Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIVITY DETECTION 147
We can now write
and
or
Then
Thus
(6-9)
(6.10)
(6.1 1)
(6.12)
R2 R1
We can conclude that vc is a constant if - = - and the amplitude of vu is proportional
R3 Rv
to R.
After rectification and smoothing of vu, the resulting potential is also proportional to
R. In order to keep the frequency of vc equal to the resonance frequency a,., a
comparator is used, which generates vc. This comparator is controlled by vu, a squarewave
voltage; in all of the above equations we have considered only the first harmonic of
this square-wave voltage.
The higher harmonics are suppressed by the circuitry applied, assuming that R is not
too small. These higher harmonics can be neglected in this case. The circuit as finally
applied is shown in Fig.6.18.
IC2 is the operational amplifier as already discussed in Fig.6.17. lC3 rectifies the
voltage vu in a single-phase d.c. mode. The pA meter measures the average value of this
rectified signal. By means of IC5, a pre-adjusted voltage can be added to this signal, whch
is then smoothed and amplified by IC4. The smoothmg capacitcr is chosen such that the
maximal time constant involved in the amplification of IC4 is 0.1 sec. The potential
recorder used in our laboratory in combination with the conductimeter also has a time
constant of ca. 0.1 sec.
Both the d.c. voltage and the amplification can be adjusted by two potentiometers
(P3 and P4), which are supplied with a set of Multi-dials (Spectro Electronics, Plainview,