Electrochemical cells, amperometry

In potentiometry, the potential of an electrochemical cell under static conditions is used to determine an analyte s concentration. As seen in the preceding section, potentiometry is an important and frequently used quantitative method of analysis. Dynamic electrochemical methods, such as coulometry, voltammetry, and amper-ometry, in which current passes through the electrochemical cell, also are important analytical techniques. In this section we consider coulometric methods of analysis. Voltammetry and amperometry are covered in Section 1 ID. 

Voltammetric methods are based on measurements made using an electrochemical cell in which electrolysis is occurring. Voltammetry, sometimes also called amperometry, involves the use of a potential applied between two electrodes (the working electrode and the reference electrode) to cause oxidation or reduction of an electroactive analyte. The loss or gain of electrons at an electrode surface causes current to flow, and the size of the current (usually measured in mA or pA) is directly proportional to the concentration of the electroactive analyte. The materials used for the working electrode must be good conductors and electrochemically inert, so that they simply transfer electrons to and from species in solution. Suitable materials include Pt, Au, Hg and glassy carbon. 

Faradaic techniques are those in which oxidation or reduction of analyte species occurs at the electrodes and therefore a measurable current is passed through the electrochemical cell. This discussion will be limited to controlled-potential techniques, primarily volta-metry and amperometry, coupled to liquid chromatography. While other Faradaic electrochemical techniques have been developed and electrochemical techniques in bulk solution are common, the use of liquid chromatography employing these two detection strategies is by far the most common electroanalytical technique in pharmaceutical studies. 

Amperometry Measurements based on current flow produced in an electrochemical cell by an applied voltage. 

Amperometric sensors are small electrochemical cells consisting of two or three electrodes that are usually combined in a single body. A constant potential is applied, i.e., the sensor operates as a Faradaic cell, and a dependence of the measured current on the analyte concentration in the sample is obtained. As in ordinary amperometry, this requires a diffusion layer on the surface of the working electrode. This diffusion layer, in which the analyte concentration is depleted, arises because the analyte is consumed in the electrode reaction. In order for this depletion to occur, the electrode kinetics has to be faster than the... 

Voltammetric MEMS biosensors (voltammetry) are based on measuring the current-voltage relationship in an electrochemical cell consisting of electrodes in a solution. After a potential is applied to the sensor, current, which is proportional to the concentration of the electroactive species of interest, is measured. Amperometry is a special case when potential is maintained so as to be constant with time. 

FIGURE 12.1 Scheme of the artificial synapse configuration for which a UME is positioned near an emitting cell. Considering that the species released can be oxidized or reduced at the electrode surface, electrochemical techniques (amperometry at constant potential or FSCV) allow one to monitor the release in real time. 

Most electroanalytical methods are based on the measurements of electrochemical cell potential, current, charge, or impedance at a fixed or controlled external applied function to the electrochemical cell. There are three main types of electrochemical sensors based on either the measurement of a redox current (amperometry), the potential (potentiometry), or the impedance or capacitance (impedance spectroscopy)... 

Amperometry involves the determination of the intensity of a current as it flows through an electrochemical cell at an applied potential. This intensity is a function of the concentration of the electroactive species being oxidized or reduced at a probe electrode, a second electrode acts as a reference. After proper calibration, these intensities can be used to measure the concentration of some of the species present. 

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. 

Electrochemical sensors with a liquid electrolyte are widely used for the detection of corrosive or toxic gases in the workplace. Portable monitors are used in short time measurements of exhaust gases as well. These sensors work amperometri-cally - an external voltage supply is connected with the electrode on both sides of the measuring cell. 

We have already briefly described a popular application of amperometry in Chapter 13. This was the electrochemical detector used in HPLC methods. In this application, the eluting mobile phase flows across the working electrode embedded in the wall of the detector flow cell. With a constant potential applied to the electrode (one sufficient to cause oxidation or reduction of mixture components), a current is detected when a mixture component elutes. This current translates into the chromatography peak... 

Mercury is the electrode material of choice for many electrochemical reductions and some unique oxidations (see Chap. 14). We have explored the use of both small mercury pools and amalgamated gold disks in thin-layer amperometry. Other workers have used pools in a capillary tube [7] and amalgamated platinum wire [8]. In 1979, Princeton Applied Research introduced a unique approach based on their model 303 static mercury drop electrode (see Sec. II.F). Our laboratories and MacCrehan et al. [9] have focused on the use of amalgamated gold disks. This approach results in an inexpensive, easily prepared, and mechanically rigid electrode that can be used in conventional thin-layer cells (Sec. II.C) of the type manufactured by Bioanalytical Systems. 

We wish only to remind readers that there are three main methods of electrochemical re-vealment conductivity, direct current (d.c.) amperometry, and integrated amperometry (pulsed amperometry is a form of integrated amperometry). In revealment by conductivity, the analytes, in ionic form, move under the effect of an electric field created inside the cell. The conductivity of the solution is proportional to the mobility of the ions in solution. Since the mobile phase is itself an electrolytical solution, in order to increase the signal/noise ratio and the response of the detector, it is very useful to have access to an ion suppressor before the revealment cell. By means of ionic exchange membranes, the suppressor replaces the counterions respectively with H+ or OH , allowing only an aqueous solution of the analytes under analysis to flow into the detector. 

Any type of detector with a flow-through cell can be used for FIA. Photometric detectors are most often used in FIA (15-18, 25). However, many other analyses using fluorimeters (28, 29), refractometers (24), atomic absorption (30, 31), and inductively coupled plasma emission spectrometers (32) have been described. Electrochemical detectors based on potentiometry with ion-selective electrodes (15, 33), anodic stripping voltammetry (15, 34), potentiometric stripping (35), and amperometry (36) have also been used. 

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-... 

This kind of amperometry is the most widely used electrochemical detection method in liquid chromatography. A constant DC potential is continuously applied to the electrodes of the detector cell. The theory of amperometry with constant working potential does not differ from the theory of hydrodynamic voltammetry, even though the applied potential remains constant. 

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