Amperometric Measurement

The applied potential affects not only the sensitivity and signal-to-noise characteristics but also die selectivity of amperometric measurements. In general, a lower potential is more selective, and a higher one more universal. Thus, compounds undergoing redox potentials at lower potentials can be detected with greater selectivity. The selection of die applied potential relies on the construction of... 

The simplest, and by far the most common, detection scheme is the measurement of the current at a constant potential. Such fixed-potential amperometric measurements have the advantage of being free of double-layer charging and surface-transient effects. As a result, extremely low detection limits—on the order of 1-100 pg (about 10 14 moles of analyte)—can be achieved, hi various situations, however, it may be desirable to change the potential during the detection (scan, pulse, etc.). 

In the direct-reading instruments the emf of the cell is led through an (operational) amplifier across a standard high resistor yielding a current that is measured by a milliammeter calibrated to be read in pH units or millovolts. So, while the null-point system provides a truly potentiometric (non-faradaic) measurement where the off-balance adjustment remains limited to an interrupted temporary current draw-off, the direct-reading system represents an amperometric measurement where a continuous steady-state current draw-off takes place as long as the meter is switched on. In fact, the latter is a deflection method as a pointer indicates the pH units or millivolts by its deflection on the meter scale. 

Amperometric cells, sensors using, 22 271 Amperometric measurements, 14 612 Amphetamine, 3 89-90 Amphibole asbestos, 1 803 3 288 crystal structure, 3 297-298 exposure limits, 3 316 fiber morphology, 3 294-295 silicate backbone, 3 296 Amphibole potassium fluorrichterite, glass- ceramics based on, 12 637 Amphiphile-oil-water-electrolyte phase diagram, 16 427-428 Amphiphile-oil-water phase diagrams,... 

Henderson or Plank formalisms. Mobilities for several ions can be seen in Table 18a. 1. Liquid junction potentials can become more problematic with voltammetric or amperometric measurements. For example, the redox potentials of a given analyte measured in different solvent systems cannot be directly compared, since the liquid junction potential will be different for each solvent system. However, the junction potential Ej can be constant and reproducible. It can also be very small (about 2-3 mV) if the anion and cation of the salt bridge have similar mobilities. As a result, for most practical measurements the liquid junction potential can be neglected [9]. 

To understand the fundamental differences between potentiometric and amperometric eleclroanalytical measurements, namely potentiometric measurements are those of the potential made at zero current (i.e. at equilibrium), while amperometric measurements are of the current in response to imposing a perturbing potential (dynamic, i.e. a non-equilibrium measurement). 

An amperometric technique relies on the current passing through a polarizable electrode. The magnitude of the current is in direct proportion to the concentration of the electroanalyte, with the most common amperometric techniques being polarography and voltammetry. The apparatus needed for amperometric measurement tends to be more expensive than those used for potentiometric measurements alone. It should also be noted that amperometric measurements can be overly sensitive to impurities such as gaseous oxygen dissolved in the solution, and to capacitance effects at the electrode. Nevertheless, amperometry is a much more versatile tool than potentiometry. 

The differences between potentiometry and amperometry are summarized in Table 1.1. It will be seen that amperometric measurements are generally more precise and more versatile than those made by using potentiometry, so the majority of this book will therefore be concerned with amperometric measurements. 

Amperometry The techniques and methodology of determining a concentration as a function of current the most popular amperometric measurement technique is voltammetry. 

Table 3.2 lists the most salient microbial sensors reported to date, together with the type of immobilized microorganism and measurement used, and the response time and dynamic range achieved in each instance. As can be seen, most of these biosensors rely on amperometric measurements. Some of them are described in detail below. 

Trom reversible voltammetry. Not determined. Trom amperometric measurements. 

For further characterization of the protein multilayer, two electrochemical techniques were employed independently. The first technique involves amperometric measurements of the GOx multilayer-modified electrode in the presence of glucose. Figure 15 plots the output current (A/) of the electrode to 1 mM glucose as a function of the deposition number. The response current depends linearly on the number... 

Electrochemical biosensors [18] Here we mean biomimetic sensors, which utilize the ability of biological materials (enzymes, antibodies, etc.) to recognize specific components and to catalyze their reactions with great specificity. Many of the biosensors are electrochemical sensors, based on potentio-metric or amperometric measurements. For example, in the case of an amperometric... 

Figure 19.8—A selective electrode designed from a MOSFET (metal oxide semiconductor field effect transistor). A specific reaction can be monitored by putting an enzyme in contact with the electrodes. This schematic shows the three electrodes used for amperometric measurement.

