Detectors

The quality of the detector employed determines and limits the qualitative and quantitative analysis of ITP.

There are two demands on the detector; firstly to accurately reflect the separation occurring in ITP and secondly to obtain the isotachopherogram from the analysis with high reproducibility.

Detectors for ITP can be divided into universal and specific types. The signal from universal detectors is directly proportional to the effective mobilities of the ionic species, and these detectors detect zones of all components separated in the narrow-bore tube. Ther-mometric, potential gradient and conductivity detectors belong to this class. The detection limit of potential gradient and conductivity detectors is sub-nanomole but that of thermometric detector is rather high. On the other hand, specific detectors such as UV spectrophotometers allow the identification of some components directly, or at least can give additional information about zones.

In Figure 3, three detection systems using a high-frequency contactless conductivity detector (HFCCD), a potential gradient detector (PGD), and an ultraviolet detector (UVD) are shown. Since the sensing electrodes of PGD directly contact the solution in the capillary, this system needs a device to isolate high voltage. In Figure 3, a photocoupler is used for this purpose (IP-1B, IP-2A, IP-3A, Shimadzu, Kyoto, Japan, production discontinued). A transformer is used in the usual (contact type) conductivity detection (a.c. method). Although the sensitivity of a contactless detector is lower than the direct contact detector, the merit of HFCCD is obviously that the detection system needs no such isolation device.

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