Contactless detection

Contactless Impedance Detection Contactless detection means that there is no galvanic (conductive) connection between the... 

A CE-ECD system mainly consists of a 30 kV high-voltage dc power supply, a piece of a fused-silica capillary, a conductivity detector, a data acquisition system, and a pair of electrodes for contact or contactless detection. 

Conductimetric detection is a less sensible, but universal detection technique that has been applied as a detection mode in ME, either in the galvanic (a pair of electrodes is placed in the separation channel for liquid impedance measurement) [12, 13] or the contactless mode (no contact between the pair of electrodes and separation channel solution) [14-16], Both formats are illustrated in Figure 12.4. Contactless detection (CCD) is preferred for three reasons (i) the electronic circuit is decoupled from the high-voltage applied for separation (no direct coupling between the electronics and the liquid in the channel), (ii) the formation of glass bubbles at the metal electrodes is prevented, and (iii) electrochemical modification or degradation of the electrode surface is prevented. 

Recently, Liu et al. developed a MCE with contactless detection for inorganics ions and heavy metals. Using this device, an average LOD of 0.4 pM for inorganics cations in water was obtained. 

Active thermography is a contactless NDE technique that consists in the detection of infrared emission after the transient thermal excitation of the inspected structure. 

The performances of ultrasound generated and detected by lasers offer a wide field of applications in industry, so that we will pursue research for the testing of very small strucmres which could only be examined by a contactless technique. 

In summary, CL can provide contactless and nondestructive analysis of a wide range of electronic properties of a variety of luminescent materials. Spatial resolution of less than 1 pm in the CL-SEM mode and detection limits of impurity concentrations down to 10 at/cm can be attained. CL depth profiling can be performed by varying the range of electron penetration that depends on the electron-beam energy the excitation depth can be varied from about 10 nm to several pm for electron-beam energies ranging between about 1 keV and 40 keV. 

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. 

FIGURE 6 An electropherogram using detection by contactless conductivity of anions with an injection of 20 mbar for 5 s. Sample concentration (in order) 0.5 ppm bromide, chloride, nitrite, nitrate, sulfate, and fluoride and 1.0 ppm phosphate. Data courtesy of TraceDec. 

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. 

FIGURE 7 Contactless coupled conductivity detection (C D) is based on two cylindrical metal electrodes, actuator, and pickup electrode, which are placed on the separation capillary. Schematically, it represents a series of a capacitor, an ohmic resistor, and a second capacitor (from Innovative Sensor Technologies GmbH). 

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. 

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... 

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. 

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]. 

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

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. 

This type of detection has achieved much development in the last few years due to its simplicity. A specific revision on conductimetric (and potentiometric) detection in conventional and microchip capillary electrophoresis can be found in Ref. [57]. It is considered a universal detection method, because the conductivity of the sample plug is compared with that of the solution and no electroactivity of the analytes is required. Two electrodes are either kept in galvanic contact with the electrolyte (contact conductivity) or are external and coupled capaci-tively to the electrolyte (contactless mode). An alternating current potential is applied across the electrodes and the current due to the conductivity of the bulk solution is measured. As the signal depends on the difference in conductivity between solution and analyte zones, the choice of the electrolyte is crucial. It is necessary that it presents different conductivity without affecting sensitivity. 

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