Frans_M_Everaerts_Isotachophoresis_378342.pdf

UV ABSORPTION METER 153
-_-.-
+-
R - V 0.1 Zero
10MR l00mV Int 1
(6.14)
The potentiometers ‘Zero’ and ‘En’ must be arranged in a position such that during the
analysis the output voltage on ‘Lin’ always remains between 0 and 100 mV. An example
of the linearity of the circuit shown in Fig.6.18 is given in Fig.6.20, in comparison with
that of the older circuits. Most of the isotachopherograms considered in this chapter were
obtained with non-linear circuitry, but the data collected for the various operational
systems that are given in Chapters 11-17 were obtained with linear circuitry.
At a resistance of 1 MR, the accuracy of the resistance determination with linear
circuitry is better than 2 kf2, whde at 10 MR the accuracy is better than 30 kR. If the
ambient temperature increases by 10°C, the difference in resistance determination at a
level of 1 Mi2 is less than 1 kR and at 10 Mi2 it is less than 20 kR.
The stepwise trace obtained during an isotachophoretic analysis when the zones pass
the conductimeter can easily be interpreted for qualitative and quantitative information,
because the steps are sufficiently sharp. This is in contrast with thermometric recording.
Two main reasons can be given why a differentiator was constructed. (1) If small
zones are present, stacked between the others, these zones can be detected much easier
and better with an electronic differentiator. (2) If electronic devices are available and
applied for measuring the time interval between two successive peaks, a pulse is needed
for the printer. A possible circuit for such a differentiator is given in Fig.6.21.
The circuit with IC6 is actually the differentiator. Its amplification can be chosen by
the potentiometer ‘Diff. The time constant is 24 msec, which is small enough to differentiate
the signal on the output ‘En’. The circuit IC7 and lCs is a double-phase rectifier.
A potential recorder with a sensitivity of 100 mV can be applied in combination with
ths differentiator. IC9 forms, together with the two resistors on the non-inverting input,
a Schmitt trigger with a very small hysteresis. The 10-pF capacitor ensures that the
input signal is equal to the first differential of the output signal of the rectifier. If this
value is zero, the Schmitt trigger is triggered. This procedure is shown schematically in
Fig.6.22.
The printed values of the time intervals between the fast moving (about 1 m/h) zone
boundaries increase the accuracy of time recording and hence the quantitative data
handling and, moreover, simplifies the latter. The accuracy in time recording is better
than 10 msec, which represents an accuracy of 5 - g-equiv./l, assuming an electric
driving current of 70 /.LA and a concentration of the leading electrolyte of lo-* g-equiv./l.
6.5. UV ABSORPTION METER
6.5.1. Introduction
Because in the steady state of an isotachophoretic separation all components move in
individual zones with identical speeds and only the counter ion is mixed with these
components, specific detectors such as the W absorption meter can offer much additional