Frans_M_Everaerts_Isotachophoresis_378342.pdf

118 DETECTION SYSTEMS
ca. 2%. The choice of the material of which the equipment is constructed influences the
electroendosmosis considerably. The length of tube needed for a complete separation
depends on the difference in the effective mobilities of the ions in the sample that are
most difficult to separate, the concentrations of the constituents of the sample and the
availability of a counterflow of electrolyte (see Chapter 7).
Because the choice of detection system determines the type of construction of the
equipment, the detection systems that are currently available for analytical isotacho-
phoresis are summarized first. Some equipment is considered in detail in Chapter 7.
The detectors that are available can be divided into three main classes: universal
detectors, specific detectors and combinations of both, and these types are considered
in the following sections.
6.1.1. Universal detectors
When a universal detector is used, the information obtained is directly proportional
to the effective mobilities of the ionic constituents [4], and the information derived
therefore has a continuous stepwise character. From the height of a step, qualitative
information can be deduced, while the length of a step provides quantitative
information.
Universal detectors may be divided into two classes:
(1) Detectors of which the sensing element is not in direct contact with the electrolytes
inside the narrow-bore tube [S] . This class can be divided into two sub-classes:
(la) Detectors with a low resolving power, e.g., temperature-recording detectors [l].
(lb) Detectors with a high resolving power, e.g., high-frequency conductivity detectors.
(2) Detectors in which the sensing element is in direct contact with the electrolytes
inside the narrow-bore tube [6]. This class can also be divided into two sub-classes:
(2a) Detectors that involve a.c. recording of the conductivity between two micro-sensing
electrodes mounted equiplanar or axially [7].
(2b) Detectors that record the potential gradient directly, making use of the direct
driving current between two axially mounted micro-sensing electrodes [8] .
6.1.2. Specific detectors
When a specific detector is used, the information obtained is not directly proportional
to the effective mobilities of the ionic constituents. A series of components, separated
isotachophoretically, may have different, non-continuous, responses on the detector, so
that the absolute value of the measuring signal may be different from zone to zone. One
can use the principle of absorption of light measured during the analysis, or polarimetric
detection can be used [9]. The succesful use of radiochemistry has so far been applied
only in analyses on strips [ 101 .