Electrode construction electrochemical detector

Electrochemical detectors are constructed with three electrodes. The electrolysis of interest takes place at the working (marked W in Fig. 4.5) electrode at a potential measured by the reference electrode. The auxiliary (or counter) electrode potential is controlled to maintain the reference potential. The reference electrode is usually saturated calomel or silver/silver chloride. Platinum or glassy carbon is generally used for the auxiliary electrode [67, 53]. 

Pulse polarographic studies have been described using a microcell of 0.5 mL capacity, which analyzed two 1,4-benzodiazepines, with the lowest detection limit reported to date being 10-20 ng/mL of blood [199]. Detailed construction of the cell and electrode assembly was also described (shown in Fig. 26.16). Further miniaturization of this type of three-electrode cell is not practical hence further increases in sensitivity will have to rely on electrochemical detector flow cells of microliter capacity such as those used in conjunction with liquid chromatography (see Chap. 27). 

The hydrodynamically well-defined conditions of flow systems are an ideal environment for electrochemical detectors, resulting in enhanced performance characteristics. The surface sensing properties of most electrochemical methods require particular attention in the construction of suitable flow-through cells. Efficient and repeatable mass transport toward the electrode surface is necessary, and dead volumes should be small. Various flow-through cells have been designed for electrochemical detection, all of which can be derived from the basic configurations depicted in Figure 4. 

In the following part of this paper I am going to present a system of organic electroactive groups or of substances which can be subjected to voltammetric analysis at solid electrodes forming thus a base for construction of electrochemical detectors in HPLC. Since electroactivity is the most important property of the substance to be studied and we are interested here only in the analytical properties, i.e. in the characteristical potentials (Ep or Ep and in the shape of the i - E or ip - E plot it will be not necessary to stress Adams observatlon[7] "Electrochemistry of organic oxidations is mainly the chemistry of follow-up reactions" (of course, similar ideas hold true with reductions). Nevertheless, electroactivity will be discussed here jointly with probable or proved interpretations of mechanisms. This is done in contrast to Bond s view[9] who considers interpretations of electrode processes and of their follow-up or preceding reaction useless in analytical chemistry. 

Even more convenient are the conditions for determining quinones[14] at platinum or carbon solid electrodes because their reversible 2-electron (or 2 one electron steps in aprotic media) reduction proceeds at considerably positive potentials. This property could be utilized in constructing an electrochemical detector for vitamin K derivatives ... 

Therefore the oxidation processes of these compounds at platinum or carbon electrodes will be presented which are, or could be, used for constructing an electrochemical detector. 

Basically, an electrochemical detector for HPLC is a flow electrochemical cell which contains all of its basic elements (working, reference and auxiliary electrodes, holder, and connections) adapted to the experimental conditions expected in the chromatographic separations. The characteristics of the materials used for the holder and the electrodes construction must be compatible with typical mobile phases, working pressures, and temperatures employed in HPLC whereas the geometric configuration must provide an easy coupling... 

Electrochemical detector LC-4C electrochemical detector (BAS, Tokyo, Japan). Electrochemical flow cell the radial flow cell (BAS) constructed from a glassy-carbon working electrode (9 6 mm), an Ag/AgCl reference electrode, and a stainless-steel counter electrode. The 25 pm thickness of the polytetrafluoroethylene (PTFE) gasket was set between the flow-cell block and the working electrode in the electrochemical cell the cell volume was therefore 0.7 pL. 

Karube and Yokoyama presented an overview on the developments in the biosensor technology [60]. The overview describes the use of micromachining fabrication techniques for the construction of detection units for FIA, electrochemical flow cells and chemiluminescence detectors. Acetylcholine microsensors using carbon fiber electrodes and glutamate microsensors for neuroscience were discussed. 

Detector E, Metrohm 1096/2, platinum working electrode +0.4 V, Ag/AgCl reference electrode following post-column reaction. The column effluent passed through an electrochemical cell (construction details in paper) and the bromide was oxidized to bromine at 3 pA. The mixture flowed through a 20 s reaction coil (3.9 m ( ) X 0.33 mm ID) to the detector. 

Several of the ion-selective membrane electrodes described in the previous section can also be employed as transducers to devise highly selective potentiometric gassensing probes. As shown in Table 4, equilibrations of gases in aqueous solution yield ionic species. Hence, a number of relatively simple and analytically useful probes can be constructed by incorporating ISE-based electrochemical cells as detectors behind outer gas permeable membranes, and choosing appropriate... 

The three-electrode setup in the Uqnid flow cell was constructed as follows the thin Cn film ( 300-nm thick) with 5 nm adhesive Cr layer was used as working electrode which was thermally deposited on the cell side of the SijN membrane (b in Fig. 5.19) a Pt wire and a Ag wire, inserted into the liquid flow cell through small holes on the sides of the PEEK body, were nsed as the counterelectrode and pseudoreference electrode, respectively (b and in Fig. 5.19). A VersaSTAT4 potentiostat (Princeton Applied Research) was used to manipulate the potential for electrochemical reactions. The in situ SXAS measuranents were recorded with the TPY mode nsing the channeltron detector under the controlled potentials (the TEY mode was not accessible in solntions). The thickness of the Cn film deposited on the SijN membrane was selected to ensure the transmission of both incident X-rays and the outgoing fluorescence photons. 

Commercially available cells with rate constant of 500 s and a cell volume of about 5 pi assure coulometric efficiency for typical HPLC flow rates with minimal extra-column band broadening. Each electrochemical unit has a central porous carbon electrode, on either side of which is situated a reference electrode and an auxiliary electrode. The characteristics of porous graphitic carbon electrode facilitate the construction of electrode arrays, lypical commercial systems include two units placed in series but arrays of up to 16 units are commercially available (Thermo Scientific, formerly ESA/Dionex). These cells have some degree of resistance to flow and with use can develop a significant back pressure. To minimize such back pressure changes, they need to be protected from particulate materials. Their intrinsic back pressure should also be borne in mind when connecting other types of HPLC detector cell in series. 

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