Amperometric cells

Aniline, methyl aniline, 1-naphthylamine, and diphenylamine at trace levels were determined using this technique and electrochemical detection. Two electrochemical detectors (a thin-layer, dual glassy-carbon electrode cell and a dual porous electrode system) were compared. The electrochemical behavior of the compounds was investigated using hydrodynamic and cyclic voltammetry. Detection limits of 15 and 1.5nmol/l were achieved using colourimetric and amperometric cells, respectively, when using an in-line preconcentration step. 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

Green tea Dilution Dionex AminoPac PAIO (250 x 2 mm) Ternary gradient eluent consisting of water/0.25M NaOH/l.OM NaOAc No ED (equipped with a thin layer type amperometric cell) [248]... 

Ion-selective electrodes and amperometric cells have had a long history of success in a wide variety of applications (8,9). A microelectronics-inspired revolution is also occurring in these devices, brought about by the advent of photolithographically defined arrays of microelectrodes on planar substrates... 

Amperometric detection, which generally offers quite high sensitivity, has not been used so far for forensic (toxicological) applications. The reason probably reflects some problems with commercially available coupling of the high-performance amperometric cell with the capillary electrophoresis device. 

Somov, S.I. and Guth, U. (1998) A parallel analysis of oxygen and combustibles in solid electrolyte amperometric cells. Sens. Actuators B, B47 (1-2), 131-8. 

Oxidation reaction of sulfur dioxide in nitric acid, in contact with the working electrode of an amperometric cell (cf Chapter 20) ... 

Electrochemical pulsed amperometric cell assembly. (Courtesy - Dionex Corp., Sunnyvale, CA)... 

The selectivity of electrochemical detection can be improved by the use of two electrodes (dual-mode detection). Basically, two different combinations are used two amperometric cells and the combination of a coulometric cell with an amperometric cell. The difference between these two cell types is that in amperometric cells only a fraction of the eluting analytes react, whereas in coulometric cells analytes (and all other eluting compounds ) may be quantitatively converted depending on the working potential. Using these two combinations a variety of different experimental setups are possible [74,271]. 

W. L. Caudill, J. O. Howell, and R. M. Wightman, Flow Rate Independent Amperometric Cell. AnaL Chem., 54 (1982) 2532. 

Electrochemical sensors include amperometric cells, especially galvanic fuel cells, and polarographic cells. Both types are available from several manufacturers world-wide in large numbers for clinical use. Higher stability with fewer technical problems makes the galvanic cell currently the most popular oxygen sensor in the operating room environment [12]. 

Fk. 31., Differential amperometric cell per Brunt and Bruins (1978). W, sampled the aomud LC effluent at A. W2 sampled the mobile phase via a low-pressure pathway at B, sans iqiector or column. Redrawn with permission from Elsevier, Amsterdam. 

Conceptually, it should be obvious that the overall conversion efficiency is a delicate function of flow rate, cell dimensions, and electrode area. All detectors can be made to bear 100% efficiency, if desired, by simply minimizing channel thickness and reducing flow rate to allow maximum cell residence time. Conversely, amperometric cells can Ik less than 100% cIVcc-tive by raising flow rate or using less electrode area. Figure 6 shows this trend... 

A typical response of an amperometric cell is illustrated in Figure 2. On increasing the voltage, a rise in current is obtained when the redox potential of the species sensed is reached. A current plateau is reached for the diffusion limited case as discussed above, since this transport process is independent of the applied voltage. If the voltage is increased further. 

Lifetime. As already mentioned above, some rugged solid-state sensors have lifetimes of several years. Certain sensors can also be regenerated when their function begins to deteriorate. The shortest lifetimes are exhibited by biosensors. Ion-selective electrodes and optical sensors based on membrane-bound recognition molecules often lose their ability to function by a leaching-out effect. In optical sensors the photobleaching effect may also reduce the lifetime to less than a year. On the other hand, amperometric cells work well for many years, albeit with restricted selectivities. 

Figure 8.13 Schematic representation of the electrode block of an amperometric cell, (a) New cell with a leveled gold working electrode and (b) used cell with a larger cell volume due to recession of the working electrode.

Apart from UV detection, there are two other alternative detection methods for trace analysis of nitrite. A very sensitive detection method for nitrite is DC amperometry at +1.1 V on a carbon paste electrode. In this case, amperometry and suppressed conductivity detection can be applied simultaneously by directing the conductivity cell effluent through the amperometric celL Figiu-e 10.17 exemplifies this with a surfiice water sample that was injected imdiluted [26]. As can be seen firom this chromatogram, nitrite concentrations in the lowest micro-gram/Liter range can be detected without any problem. 

To avoid the flow dependency of amperometry at uncovered electrodes a defined diffusion layer thickness is often established by covering the electrode surface with a solid membrane which enables the EAS passage by solution diffusion or diffusion through perforation(s) [14]. An example of a membrane covered two-electrode amperometric cell is shown in Fig. 3. 

The kind of cell shown in Fig. 3 can be used in principle also for measurements in gases. Basic equations given in chapter 1.3.1 are valid also for this application. Another different form of amperometric cell that is extensively used in gases is based on solid instead of liquid electrolytes. To establish appropriate ion conductivity levels these electrolytes have to be operated at elevated temperatures in the range 500-800 °C [17]. To withstand those temperatures the diffusion barrier of these cells is often made of perforated or porous ceramics as indicated in Fig. 4. 

The response time of membrane-covered amperometric sensors depends on the material of the membrane and its thickness. For example, a sensor with a 20 pm thick PP membrane has a response time of about tgo = 30 s. Details on the theoretical background of the temporal behaviour of membrane covered amperometric cells were published by Mancy et al. [24]. 

In a modified membrane-covered amperometric cell a Pb anode is combined with an Ag cathode in a KOH solution [26, 27]. In this sensor, which is usually called Mackereth sensor, the following reactions take place ... 

Amperometry, Fig. 6 (a) Measuring circuits for amper-omelric sensors. Two-electrode amperometric cell, (b) Measuring circuits for amperometric sensors. Three-electrode amperometric cell... 

Liquid Electrolyte Sensors The construction of amperometric cells with liquid electrolytes for gas analysis does not differ principally from those for measurements in liquids. Differences can be found in the utilized membranes, electrolytes and electrode materials. Gases as carbon monoxide or nitrogen oxide can be determined only with catalytically activated... 

Optimization of miniaturized amperometric cells for multicomponent lab-on-a-chip systems [47],... 

The enzyme modified electrode was fitted into a Teflon holder and inserted into a flow-through wall-jet amperometric cell (28), The enzjmie electrode was used as the working electrode, an A AgCl (0.1 M KCl) electrode as reference electrode, and a platinum wire served as the auxiliary electrode. The electrodes were connected to a three-electrode potentiostat (Zata Elektronik, Lund, Sweden), controlling the applied... 

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