Thin-layer cell/configuration

A mixed self-assembled monolayer was used for the aptamer immobilization on the gold electrode. The aptamer-modified electrodes were then incubated for 1 h at 37°C with thrombin (18 ag/mL). Electrochemical measurements were recorded in the thin-layer cell configured to contain a total volume of 20 [xL. Thrombin chromogenic substrate (/3-Ala-Gly-Arg-p-nitroaniline) was injected into the cell and differential pulse voltammetry (DPV) measurements between -0.2 and -1V with a pulse height of—0.05 V and pulse duration of 70 ms were carried out. The DPV measurements showed that /3-Ala-Gly-Arg-p-nitroaniline substrate and the p-nitroaniline product have different redox potentials. Moreover, the DPV experiments showed a current peak at -0.45 V in the presence of the thrombin substrate. After 5 min, the peak at —0.45 V decreased and a new peak was detected at -0.70 V, indicating the formation of p-nitroaniline. The same measurements carried out on a control electrode in order to test the specificity of the assay in this experiment bovine serum albumin (BSA) substituted thrombin and in this case only the peak at 0.45 V was measured. 

Some of the comparisons are described below, but the results must be interpreted with due caution particularly in view of continuing improvements in detector design. One report compared two coulometric detectors for catecholamine analysis and concluded that they were equivalent to current amperometric detectors. Another study compared wall-jet and thin-layer cell configurations. Other more comprehensive studies have been controversial. Forzy et al compared 11 detectors for the analysis of 5-hydroxyindoleactic acid (5-HIAA). They used the same HPLC system with each detector to determine linearity, repeatability, absolute sensitivity, limit of detection and stabilisation time. Driebergen and Benders evaluated 10 detectors with respect to their suitability for routine use in a pharmaceutical company using tetramethylbenzidine as the test compound. Both of these reports found similar relative results with respect to sensitivity and ease of use but stressed the importance of matching instrument to application. 

Another route to exhaustive electrolysis again maximizes the A/V ratio by shrinking the electrolyte volume. A variety of such thin-layer cell configurations have been described [50-55]. The solution volume typically is few microliters in these configurations as opposed to a few milliliters in the usual scenarios. As long as the cell thickness is smaller than the diffusion layer thickness for a given experimental time [(2 DtY ], mass transfer within the cell can be neglected. 

Prom a more general point of view, Keddam et al. also demonstrated that the impedance measured in a thin-layer-cell configuration is significantly modified by the existence of a radial potential... 

As discussed above, if we assume that diffusion processes occur only normal to the electrode surface (one-dimensional model), the theoretical diffusion impedance associated with the transport of an electroactive species can be derived analytically in a thin-layer-cell configuration. For a thin-layer cell of thickness e, the expression of this impedance will depend on the condition considered at the boundary. 

The most widely used amperometric detectors are based on the thin-layer and wall-jet configurations (Figure 3-22). The thin-layer cell relies on a thin layer of solution that flows parallel to the planar electrode surface, which is imbedded in a... 

Figure 9.7 Three configurations for sandwich-type thin-layer cells. (A) Minigrid suspended between two spacers. (B) Twin-electrode cell using metal films on glass. (C) Single-electrode cell, barrier plate and electrode plate, s, Sample solution.

Film electrodes can be conveniently prepared from numerous types of materials on a wide variety of substrates in a bewildering variety of configurations, with high purity and chemical and spatial reproducibility. Some applications rely on the convenience of preparing uniform electrochemical films on optical quality substrates for reflection spectrometry [2], or for electrodes in thin-layer cells, where flatness and parallelism of electrodes to tolerances of a few micrometers is necessary [3,4]. Film electrodes are useful for many other applications. In... 

Insert an immunosensor into the thin-layer cell flow system setup shown in Fig. 35.3. The cell is configured as a three-electrode system with a Ag/AgCl reference and stainless-steel counter electrode. The gasket thickness should be >500 pm. Details are given in Ref. [2]. 

This section analyzes the response of a charge transfer process under conditions of finite linear diffusion which corresponds to a thin layer cell. This type of cell can be achieved by miniaturization process for obtaining a very high Area/Volume ratio, i.e., a maximum distance between the working and counter electrodes that is even smaller than the diffusion layer [31], In these cells it is easy to carry out a bulk electrolysis of the electroactive species even with no convection. Two different cell configurations can be described a cell with two working electrodes or a working electrode versus an electro-inactive wall separated at distance / (see Fig. 2.23). 

Figure 1 is representative of one choice that meets these criteria. Such a cell is normally described as a thin-layer sandwich configuration. The working electrode(s) is in the form of an interchangeable block. Electrodes of different sizes, shapes, or materials can be accommodated with a flow pattern established by a gasket shape and thickness. Such cells can easily be adapted for LC flow rates of from 5 to 5000 /rL/min. Different designs are used for capillary separation tools such as capillary electrophoresis (CE). 

A thin layer cell is comprised of an isolated plane at a very short distance e from a working electrode. By considering the system with a cylindrical sjnnmetry, calculated the impedance of a disk electrode in this configuration. 

Cells are classified according to how the working electrode is positioned relative to the flow stream. There are three major configurations tubular, thin layer, and wall jet. The tubular cell (open or packed) with its greater working electrode surface area is used for coulometric detection. The thin layer and wall jet designs are used for amperometric detector cells. In thin layer cells, the eluent flow is in the same plane as... 

A system somewhat similar to that of Hubbard was built by Yeager et al. [17, 18] and is shown schematically in Fig. 5. This uses a flat electrode and a thin layer cell as in Hubbard s first design, Fig. 2, but has two separate chambers for electrochemistry and UHV analysis, the sample holder being transferred between two transfer rods and a manipulator. This allows the use of a carousel on which several electrodes may be mounted. Ross and Wagner also adopted a thin layer configuration for the electrochemical cell [12, 19] with a palladium/hydrogen counter/reference electrode. 

A number of electrochemical cell designs have been described but the most popular configurations are the three-electrode thin-layer cell and the wall-jet cell. Figure 5.21[20,102,166-171,189]. The eolumn eluent is introduced either parallel to... 

Elbicki etal. 984) reviewed the optimum configurations for each of the above electrodes (thin-layer, tubular, and wall-jet) based on a mathematical treatment of the diffusive and convective phenomena in force. Boundary conditions on such physical restraints as electrode area, cell dimensions, and inlet configuration were established. Some confusion in the past has resulted from misinterpreting these equations (Weber, 1983) they are derived for cells in which the boundary layer may freely grow unencumbered. In certain cells (e.g., low-volume wall-jet or long-channel electrodes), walls, nozzles, etc. may impede the growth of the diffusion layer and bias the output current expected. Under these conditions, the wall-jet electrode behaves virtually as a thin-layer cell (if the nozzle spacing is small and the nozzle acts as a point source). Both detectors were concluded to yield output currents of... 

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