The handling and disposal problems associated with the use of liquid solvent extractors have resulted in increased attention to the separation and preconcentration of organic compounds in water by collection in synthetic polymers followed by elution with an organic solvent (2). For example, selective collection and concentration of organic bases on methylacrylic ester resin from dilute water samples have been reported (3). Such collection techniques are especially well-suited to flow-injection measurement techniques. In this study, ionizable organic analytes such as salicylic acid and 8-hydroxyquinoline (oxine) were extracted into a polymer and then back extracted by an aqueous solution. Amperometric measurement using a flow-injection technique was employed to monitor the process. 

AT-Phcnyl-AT,-(2-thiazolyl)guanidine forms 2 1 complexes with Cu11 and Hg11 and 1 1 complexes with WV1 and Movl as indicated by absorption spectra, conductometric and amperometric measurements.171 Room temperature Mossbauer parameters have been tabulated for dehydro-A, A, A"-trialkylguanidinohexacarbonyldiiron(O) and related compounds.172... 

For in vivo electrochemical measurement, the placement of the electrodes is important. In the potentiometric measurement (e.g., measurement of the pH of gastric juices) both the working electrode and the reference electrode are placed in the stomach. On the other hand, when amperometric measurement is done, the working electrode is in the place where you are getting the information from, but the auxiliary (reference) electrode can be anywhere, even on the skin. 

What will happen when the electrodes are reversed in amperometric measurement ... 

Electroanalytical sensors based on amperometric measurements at chemically modified electrodes are in the early stages of development. The modes of modification can take many forms, but the most common approach at the present time is the immobilization of ions and molecules in polymer films which are applied to bare metal, semiconductor, and carbon electrodes. Such surface-modified electrodes exhibit unique electrochemical behavior which has been exploited for a variety of applications. 

Determination of the substance concentration with a potentiometric ISSs and C02-GSS is realized through the measurement of the electromotive force (EMF) of the galvanic cell (potentiometric measurement). In the case of the 02-GSS, however, the current is measured (amperometric measurement). 

OPH-based biosensors have been fully discussed in previous reviews [2,165]. AChE-based biosensors are based on the principle that OP pesticides have an inhibitory effect on the activity of AChE that may be permanent or partially reversible. The extent of the inhibition is directly related to the concentration of the pesticide and therefore enzyme activity may be used as a measure of the inhibition [166]. The amperometric measurement of AChE activity can be based on the measurement of any of the following three mechanisms [167] (1) production of hydrogen peroxide from choline, (2) oxygen consumption during the enzyme reaction or (3) production of electroactive compounds directly from the oxidation of acetylthiocholine chloride such as thiocholine. The measurement of hydrogen peroxide and oxygen consumption has been described in more details in other reviews [167]. 

Another class of enzymes that has found wide application in the biosensor field in the last decades is that of the cholinesterases which have been mainly used for the detection of pesticides. For the amperometric detection of cholinesterase activity, both the substrates acetylcholine and acetylthiocholine have been extensively used [6-9], the latter being preferred because this avoids the use of another enzyme, choline oxidase, which is usually coupled with acetylcholinesterase. However, the amperometric measurement of thiocholine, produced by... 

The analytical parameters with respect to H202 amperometric measurement were evaluated in both batch and continuous flow mode and their stability, especially at basic pHs, was studied. 

The modified electrode showed a very good stability even at pH 9. Although at this pH a decrease in the redox peaks in CV is observed after 250 cycles, the PB layer is still highly electroactive and sufficient to catalyse the H2O2 reduction. In fact, stirred batch amperometric measurements of H202 (10 pm oil ) were carried out before and after the continuous cycling at pH 9, and the decrease of the signal was only 10% of the initial value. 

In the last step of an assay, i.e. during the electrochemical detection of the electroactive species of interest, a new window appears in order to let the experimenter follow the detection on-line. As presented in Fig. 36.8, the software has been designed to perform chrono-amperometric measurements of the desired analyte. On the left part of the window the raw measurement data that provide the evolution of the current for each microchannel that is measured during a time period of generally 2 s appears. With the multi-potentiostat used, the eight channels are... 

All the amperometric measurements were accomplished in a solution of 0.05mol 1 1 phosphate buffer+O.lmol 1 , KC1, pH 7.4. Hydrogen peroxide solutions were prepared using the same buffer solution. 

Electropolymerization and cyclic voltammetric experiments are performed with an EG G PARC, Model 173 potentiostat equipped with a Model 175 universal programmer and a Model 179 digital coulometer in conjunction with a Kipp and Zonen BD 91 XY/t recorder. All experiments are carried out using a conventional three-electrode cell. Instrumental setup for amperometric measurements ... 

Electrochemical transducers can be divided into conductometric, potentio-metric and amperometric measuring principles. 

In contrast to the potentiometric principle where nearly no transport of analyte to the sensor occurs, the amperometric measurement requires the control of faradaic current at the electrodes and the diffusion of an analyte towards the electrode. One typical example for an electrochemical measuring device is the Clark electrode which uses the reduction of oxygen [58]. 

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