Detectors contactless detector

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]. 

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. 

J. Muzikar, T. van de Goor, B. Gas and E. Kenndler, Extension of the application range of UV-absorbing organic solvents in capillary electrophoresis by the use of a contactless conductivity detector. J. Chromatogr.A 924 (2001) 147-154. 

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. 

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. 

Kuban, P., and Hauser, P. C. (2005). Application of an external contactless conductivity detector for the analysis of beverages by microchip capillary electrophoresis. Electrophoresis 26, 3169—3178. 

The contactless conductivity microchip detection system, developed in our laboratory [31], has been particularly useful for this task. Its popularity has grown rapidly in recent years. Conductivity is a universal detection technique for CE microchips, as it relies on the same property of the analyte as the separation itself, namely the mobility of ions under the influence of an electrical field. Such a detector can thus sense all ionic species having conductivity different from the background electrolyte. 

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... 

Figure 13.6 Schematic diagram of the dual-end injection CE microchip system with the movable conductivity detector for simultaneous measurements of explosive-related anions and cations, (a) injection mode and (b) separation mode. (a,e) Running buffer reservoirs, (b,d) unused reservoirs, (c, f) sample reservoirs, (g) injected cation plug, (h) injected anion plug, (i) movable contactless conductivity detector, (j—1) cations 1-3, (m-o) anions 1-3. (Reprinted in part with permission from [33]. Copyright 2003 Wiley Interscience.)...

Wang, J., G. Chen, and A. Muck, Jr. Movable contactless conductivity detector for microchip capillary electrophoresis. Anal. Chem. 75, 4475-4479 (2003). 

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,... 

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. 

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]. 

There are two main varieties of bulk conductivity detectors contact and contactless. In a contact conductivity detector, the electrodes contact the column effluent directly. The electrodes are usually made of stainless steel, platinum, or gold in order to minimize electrochemical reactions, but they are still subject to fouling over time. In the absence of electrochemical reactions, there is no charge transfer between the solution and the electrodes, so the conductivity measurement is made with an oscillating or alternating voltage. 

The measurement electrodes can be wrapped around, threaded onto, or painted over a standard capillary. The use of a grounded shield in between the measurement electrodes greatly reduces stray capacitance. (B) Simplified circuit diagram for a contactless conductivity detector. includes double layer capacitance Cjj as well as the capacitance across the capillary wall. 

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. 

Fig. 35.4. Electrophoregrams showing the simultaneous measurement of inorganic and nitroaromatic explosives, as recorded with (a) the contactless conductivity and (b) amperometric detectors. Analytes (1) ammonium, (2) methylammonium, (3) sodium, (4) TNB, (5) TNT, (6) 2,4-DNB and (7) 2-Am-4,6-DNB SP system peak. Reprinted with permission from Ref. [30].

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,... 

A contactless conductivity detector was also constructed using 10-p.m A1 foil strips fixed on a 125-p.m-thick PMMA chip with epoxy. Here, the PMMA plate was the dielectric layer. The electrodes were arranged in an antiparallel fashion to minimize stray capacitance between them [772]. 

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],... 

Human IgM was determined using a homogeneous immunoassay by mouse anti-human IgG on a PMMA chip. CE separation of IgM, IgG, and their complex was achieved and they were detected using a contactless conductivity detector (see Figure 10.3). A LOD value of 34 ng/mL IgM was obtained on the chip, which was different from as 0.15 ng/mL obtained in a capillary tube [1010]. 

Describe two advantages and one disadvantage of the contactless and contact conductivity detectors. [1159] (3 marks)... 

Lichtenberg, J., Verpoorte, E., De Rooij, N.F., Operating parameters for an inplane, contactless conductivity detector for microchip-based separation methods. Micro Total Analysis Systems, Proceedings 5th i7AS Symposium, Monterey, CA, Oct. 21-25, 2001, 323-324. 

Wang, J., Pumera, M., Collins, G.E., Mulchandani, A., Measurements of chemical warfare agent degradation products using an electrophoresis microchip with contactless conductivity detector Anal. Chem. 2002, 74(23), 6121-6125. 

---