Instrumentation voltammetric techniques

Voltammetry. The voltammetric techniques are based on the current-voltagetime relationship at microelectrodes. To perform voltammetry, the oil/antioxidant sample is dissolved in a solvent containing an electrolyte and a three-electrode system (glassy carbon working electrode, a platinum wire reference electrode, and platinum wire auxiliary electrode) is inserted into an oil/solvent solution. A fresh oil typical of the application (100% standard) and the solvent system (0 % standard) is used to calibrate the voltammetric instrument for % remaining antioxidant determination (Kauffman, 1989 and 1991). 

A comparison of voltammetric techniques 96 A quantitative comparison of the kinetic discrimination of common electrode geometries at steady state 97 Steady-state vs. transient experiments 102 Current and future directions of voltammetry 104 Instrumentation 104 Electrodes 105 Voltammetric simulations 108... 

Electrochemical instruments are normally based on the changes in electrical energy that occur when a chemical reaction takes place, for example ion-selective electrodes (potentiometry) and voltammetric techniques. These can be measured in different ways and can give various qualitative and quantitative information about the reactants or products. In the case of conductivity measurements, changes in ionic content are monitored and, although nonspecific, can give useful data. 

Wet ashing under pressure is useful for mineralising the milk specimens for measurement by most instrumental techniques. Its effectiveness for voltammetric techniques has been demonstrated using a mixture of nitric, perchloric and sulphuric acid (Hasse and Schramel, 1983). The use of strong oxidants, e.g. vanadium pentoxide and perchloric acid, should be made with caution and preferably after preliminary ashing of the samples in a furnace or prolonged treatment with concentrated nitric acid. 

Perhaps the most important advantage of the voltammetric techniques over the atomic spectroscopic techniques is the ability of the voltammetric techniques to differentiate between the different oxidation states of the metal, and hence give environmentally more relevant information. As was briefly stated (5.1) this applies only to the final solution used in the instrumental determination step. However it does mean that a simpler separation step can be used prior to the voltammetric procedure and still allow quantitative specia-tion. In some cases, involving effluent or natural water samples, the separation step can even be eliminated. 

Voltammetric techniques that can be applied in the stripping step are staircase, pulse, differential pulse, and square-wave voltammetry. Each of them has been described in detail in previous chapters. Their common characteristic is a bell-shaped form of the response caused by the definite amount of accumulated substance. Staircase voltammetry is provided by computer-controlled instruments as a substitution for the classical linear scan voltammetry [102]. Normal pulse stripping voltammetry is sometimes called reverse pulse voltammetry. Its favorable property is the re-plating of the electroactive substance in between the pulses [103]. Differential pulse voltammetry has the most rigorously discriminating capacitive current, whereas square-wave voltammetry is the fastest stripping technique. All four techniques are insensitive to fast and reversible surface reactions in which both the reactant and product are immobilized on the electrode surface [104,105]. In all techniques mentioned above, the maximum response, or the peak current, depends linearly on the surface, or volume, concentration of the accumulated substance. The factor of this linear proportionality is the amperometric constant of the voltammetric technique. It determines the sensitivity of the method. The lowest detectable concentration of the analyte depends on the smallest peak current that can be reliably measured and on the efficacy of accumulation. For instance, in linear scan voltammetry of the reversible surface reaction i ads + ne Pads, the peak current is [52]... 

Pulse voltammetric techniques, most used in electrochemistry, are normal pulse voltammetry (NPV) and differential pulse voltammetry (DPV). In square wave voltammetry (SWV), there may be a non-faradaic contribution to the individual currents but the current sampling strategy essentially eliminates this through subtraction, as will be seen in Sect. 2.2.4.3. SWV was pioneered by Barker [1] in the 1950s, but due to instrumentation development only 40 years... 

These developments were all based on the dropping mercury electrode and in each case the central feature is the instrument which is the embodiment of the technique. The developments of Barker are particularly significant because his was the first electronic instrument and because it was soon commercialized by Mervyn Instruments as the Mervyn-Harwell Square Wave Polarograph. A photograph of this instrument is shown in Figure 2. This pattern was to prove increasingly important because the electronic implementation of pulse voltammetric techniques required expertise and time outside the reach of the average scientist. Thus the use of these techniques would depend on the availability at an acceptable price of reliable commercial instruments. 

Brown ER, McCord TO, Smith DE, DeFord DD (1966) Some investigations on instrumental compensation of nonfaradaic effects in voltammetric techniques. Anal Chem 38 1119-1129. doi 10.1021/ ac60241a004... 

The aim of this paper is to describe the principal considerations for setting-up an electrochemical detection system for HPLC, including the determination of electroactivity of organic compounds using a novel microscale voltammetric technique, the choice of mobile phase and the selection of instrumental approaches for optimum performance. The application of electrochemical detection to several classes of less easily oxidized compounds, including sterols, nonionic surfactants and organic acids, is illustrated. 

Voltammetric and amperometric measurements in BIA can be performed with the same electrochemical cells and detectors utilized in FIA, including wall-jet arrangement with injection of sample volume of a minimum 10 pi and the use of microelectrode arrays [104, 105]. Special attention has been attributed to the monitoring in agriculture due to the possibility of exploring portable instruments for in situ analysis. It occurs mainly through association between BIA and stripping voltammetric techniques. 

As it can provide some of the most basic electrochemical information related to the reactivity of the selected analyte (peak potential and peak current) most instruments that perform amperometry can also perform some of the most basic voltammetric techniques. These techniques determine the current as a function of the potential applied to the WE (in a conventional three-electrode cell) and can be performed with relatively simple instrumentation [105,106]. As different signals can be combined in the input ports of the instrument, multiple variations of the technique have been developed including cyclic voltammetry, linear sweep voltammetry, linear sweep stripping voltammetry, stripping voltammetry [107, 108], fast-scan cyclic voltammetry [109], square-wave voltammetry [110],and sinusoidal voltammetry [111]. 

Following this initial introductory chapter, the book is structured in three parts. The first part discusses different Electroanalytical Techniques in Batch and Continuous Systems as highly remarkable tools in the agricultural and food field. In this sense. Chapter 2 deeply explores the sweep potential electroanalytical techniques, while Chapter 3 allows the readers to obtain fundamental information on voltammetric techniques coupled to flow systems which could proportionate faster analysis, reproducible results, high sensitivity, with additional advantages such as the requirement of less sample, and the use of simpler instrumentation. 

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