Working amperometry

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. 

Ampcromctry The final voltammetric technique to be considered is amperome-try, in which a constant potential is applied to the working electrode, and current is measured as a function of time. Since the potential is not scanned, amperometry does not lead to a voltammogram. 

One important application of amperometry is in the construction of chemical sensors. One of the first amperometric sensors to be developed was for dissolved O2 in blood, which was developed in 1956 by L. C. Clark. The design of the amperometric sensor is shown in Figure 11.38 and is similar to potentiometric membrane electrodes. A gas-permeable membrane is stretched across the end of the sensor and is separated from the working and counter electrodes by a thin solution of KCl. The working electrode is a Pt disk cathode, and an Ag ring anode is the... 

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. 

Coulometry and amperometry can be distinguished by the extent to which the analyte undergoes a Faradaic reaction at the working electrode, namely complete and partial, respectively. Coulometry is essentially high-efficiency amperometry with working electrodes of large surface area. Successful coulometric or amperometric detection can result only if the applied potential is chosen correctly. 

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... 

In amperometry, the current at the working electrode is proportional to analyte concentration. The amperometric glucose monitor generates H202 by enzymatic oxidation of glucose and the H202 is measured by oxidation at an electrode. A mediator is employed to rapidly shuttle electrons between electrode and analyte. 

The simplest controlled potential experiment is the potential step [34] illustrated in Fig. 15. Such experiments are sometimes termed chrono-amperometry , signifying that the current (-ampero-) is measured (-metry) as a function of time (chrono-). Sometimes, two steps, as in a double-step experiment [34] [Fig. 16(a)], or a sequence of small steps, as in staircase voltammetry [35—37] [Fig. 16(b)], are applied. When the potential of the working electrode is changed by a step for only a brief period of time before being returned to its original (or near to its original) value, we speak of a pulse . There are many varieties of pulse voltammetry [38—41], some of which are discussed in Chap. 4. 

When tested in stirred batch amperometry, the PB-modified screen-printed electrodes showed no loss of signal for H202 (20 pm oil ) after 50 h at pH 7.4 in batch amperometry. Other experiments performed at pH 9 confirmed the high pH stability of the PB-modified SPEs produced. At pH 9 there was still 80% of the residual activity of PB recorded after 16 h of continuous work in a solution of H2O2 (20pmoll 1) (Table 24.2). 

Amperometry is the most widely reported EC detection mode for CE microchips, which primarily relies on oxidation or reduction of elect-rochemically active species by applying a constant potential to a working electrode. The current is then monitored as a function of time. Since it is based on the redox reaction that occurs at the electrode surface, electrodes can be miniaturised without loss in sensitivity. The relevance of this simple technique is reported in several reviews [48,74], In this section, a general overview of the combination of this detection technique to CE microchips together with special sections for different amperometric techniques and electrode materials and types are considered. 

When the purpose of the working electrode is to act as an inert electron sink, as in the detection of catecholamines, carbon is the preferred electrode material. On occasions when the electrode plays a direct role in the reaction, the precious metals are chosen. For example, silver can be oxidized to silver cyanide in the presence of cyanide ions. A major consideration when choosing an electrode material is its ability to maintain an active surface. Electrodes will develop a layer of surface oxide at positive applied potentials. The oxide layer will inhibit the oxidation of the analyte, and the response will decrease with repeated injections. The active surface can be renewed by polishing the electrode. Since glassy carbon electrodes are more resistant to poisoning by oxide formation, they are the electrode of choice for direct current amperometry. 

Direct current (DC) amperometry is used for the analysis of catecholamines, phenols, and anilines, which are easy to oxidize. A single potential is applied, and the current is measured. The current resulting from the oxidation or reduction of analyte molecules is dependent on many factors, including the concentration of the analyte, temperature, the surface area of the working electrode, and the linear velocity of the flowing stream over the surface of the working electrode. 

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. 

