Amperometric detector cells

Dionex Corporation 1228 Titan Way P.O. Box 3603 Sunnyvale, CA 94088-3603 Pulsed amperometric detector cells and instrumentation... 

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... 

Bioanalytical Systems were the first to manufacture metal electrodes that would interchange with their standard GCE blocks in their amperometric detector cell. Thiols could be detected by forming an amalgam on a gold electrode. The static electrode had increased sensitivity relative to the mercury-pool electrode requiring similar low potentials but importantly was easier to operate. Nobel metal electrodes work at potentials intermediate between those of mercury-based electrodes and those required by a GCE (Figure 5.1). With the development of these metal electrodes the availability and the applicability of the methods described above were extended. 

Figure 18. Schematics of the most common amperometric detector cells A) Wall-jet cell B) Thin-layer cell a) Working electrode...

In amperometric detector cells, the analyte is transported to the working electrode surface by diffusion as well as convection migration is suppressed by the supporting electrolyte. Electrolyte concentrations in the eluent of 0.01 - 0.1 mol/L are sufficient. Several cell geometries employing solid electrodes have been designed and tested. The two types shown in Figure 18 are the most commonly used. 

The thin-layer cell is properly the most common and commercially successfully amperometric detector cell. These have been constructed in number of ways, utilising differing materials, but are characterised by a fluid flow that is contained parallel to the working electrode (normally 2-... 

Electrochemical Detectors Another common group of HPLC detectors are those based on electrochemical measurements such as amperometry, voltammetry, coulometry, and conductivity. Figure 12.29b, for example, shows an amperometric flow cell. Effluent from the column passes over the working electrode, which is held at a potential favorable for oxidizing or reducing the analytes. The potential is held constant relative to a downstream reference electrode, and the current flowing between the working and auxiliary electrodes is measured. Detection limits for amperometric electrochemical detection are 10 pg-1 ng of injected analyte. 

The amperometric detector is currently the most widely used electrochemical detector, having the advantages of high sensitivity and very small internal cell volume. Three electrodes are used ... 

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 3.8 Amperometric detectors (a) measure the current that flows between the working electrode, usually a glassy carbon electrode, and a reference electrode, at a fixed voltage, usually close to the discharge potential for the compound. Coulometric detectors (b) are less common and are designed with a porous carbon flow cell so that all the analyte reacts in the cell, the amount of current consumed during the process being proportional to the amount of the substance.

Amperometric detectors can operate over a range of conversion efficiencies from nearly 0% to nearly 100%. From a mathematical point of view, a classical amperometric determination (conversion of analyte is negligible) is one where the current output is dependent on the cube root of the linear velocity across the electrode surface as described by Levich s hydrodynamic equations for laminar flow. Conversely, the current response for a cell with 100% conversion is directly proportional to the velocity of the flowing solution. While the mathematics describing intermediate cases is quite interesting, it is beyond the scope of this chapter. 

Packed-bed electrodes need not be cylindrical. Takata and Muto reported on a rectangular design some years ago in which a bed of carbon fibers was used for the electrode material [15]. While a number of innovative applications were reported, for the reasons described earlier, cells of this type do not provide detection limits competitive with those that can be achieved using more conventional amperometric detectors. 

Fig. 23 Cell construction of an amperometric detector for capillary LC. 1 column 2 and 5 fluoroplastic body 3 working electrode 4 reference electrode. (Reproduced with permission from Elsevier.)...

The ClinLab flow cell Sputnik [123] was originally developed for H PLC applications but is a separate device which in principle can be combined with every amperometric detector which is designed for three-electrode operation (Figure 4.74). Two PEEK capillaries and three electronic plugs are the only external interfaces. 

Electrochemical detection was performed with an Ion Chrom/Amperometric Detector (P/N 35221). The cell consists of a silver rod working electrode 1.3cm longxO. 178cm in diameter, an Ag/AgCl reference... 

The three most common modes of operation of electrochemical detection are amperometric, coulometric, and potentiometric. An amperometric detector is an electrochemical cell that produces a signal proportional to the analyte concentration usually the percentage of the analyte that undergoes the redox reaction is very low, about 5%. 

There are a number of electrochemical interactions which may be useful as the basis for detection in HPLC the most commonly used electrochemical detectors are based on amperometric measurements. The principle of operation of an amperometric detector is the oxidation or reduction of analyte in a flow-through electrolysis cell with a constant applied electrical potential, e.g. the oxidation of hydroquinone. 

To overcome the problem of detection in CE, many workers have used inductively coupled plasma-mass spectrometry (ICP-MS) as the method of detection. " Electrochemical detection in CE includes conductivity, amperometry, and potentiometry detection. The detection limit of amperometric detectors has been reported to be up to 10 M. A special design of the conductivity cell has been described by many workers. The pulsed-amperometric and cyclic voltametry waveforms, as well as multi step waveforms, have been used as detection systems for various pollutants. Potentiometric detection in CE was first introduced in 1991 and was further developed by various workers.8-Hydroxyquino-line-5-sulfonic acid and lumogallion exhibit fluorescent properties and, hence, have been used for metal ion detection in CE by fluorescence detectors.Over-... 

With the development of the pulsed amperometric detector (PAD) [13] a new detector cell was also designed. It is schematically shown in Fig. 6-10. To facilitate replacing the... 

Amperometric detectors are some of the most selective and sensitive detectors used to monitor ion chromatography separations. They are selective because they operate on the prineiples of oxidation or reduction of substances at an electrode. The potential needed to induce electrolysis differs for each ion. Detector selectivity is controlled by controlling the magnitude of the potential applied to the cell, the detector electrode material and the solution pH. The detectors may operate down to the picoe-quivalent sample concentration level which requires nanoamperc current measurement... 

Potentiometric electrodes of all types In flow-injection analysis (FIA) glass, ion-selective, amperometric electrodes, etc., can all theoretically be used in a detector cell to quantify some chemical substance. 

Figure 20.5 Amperometric detection in HPLC and HPCE. (a) Two models of detector cells. The porous graphite working electrode have a large surface and operates under coulometric conditions. The flow of the mobile phase at the working electrode ensures renewal of the electroactive species (b) Detail of the end of a capillary in HPCE. The working electrode, placed on the cathodic side of the apparatus, is bathed by ions exiting the capillary. Apart from phenols, aromatic amines and thiols, few analytically important molecules are electroactive.

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