Frans_M_Everaerts_Isotachophoresis_378342.pdf

CONDUCTIVITY DETECTION
Fig.6.12. Comparison of proffles obtained from thermometric recording and detection with a
conductivity probe. The dotted curve is the profile of the boundary choride/glutamate determined
with the conductivity probe, and the solid curve is the thermometxjc profile. The current was
stabilized at 70 MA. The speed of the recorder paper was 6 cm/min in both instances. The analysis
was performed in the operational system at pH 6, and the recording was made simultaneously in a
PTFE narrow-bore tube (I.D. 0.45 mm, O.D. 0.75 mm). R = Increasing electric resistance; T=
increasing temperature; t = time.
resistance in sections with different values of resistance per unit length. On passage of a
boundary, this approximation procedure leads to a resistance versus time curve as shown
in Fig.6.13(1), curve A. The correspondence with the measured curve B is acceptable,
apart from the deviations at the top of the curves. These deviations probably have three
causes: (a) the model is highly simplified; (b) in the region directly behind a boundary,
both the composition and the temperature of a zone may not be completely homogeneous;
and (c) parts of the measuring equipment introduced a certain time delay.
The detectox discussed in this section has also been applied in experiments where
coatings and additives were studied, which are described later in this chapter. The
influence of a coating on the micro-sensing electrodes can be illustrated by comparing
results from both the d.c. and a.c. methods of resistance determination during isotacho-
phoretic analyses. A small coating on the micro-sensing electrodes only slightly influences
the signal derived from the detector if the d.c. method is applied.
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