There are many substances which would appear to be good candidates for LC-EC from a thermodynamic point of view but which do not behave well due to kinetic limitations. Johnson and co-workers at Iowa State University used some fundamental ideas about electrocatalysis to revolutionize the determination of carbohydrates, nearly intractable substances which do not readily lend themselves to ultraviolet absorption (LC-UV), fluorescence (LC-F), or traditional DC amperometry (LC-EC) [2], At the time that this work began, the EC of carbohydrates was more or less relegated to refractive index detection (LC-RI) of microgram amounts. The importance of polysaccharides and glycoproteins, as well as traditional sugars, has focused a lot of attention on pulsed electrochemical detection (FED) methodology. The detection limits are not competitive with DC amperometry of more easily oxidized substances such as phenols and aromatic amines however, they are far superior to optical detection approaches. 

In electrochemical detection, the potential of a working electrode can be measured versus a reference electrode, usually while no net current is flowing between the electrodes. This type of detection is referred to as potentiometry. Alternatively, a potential is applied to the working electrode with respect to the reference electrode while the generated oxidation or reduction current is measured. This technique is referred to as amperometry. When applying a negative po-... 

Amperometry. Amperometric methods measure the current produced at a working electrode in response to an applied potential. Amperometric enzyme assays rely on the production of an oxidizable or reducible species from an enzyme-catalyzed reaction. The applied potential is extreme enough to completely oxidize (at positive potentials) or reduce (at negative potentials) any analyte that contacts the working electrode. In stirred or unstirred solutions, the current produced under such mass-transport-controlled conditions is directly proportional to analyte concentration. 

Fig. 3-49. Separation of sulfide and cyanide. — Separator column IonPac AS3 eluent 0.001 mol/L Na2C03 + 0.01 mol/L NaH2B03 + 0.015 mol/L ethylenediamine flow rate 2.3 mL/min detection amperometry on a Ag working electrode injection volume 50 pL solute concentrations 0.5 ppm sulfide and 1 ppm cyanide.

Fig. 3-105. Separation of various sugar alcohols and saccharides. - Separator column CarboPac PA-1 eluent 0.15 mol/L NaOH flow rate 1 mL/min detection pulsed amperometry at a Au working electrode injection volume 50 pL solute concentrations 10 ppm xylitol, 5 ppm sorbitol, 20 ppm each of rhamnose, arabinose, glucose, fructose, and lactose, 100 ppm sucrose and raffmose, 50 ppm maltose.

Fig. 3-110. Gradient elution of various mono- and disaccharides. - Separator column IonPac AS6A eluent (A) water, (B) 0.05 mol/L NaOH gradient linear, from 7% B to 100% B in 15 min flow rate 0.8 mL/min detection pulsed amperometry at a Au working electrode with post-column addition of NaOH injection volume 50 pL solute concentrations 15 ppm inositol (1), 40 ppm sorbitol (2), 25 ppm fucose (3) and deoxyribose (4), 20 ppm deoxyglucose (5), 25 ppm arabinose (6), rhamnose (7), galactose (8), glucose (9), xylose (10), mannose (11), fructose (12), melibiose (13), isomaltose (14), gentiobiose (15), and cellubiose (16), 50 ppm turanose (17), and maltose (18).

Fig. 4-5. Analysis of arsenite with amperometric detection. - Separator column IonPac ICE-AS1 eluent 0.0005 mol/L HC1 flow rate 1 mL/min detection D.C. amperometry on a Pt working electrode oxidation potential +0.95 V injection volume 50 pL solute concentration 10 ppm arsenite.

Information about suitable working potentials for the amperometric detection of electroactive species are obtained in voltammetric experiments. The term voltammetry refers to the investigation of current-voltage curves in dependence of the electrode reactions, the concentrations and its exploitation for analytical chemistry. Of the different types of voltammetry, information from the hydrodynamic and pulsed voltammetry can best be applied to amperometry. In both cases, the analyte ions are dissolved in a supporting electrolyte which has several functions ... 

While the working potential required for the desired electrochemical reaction may be determined with voltammetric experiments, amperometry is used as the detection method in ion chromatography. A distinction is made between amperometry with constant working potential and pulsed amperometry. 

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