Contactless coupled conductivity detector

Two types of conductivity detectors exist the contact conductivity detector, where the electrodes are in direct contact with the electrolyte, and the contactless coupled conductivity detector (C D also called oscillometric detector). With this detector, two stainless-steel tubes that act as electrodes are mounted on a capillary at a certain distance from each other. By applying an oscillation frequency, a capacitive transition occurs between the actuator electrode and the liquid inside the capillary. After having passed the detection gap between the electrodes, a second capacitive transition between the electrolyte and the pickup electrode occurs (see Figures 7 and 8 which is an example of separation of cations). In different reviews, Zemann and Kuban and Hauser discuss the advantages of this technique which include rather simple mechanical parts and electronics, and Kuban et al. compared several C D detectors. This technique has also been used as a detector for analysis by microchip CE. C" D detectors are available to be mounted on existing CE instruments. 

Electrodes in a capacitively coupled conductivity detector were made by injection molding carbon-filled polymer into a preformed PS chip. The polymer consisted of three conducting formulations 8% carbon black filled PS, 40% C fiber filled nylon-6,6, and 40% C fiber filled high-impact PS [774]. In another report, a movable contactless conductivity detector was also developed to allow the distance of the electrode to be adjustable [775],... 

Figure 4,2. Schematic drawing of the contactless capacitively coupled conductivity detector [8].

A dual electrochemical microchip detection system, based on the coupling of conductivity and amperometric detection schemes, was developed for simultaneous measurements of both nitroaromatic and ionic explosives [34], The microsystem relied on the combination of a contactless conductivity detector with an end-column thick-film carbon amperometric detector. Such ability to monitor both redox-active nitroaromatic and ionic explosives is demonstrated in Figure 13.7, which shows typical dual-detection electropherograms for a sample mixture containing the nitroaromatic explosives trinitrobenzene (TNB) (4), TNT (5), 2,4-DNB (6), and 2-Am-4,6-DNB (7), as well as the explosive-related ammonium... 

Bioluminescence detector Charge-coupled device Contactless-conductivity detector Capillary electrophoresis Capillary electrophoresis-Electrochemistry Collision-induced dissociation Chemiluminescence detector Sodium chlorate-nitrobenzene Commercial off-the-shelf (U.S. Army) Cold Regions Research and Development Center Croatian Mine Action Center Council of Scientific and Industrial Research,... 

Other electrode configurations, such as the radial arrangement consisting of four thin wires placed perpendicularly around the circumference of the separation capillary column, have found less application due to more complicated construction and restriction in space and diameter of the separation capillary [56]. Due to its low cost, robustness, minimal maintenance demands, possibility to be freely moved along the capillary [57], or combined with either UV-absorbance [58] or fluorescence [59] detection, the capacitively coupled contactless conductivity detector has recently gained wide acceptance not only for the determination of inorganic ions but also for biomolecules and organic ions, as it has been recently comprehensively reviewed by Kuban and Hauser [1]. 

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. 

Guijt et al. [69] reported four-electrode capacitively coupled conductivity detection in NCE. The glass microchip consisted of a 6 cm etched channel (20 x 70 pm cross-section) with silicon nitride covered walls. Laugere et al. [70] described chip-based, contactless four-electrode conductivity detection in NCE. A 6 cm long, 70 pm wide, and 20 pm deep channel was etched on a glass substrate. Experimental results confirmed the improved characteristics of the four-electrode configuration over the classical two-electrode detection set up. Jiang et al. [71] reported a mini-electrochemical detector in NCE,... 

Conductivity detection performed with a contactless conductivity detector through the packed bed was shown to be a more versatile and sensitive method than indirect UV detection [77]. Chen et al. employed inductively couple plasma mass spectrometry (ICP MS) as a detection tool for the simultaneous analysis of various ionic species of arsenic, chromium, and selenium [79]. The separation was achieved on an OT CEC column prepared by bonding a macrocyclic polyamine medium on the walls of the capillary. 

Capacitive coupling contactless conductivity detectors (C4D) which avoid contact with the solutions using high frequencies (>1 MHz) have gained popularity due to their applications in capillary electrophoresis and ionic chromatography [20,21,29,30]. These have also been coupled to flow... 

FIGURE 24.7 Sequential injection analysis-capillary electrophoresis manifold with contactless conductivity detection (SIA-CE-C D). C D capacitively coupled contactless conductivity detector DW deionized water ES electrolyte solution GI grounded interface HC holding coil HV high-voltage power supply ND needle valve S sample SC separation capillary SI syringe SLl 1 M NaOH SL2 1 M HCl SP syringe pump SS safety switch SV selection valve VI and V2 solenoid valves W waste. 

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. 

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. 

A four-electrode capacitively coupled (contactless) detector has been integrated on a Pyrex glass chip for detection of peptides (1 mM) and cations (5 mM K+, Na+, Li+). The A1 electrode (500 nm Al/100 nm Ti) was deposited in a 600-nm-deep trench and was covered with a thin dielectric layer (30-nm SiC). The other parts of the channel were covered and insulated with Si3 N4 (160 nm). To avoid gas bubble formation after dielectric breakdown, the electric field for separation was limited to 50 V/cm [145]. This four-electrode configuration allows for sensitive detection at different background conductivities without the need of adjusting the measurement frequency [328]. 

Capillary electrophoresis Capillary electrophoresis (CE) is used to analyze sodium, potassium, calcium, and magnesium in water samples. The detection is conducted by reverse absorbance measurements. Sufficient separation of the four cations is established with an electrolyte solution of 5 mM imidazole/6.5 mM a-hydroxyisobutyric acid/2 mM 18-crown-6 ether of pEI 4.1 [42]. CE with a contactless conductometric detector is used to determine small anions and cations in water samples from different sources. 2-(N-Morpholino)ethanesulfonic acid/histidine-based (Mes/Elis) electrolytes are used for direct conductivity detection of anions and cations, while ammonium acetate is used for indirect conductivity determination of alkylammonium salts. Eor the simultaneous separation procedure, involving dual-opposite end injection, an electrolyte consisting of 20 mM Mes/EIis, 1.5 mM 18-crown-6 and 20 mM cetyltri-methylammonium bromide provides baseline separation of 13 anions and cations in less than 6 min [43]. Also CE with a capacitively coupled... 

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