Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIVITY DETECTION 137
Hi V
1kR
15~F
330pF 4.7 k n
Fig.6.11. Electronic circuit for the determination of the conductivity of various zones by the d.c.
method (potential gradient measurement). A much better device is shown in Fig.6.14.
the two discs, the resulting potential between them is a direct measure of the resistance,
and a correction must be made for variations in the electric driving current. The input
impedance of the 272 J amplifier was found to be high enough to prevent the formation
of disturbing gas bubbles, but other electrode reactions were not completely suppressed.
Because the maximum voltage on the input of the 272 J amplifier is limited, an attenuator
(20 and 5 Ma) has to be applied. If a 272 J operational amplifier is used, only those
HSP stabilized power supplies which are equipped with a polarity switch for the case
when both anions and cations need to be separated can be chosen. One can only select
the high voltage side to be as far away from the measuring cell as possible. This decreases
the chance of destruction of the operational amplifier, which has a limited voltage
(maximal value) of 5 kV. In addition to this advantage, the electrioleak currents via the
micro-sensing electrodes will be as small as possible because the electrodes are always at
relatively low potentials. As already mentioned, these leak currents may cause a build-up
of a layer on the electrodes that may obscure the detection, especially in the a.c. method
of resistance determination, which is often applied simultaneously with the d.c. method.
In order to measure the small difference in d.c. voltage that will be obtained after a zone
boundary on the micro-sensing electrodes, on the output of the 272 J amplifier, a certain
voltage has to be subtracted for zero adjustment. Experimentally, we found that the
Diff