Frans_M_Everaerts_Isotachophoresis_378342.pdf

THERMOMETRIC RECORDING
6.1.3. Combinations of universal and specific detectors
There are two main possibilities for combining universal and specific detectors:
(1) Specific detectors and universal detectors both mounted in a similar piece of
equipment. This will be discussed in Chapter 7, where the construction of the equipment
is dealt with.
(2a) A universal detector may serve simultaneously as a specific detector. Particularly
if micro-sensing electrodes, which are in direct contact with the electrolyte inside the
narrow-bore tube, are coated with a suitable polymer [7], these electrodes give specific
infor mation.
(2b) A specific detector may serve simultaneously as a universal detector. If, for
instance, a W-absorbing component, for which the electrophoretic migration in the
operational system applied is almost zero and the UV absorption is a function of the
pH, is added to the leading electrolyte, the W detector gains ‘universal characteristics’
if non-UV-absorbing species are present. This effect is due to the different pH values
characteristic of each zone [l 11 .
Before the detectors are discussed in detad in the following sections, a survey is given
of the phenomena that occur during isotachophoretic analyses (Table 6.1).
A survey of the detectors used in analytical isotachophoresis is presented in Table 6.2.
6.2. THERMOMETRIC RECORDING
6.2.1. Introduction
The electric field strength (V/cm) varies from one zone to another and is inversely
proportional to the effective mobilities of the ionic species, giving all zones the same
speed. If a stabilized electric current is applied, the heat production increases from the
front side towards the rear in a well defined way. Consequently, the zone boundaries
are characterized by sharp changes in temperature [l, 121. This effect can serve to
indicate the positions of the zone boundaries or to characterize the zones themselves if
the actual temperature is measured simultaneously. While the zone length provides
quantitative informtion, the actual temperature characterizes the ionic species in a
particular zone. Hence thermometric recording in an isotachophoretic analysis provides
all the qualitative and quantitative information required.
6.2.2. Construction
The temperature of the individual zones can be measured with micro-thermocouples
[13, 141 or micro-beat thermistors [15]. Both types of detectors can be mounted around
the narrow-bore tube with a suitable adhesive (elastic type). This adhesive, in addition to
fixing the detector, also improves the thermal contact of the detector with the narrow-bore
tube.
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