Contactless conductivity cells

Conductivity detection is a universal detection mode in which the conductivity between two inert electrodes comprising the detector cell is measured. The different arrangements employed for the construction of these detectors include apparatus with a galvanic contact of the solution with the sensing electrodes (contact conductivity detection) [51] and detection systems without galvanic contact of the solution with the sensing electrodes (contactless conductivity detection) [1]. 

Contactless conductivity detection mode, based on an alternating voltage capacitively coupled into the detection cell, is the practical and robust arrangement nowadays employed in commercially available detectors that has been independently developed in 1998 by Zemann et al. [54] and by Freacassi da Silva and do Lago [55]. This detection mode is based on two tubular electrodes. 

FIGURE 6.5 Schematic representation of contactless conductivity detection cell. (1) Capillary, (2) actuator electrode, and (3) pickup electrode. 

FIGURE 7.8 (A) Diagram for a laboratory-built detection cell for contactless conductivity measurements. 

Fig. 34.3. Schematic drawing of the cell arrangement for contactless conductivity detection in CE microchip. Reprinted in part with permission from Ref. [158]. Copyright (2004) American Chemical Society.

P. Kuban and P.C. Hauser, Effects of the cell geometry and operating parameters on the performance of an external contactless conductivity detector for microchip electrophoresis, Lab Chip, 5 (2005) 407-415. J.G.A. Brito-Neto, J.A.F. da Silva, L. Blanes and C.L. do Lago, Understanding capacitively coupled contactless conductivity detection in capillary and microchip electrophoresis. Part 2. Peak shape, stray capacitance, noise, and actual electronics, Electroanalysis, 17 (2005) 1207-1214. 

P. Kuban and P.C. Hauser, Fundamental aspects of contactless conductivity detection for capillary electrophoresis. Part I Frequency behavior and cell geometry, Electrophoresis, 25 (2004) 3387-3397. 

Kubnm P, Hauser PC (2005) Effects of the cell geran-etry and operating parameters on the ptaformance of an external contactless conductivity detector for microchip electrophoresis. Lab Chip 5(4) 407-415... 

S. Zhang, P. R. Haddad. Miniaturized movable contactless conductivity detection cell for capillary electrophoresis. Electrophoresis 24, 2144, 2003. 

Z. Hohercakova, F. Opekar, A contactless conductivity detection cell for flow injection analysis determination of total inorganic carbon. Anal. Chim. Acta 551 (2005) 132—136. 

Two kinds of conductivity detector are distinguished contact detectors and contactless detectors. Both types were originally developed for isotachophoresis in 0.2-0.5-mm-inner diameter (i.d.) PTFE tubes. Contactless detectors are based on the measurement of high-frequency cell resistance and, as such, inversely proportional to the conductivity. The advantage is that electrodes do not make contact with the buffer solution and are, therefore, outside the electric field. As these types of detectors are difficult to miniaturize down to the usual 50-75-jU.m capillar inner diameter, their actual application in capillary electrophoresis (CE) is limited. 

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