, and E4), which are not in direct contact with the electrolytes inside the narrow-bore
tube, are mounted equiplanar. By using screening, the resolution is improved, although a much higher
amplification is necessary, which decreases the signal to noise ratio. A = Amplifier; G = generator
(cu. 1 MHz).
The emitting electrodes El and E2 are in contact with the receiving electrodes E3 and
E4 via the capacity of the narrow-bore tube. The electrodes El, E2, E3 and E4 are
mounted equiplanar. In order to prevent a fan-shaped high-frequency signal between the
emitting and receiving electrodes, which of course would decrease the resolution,
shielding is necessary, as shown in Fig.6.8. The signals picked up by the electrodes E3
and E4 are fed to the second symmetry transformer (coil ratio 1 : 1) and, after arnplifica-
tion and rectification, are recorded with a potentiometric recorder. The signals derived
from the probe are directly proportional to the effective mobilities of the ionic material
at the position of the electrodes El, E2, E3, E4 and between the shielding.
For optimal operation of the conductivity probe, a good symmetry of the emitting
electrodes El and E2 to earth is necessary. If this symmetry is not correct, boundary
passages of various shapes are recorded, which, of course, do not exist in reality. It was
even found that some boundary passages were not recorded at all. Poor symmetry to
earth of the receiving electrodes E3 and E4 influences only the final amplification, and
does not influence the shape of the recorded transition.
In some isotachophoretic analyses using the high-frequency detector, the reproduc-
ibility was found to be poor. Particularly when a series of zones needed to be detected, the
resolution was far lower than that with a conductivity detector with the micro-sensing