Contactless conductivity detection

Zemann and coworkers have developed a novel contactless conductivity detector [6-12]. Contactless conductivity detection offers the advantage of avoiding detection dead volumes. This is especially important for miniaturized chromatographic and electrophoresis systems. 

A high oscillating frequency of 40-100 kHz is applied to one of the electrodes. A signal is produced on the other electrode as soon as an analyte zone with a different conductivity compared to the background passes through the detection gap. An amplifier and rectifier are connected to the second electrode to measure resistance between the two electrodes. To isolate the two capacitors associated with each electrode, a thin piece of copper is placed between the electrodes and grounded. 

---

Organic modifiers have been frequently employed in CE to increase the solubility of hydrophobic solutes in the aqueous buffer system. Unfortunately, many organic modifiers are UV absorbent and cannot be used without considerable loss of sensitivity of detection. A contactless conductivity detection system has been developed which extends the application range of UV-absorbing solvents [ 119]. As both natural pigments and synthetic dyes absorb in the visible part of the spectra, the application of UV-absorbing organic modifiers in their CE analysis does not cause detection problems. 

For systems with moderate-to-low probability, CE might not be the chromatographic quantification method of choice, and other alternatives, such as HPLC and GC, should be considered. However, specific procedures (e.g., off-line concentration, stacking techniques, extended light path capillaries) and detectors may be applied to increase solubility and sensitivity of detection, such as derivatization (e.g., carbohydrates, amino acids, amines, etc.) or the use of a specific detector (e.g., contactless conductivity detection, coupling with mass spectrometry, etc.). However, increasing the complexity of the methodology may be counterproductive if it leads to a lower robustness and transferability of the system. 

Zemann, A. J. (2003). Capacity coupled contactless conductivity detection In capillary electrophoresis. Electrophoresis 24, 2125—2137. 

Kuban, P., and Hauser, P. C. (2004). Contactless conductivity detection In capillary electrophoresis a review. Electroanalysis 16, 2009—2021. 

Henchoz, Y, Schappler, J., Geiser, L., Prat, J., Carrupt, P.A. and Veuthey, J.L. (2007) Rapid determination of pKa values of 20 amino adds by CZE with UV and capadtively coupled contactless conductivity detections. Analytical and Bioanalytical Chemistry, 389, 1869-1878. 

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. 

Zemann AJ, Schnell E, Volgger D, Bonn GK. Contactless conductivity detection for capillary electrophoresis. Analytical Chemistry 70, 563-567, 1998. 

Kuban P, Abad-Villar EM, Hauser PC. Evaluation of contactless conductivity detection for the determination of UV absorbing and non-UV absorbing species in reversed-phase high-performance liquid chromatography. Journal of Chromatography A 1107, 159-164, 2006. 

Borissova, M., Gorbatsova, J., Ebber, A., Kaljurand, M., Koel, M., and Vaher, M., Non-aqueous capillary electrophoresis using contactless conductivity detection and ionic liquids as background electrolytes in acetonitrile. Electrophoresis, 28, 3600-3605,2007. 

Reduction in the noise and avoiding the creation of bubbles make the contactless mode (CCD—contactless conductivity detection) more... 

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.

A. Berthold, F. Laugere, H. Schellevis, C.R. de Boer, M. Laros, R.M. Guijt, P.M. Sarro and M.J. Vellekoop, Fabrication of a glass-implemented microcapillary electrophoresis device with integrated contactless conductivity detection, Electrophoresis, 23 (2002) 3511-3519. 

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 1. Fundamentals, Electroanalysis, 17 (2005) 1198-1206. 

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. 

J. Tanyanyiwa and P.C. Hauser, High-voltage contactless conductivity detection of metal ions in capillary electrophoresis, Electrophoresis, 23 (2002) 3781-3786. 

Schulze et al. [135] developed fused-silica chips dynamically coated with hydroxypropylmethyl cellulose and utilized them for the separation of aromatic low molecular weight compounds such as serotonin, propranolol, a diol, and tryptophan. The authors used deep UV laser-induced fluorescence detection for these compounds. Schuchert-Shi et al. [136] identified ethanol, glucose, ethyl acetate, and ethyl butyrate, byproducts obtained in enzymatic conversions using hexokinase, glucose oxidase, alcohol dehydrogenase, and esterase. The authors reported that the quantification for ethyl acetate was possible using contactless conductivity detection. Hu et al. [137] described the separation of reaction products of (3-thalassemia in a multiplex primer-extension reaction using NCE. The method developed was used for patient samples and the results coincided with those of a detection kit. 

Bachmann S, Huck CW, Bakry R and Bonn GK, Analysis of flavonoids by CE using capac-itively coupled contactless conductivity detection. Electrophoresis 28 799-805 (2007). 

To avoid electrolysis and electrode fouling when the solution was in contact with the measurement electrodes, contactless conductivity detection was proposed. This non-contact method relied on the capacitive coupling of the electrolyte in the channel, and the method has been used to detect inorganic ions that alter the conductivity and capacitance in the electrolyte [277,638]. 

FIGURE 7.29 Microchip CE separation of a sample containing 100 pM of K+, Li+ and Na+ (prepared in the running buffer). The measured plate number for K+ is 43,200 plates/m, with an estimated limit of detection of 18 lM. Running buffer, 10 mM MES/His at pH 6.0 separation conditions, 280 V/cm effective separation length, 3.4 cm contactless conductivity detection at 58 kHz (as optimized in Figure 7.28) [141]. Reprinted with permission from Wiley-VCH Verlag. 

Why are the electrodes used in contactless conductivity detection arranged in an anti-parallel fashion [1153] (2 marks)... 

Lichtenberg, J., de Rooij, N.F., Verpoorte, E., A microchip electrophoresis system with integrated in-plane electrodes for contactless conductivity detection. Electrophoresis 2002, 23, 3769-3780. 

Tanyanyiwa, J., Hauser, P.C., High-voltage capacitively coupled contactless conductivity detection for microchip capillary electrophoresis. Anal. Chem. 2002, 74(24), 6378-6382. 

Weber, G., Johnck, M., Siepe, D., Neyer, A., Hergenroder, R., Capillary electrophoresis with direct and contactless conductivity detection on a polymer microchip. Micro Total Analysis Systems, Proceedings of the 4th pTAS1 Symposium, Enschede, Netherlands, May 14-18, 2000, 383-386. 

GiUespie, E. et al. Evaluation of capillary ion exchange stationary phase coating dis-tiihution and stability using radial capillary column contactless conductivity detection. Analyst 2006, 131, 886-888. 

Tanyanyiwa J, Abad-Vihar EM, Hauser PC. Contactless conductivity detection of selected organic ions in on-chip electrophoresis. Electrophoresis 2004 25 ... 

K. Mayrhofer, A. J. Zemann, E. Schnell, and G. K. Bonn, Capillary electrophoresis and contactless conductivity detection of ions in narrow inner diameter capillaries. Anal. Chem., 71, 3828, 1999. 

The difficulties encountered in the separation of amino acids can be attributed to the altered structural profile of the amino acid once derivatized. Underivatized amino acids have been separated previously, but require contactless conductivity detection to identify all amino acids (11).The reaction is shown in Figure 7.1. The attachment of the fluorescent/UV label minimized the structural differences in side chains. Therefore, as the mass-to-charge ratios were similar, the addition of a surfactant such as SDS did not offer sufficient selectivity (13,14). Nevertheless, the application of an ANN to the optimization of this separation rapidly arrived at the optimum conditions. 

---