Detector electrode

Figure Bl.7.18. (a) Schematic diagram of the trapping cell in an ion cyclotron resonance mass spectrometer excitation plates (E) detector plates (D) trapping plates (T). (b) The magnetron motion due to tire crossing of the magnetic and electric trapping fields is superimposed on the circular cyclotron motion aj taken up by the ions in the magnetic field. Excitation of the cyclotron frequency results in an image current being detected by the detector electrodes which can be Fourier transfonned into a secular frequency related to the m/z ratio of the trapped ion(s).

To ensure that the detector electrode used in MEMED is a noninvasive probe of the concentration boundary layer that develops adjacent to the droplet, it is usually necessary to employ a small-sized UME (less than 2 /rm diameter). This is essential for amperometric detection protocols, although larger electrodes, up to 50/rm across, can be employed in potentiometric detection mode [73]. A key strength of the technique is that the electrode measures directly the concentration profile of a target species involved in the reaction at the interface, i.e., the spatial distribution of a product or reactant, on the receptor phase side. The shape of this concentration profile is sensitive to the mass transport characteristics for the growing drop, and to the interfacial reaction kinetics. A schematic of the apparatus for MEMED is shown in Fig. 14. 

The creation of nanostructured surfaces is one thing, the study of electrochemical reactions on such nanostructures is another one. Especially in electrocatalysis, where size effects on reactivity are often discussed, there have been attempts to use the tip of an STM as a detector electrode for reaction products from, say, catalytically active metal nanoclusters [84]. Flowever, such ring-disk-type approaches are questionable,... 

The rise in Br2 concentration is detected by measuring the current between the two detector electrodes at the right in Figure 17-8. A voltage of 0.25 V applied between these two electrodes is not enough to electrolyze any solute, so only a tiny current of < 1 p,A flows... 

Fig. 8. Typical tubular electrode assemblies, (a) Integral construction. A, Generator electrode B, detector electrode C, reference electrode D, counter electrode E, porous frits F, ball and socket joints G, epoxy resin, (b) Demountable type. A, Generator electrode B, counter electrode C, Teflon spacers D, reference electrode E, Teflon cell body F, brass thread. (From ref. 128.)...

The first combined HPLC-electrochemical measurements of vitamin K used the reductive mode, but this technique suffered from interference from the reduction of oxygen. A redox method was later developed that eliminated this interference, and provided a 10-fold increase in sensitivity over photometric detection and an improved selectivity. The coulometric detector employed in the redox mode is equipped with a dual-electrode cell in which phylloquinone is first reduced upstream at the generator electrode and the hydroquinone is reoxidized downstream at the detector electrode. 

Fig. 14 Analytical HPLC of the phylloquinone fraction from an extracted sample of brown rice isolated by semipreparative HPLC. Column, Spherisorb C8 (octyl) mobile phase, methanol/50 mM acetate buffer pH 3.0 (97 3) containing 0.1 mM EDTA, dual-electrode coulometric detection (redox mode), porous graphite electrodes, — 1.5 V (generator electrode), +0.05 V (detector electrode). The arrows signify the fraction containing tritiated phylloquinone 2,3-epoxide (internal standard) and phylloquinone (analyte) that is collected for quantitation by radioisotopic dilution. (Courtesy of M. J. Shearer.)...

Hydrophobic matrix components such as fats can foul electrochemical detector electrodes by blocking the active metal surface and inhibiting interaction between the analytes and the metal surface of the electrode. This problem is usually see as fairly rapid loss in peak area, injection to injection. 

The mass transfer by convection and diffusion within the channel is precisely calculable. Numerical methods are used to fit measured concentrations at the detector electrode to the fluid flow in the cell, and to the reaction kinetics at the surface and in solution [24]. The technique was extensively developed during a study of the dissolution of calcite in dilute aqueous acid [25], and has latterly been applied to a number of organic reactions. 

