Multiple electrode systems

Electrophysiological teclmiques have high temporal (millisecond) and spatial (micron) resolution. They are carried out at discrete electrodes and so obtain information from a single spatial location or from a finite number of locations if a multiple-electrode system is used. These recording teclmiques... 

Electrochemical (EC) detection is particularly well suited to capillary electrophoresis. In contrast to many optical methods, EC detection can be miniaturized without a loss of sensitivity. It is very selective because there are few compounds in biological samples that are electroactive. The selectivity is tunable and dependent on the electrode material and the applied potential. Modified electrodes and multiple electrode systems can be used to increase selectivity for certain classes of analytes. Electrodes and potentiostats can also be integrated into chip-based CE systems using procedures similar to those employed for the manufacture of the analytical chips. 

Another but less precise method of measurement of the surface conductivity is the use of suitable multiple-electrode systems. If we lay a metallic ring round a circular electrode, that lies at the same potential as the flat extended counterelectrode or the circular electrode, it is possible to separate the surface current. Cf. here [641]. 

Multiple electrodes have been used to obtain selectivity in electrochemical detection. An early example involved the separation of catecholamines from human plasma using a Vydac (The Separation Group Hesperia, CA) SCX cation exchange column eluted with phosphate-EDTA.61 A sensor array using metal oxide-modified surfaces was used with flow injection to analyze multicomponent mixtures of amino acids and sugars.62 An example of the selectivity provided by a multi-electrode system is shown in Figure 2.63... 

In the case of mineral-mineral interactions, a mineral with higher potential acts as a cathode, while a mineral with lower potential acts as an anode. For a multiple mineral/grinding media(steel)system. The galvanic interactions become more complex than the two-electrode systems. The galvanic reactions among multielectrode systems are also governed by the mixed potential principle as shown in an example of polarization curves involving pyrite, pyrrhotite and mild steel in Fig. 1.9 (Pozzo and Iwasaki, 1987). 

MesoScale Discovery (MSD) succeeded in introducing product with a similar technology approach based upon ruthenium redox-mediated electrochemical detection (Figure 2.14). MSD is a joint venture of its parent company, MesoScale, and IGEN, a company that pioneered much of fhe work on electrochemical detechon based on the ruthenium redox system. MSD s Multi-Spot plates contain antibodies immobilized on multiple working electrode pads within each well, allowing each spot within the well to serve as an individual assay. Multiplexed cytokine immxmoassays can be performed in 96-well (4,7, or 10 spots per well) patterns with detection limits of 1 to 10 pg/mL and a linear dynamic range up to 3,000 pg/mL. Both 24-and 384-well electrode systems are available. 

Microelectrode arrays containing AChE were also utilised within a flow injection system [40]. A system was developed where a sample was separated and flushed simultaneously through eight cells, each containing a screen-printed electrode and fitted with a separate bespoke mini-potentiostat (Fig. 15.3). This allowed multiple measurements to be made on a single water sample using multiple electrodes, each specific for a different pesticide due to inclusions of different AChE mutants in each of the electrodes. Pattern-recognition software could then be utilised to deduce the pesticide levels in a potentially complex sample. 

If a microsystem should interface a regenerating nerve, some basic demands have to be taken into consideration the system must be absolutely non toxic, it must be stable in the physiologic environment for a long time, and the mechanical load on the severed nerve should be as minimal as possible during muscle contraction and limb movement. The neural prostheses should be equipped with multiple electrodes for recording nerve signals and for stimulation of different portions of the nerve. 

The ability to switch the operation of electrochemical metal sensors between active and passive modes on demand offers substantial improvements in their stability in the presence of common surfactants, as demonstrated in stripping-voltammetric signals obtained from cadmium in the presence of gelatin and Tween 80. Bare electrodes display a substantial diminution of the cadmium peak in the presence of both surfactants. In contrast, the adaptive-nanowire electrode system exhibits a highly stable response with a negligible change of the peak current over multiple measurements. 

The operating frequencies are determined by the capacitance of the electrode dielectric system in general multiple electrodes require a lower frequency. If possible it is preferable to use more than one electrode—as this gives a more uniform discharge, more even treatment and a lower gap current—enabling the dissipation of more power and so lengthening the lives of electrodes and dielectric. 

The first method is a sure way to expose the surface of powder uniformly if one pass is sufficient to achieve the surface modification, but it is not easy to recycle the substrate in the luminous gas phase in vacuum. Therefore, the main issue in this approach is how to repeat the interaction of surface with the luminous gas phase efficiently, which entirely depends on the flow dynamics of powders. Multiple-step operation requires multiple discharge systems or repeated operation. The generation of discharge is more or less the same as the conventional modes used in LCVD reactors. External radio frequency electrodes or coil with glass tube is the most... 

The standard-addition method (see Section 8C-3) involves determining the potential of the electrode system before and after a measured volume of a standard has been added to a known volume of the analyte solution. Multiple additions can also be made. Often, an excess of an electrolyte is incorporated into the analyte solution at the outset to prevent any major shift in ionic strength that might accompany the addition of standard. It is also necessary to assume that the junction potential remains constant during the measurements. 

Oxygen ion conductors are used in amperometric sensors for a variety of gas species. The selectivity is controlled by selecting electrode materials that catalyze particular reactions hence, multiple electrodes are required for some multicomponent gas mixtures. In such cases, the system is designed so that each electrode removes a particular gas from the gas stream. The particular gas removed by a particular electrode can also be controlled by the voltage at the electrode, just as in cyclic voltammetry, which has also been used in solid-electrolyte based sensors [34]. In addition to providing selectivity, control of the applied voltage can be used to improve the magnitude of the response [35]. 

A catheter tip or probe electrode is placed in a naturally occurring cavity in the body, such as in the gastrointestinal system. A metal tip or segment on a catheter makes up the electrode. In the case where there is no hollow lumen, the catheter or probe is inserted into the cavity so that the metal electrode makes contact with the tissue. A lead wire down the lumen of the catheter or down the center of the probe connects the electrode to the external circuitry. Multiple electrodes can be placed along the probe as illustrated in Figure 4.5a. 

A mapping of the immittance distribution in a tissue layer (tomography) is possible with an electrode system of multiple skin surface electrodes. In EIT, a current (about 1 mA) is typically injected in one electrode pair and the voltages between other electrodes are recorded (Resell et al., 1988b Bayford and Tizzard, 2012). Current injection is then successively shifted so that all electrode pairs are used. The reciprocal theorem can serve as a control of system linearity. A frequency on the order of 50 kHz is conunonly used, so a complete set of measurements with, for example, around 50 electrodes can be performed in less than 0.1 s. The images obtained have a resolution of about 1 cm at 10 cm tissue depth. 

Figure 11.2 A current density versus electrode potential curve for a multiple reaction system, of oxygen reduction, Fe(III) ion reduction and metal deposition.

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