From the above equation, the measured capacitance between the source and the detector electrodes of the electrode pair i is determined from the given dielectric constant (permittivity) distribution of the medium under investigation. The processes of finding the capacitance for a given permittivity distribution is referred to as the forward problem. On the other hand, the process of finding the permittivity distribution from a set of capacitance measurements is referred to as the inverse problem. 

After the etching, the masking layer is removed and a passivation layer 26 and detector electrodes 28 and 29 are provided. A plurality of detectors can be formed in the body. Alternatively, the whole surface of the p-type HgCdTe body can be converted into n-type material before individual detectors are formed by mesa-etching. 

Sample preparation pooled urine (pH 2) spiked to SO ppb tetrachlorohydroquinone, hydrolyzed 30 min at 90°-100°C 5-mL volume extracted three times with ether, combined extracts evaporated and redissolved in 0.5 mL 40% methanol, 0.1 mL injected. Column 15 cm X 4.6 mm Waters fi Bondapak CIS mobile phase 0.1 M ammonium acetate pH 6, 38% methanol, 1.0 mL/min detector electrode glassy carbon, TL-5 2 mil gasket potentials vs. Ag/AgCl/3M NaCI reference electrode. 

Values of A are approximate B and B are functions of r C is a function of radius and ring potential C is a function of electrode length and detector electrode potential. 

C can be equal to A. The fraction of B which reaches the detector electrode is always less than unity because a part, after diffusing from the electrode to distance <5, is transported by convection to bulk solution and does not reach the detector electrode. The fraction of B reaching the detector electrode under these conditions is called the steady-state collection efficiency, N0. 

Experimentally, the generator electrode current, /gen, is controlled (galvanostatic control), usually being slowly increased from zero in an anodic or cathodic direction depending on the electrode reaction, and the detector electrode is held at a potential such that all the B reaching it is converted into C, i.e. a potential in the limiting current region for B and C, and passes current /det. 

The limiting current at the detector electrode with the generator electrode connected, IUdct, is then... 

In fact (N0f 213) < 1 and the term in brackets, the shielding factor, is always positive. The shielding factor is the maximum reduction in the detector electrode current that can be caused by the generator electrode its use is to remove an unwanted electroactive species from solution that interferes with the reaction under study. To maximize the reduction in current (minimize the shielding factor) we want an electrode geometry such that (A/o/3-2/3)—> 1, which corresponds to a very small interelectrode gap, (r2 — rf), and a thin detector electrode. 

The fraction of species B obtained at the detector electrode is Nk, the kinetic collection efficiency. An example of a first-order reaction is the bromination of anisole19, and of a second-order reaction the bromination of some proteins20. 

Experimentally, as for the steady-state collection efficiency, the generator electrode current is controlled and the potential of the detector electrode is held at a value corresponding to mass-transport-limited conversion of B to C. 

Fig. 8.11. Diffusion layer titration curve at a double hydrodynamic electrode (second order homogeneous reaction), (a) /det begins to rise when excess of B that did not react homogeneously reaches the detector electrode and A// /det = G(l/ar). (b) Assuming fast kinetics this is where linearity commences. From here...

In this technique the upstream electrode (generator) is galvanostati-cally controlled to generate a species that reacts with species X in solution in a second-order homogeneous reaction, the detector electrode being used to quantify the fraction of the electrogenerated species that did not react ... 

Figure 14.7 shows the technique of anodic stripping voltammetry with collection at a double hydrodynamic electrode30. The fact that it is possible to control the potentials of generator and detector electrodes independently is used to increase sensitivity. This procedure has been used with success at rotating and wall-jet electrodes4,27... 

The electronic system of the electrical conductivity detector is comprised of a 1 kHz frequency generator, the output of which is fed via a suitable impedance to the detector electrodes. The voltage across the electrodes is fed to a precision rectifier to provide a DC signal that is related to the conductivity of the fluid between the sensor plates. The DC signal is then passed to a signal modifying amplifier to... 

HPLC with amperometric detector wax impregnated carbon paste detector electrode 270... 

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