Voltammetric analysis methods

The great attraction of SV lies in the effect of pre-concentration of the analyte at the electrode with, as a consequence for the stripping current, a very high ratio of faraday current to charging and impurity currents it is this high ratio which has made SV the most sensitive voltammetric analysis method to date. 

The key factor in voltammetry (and polarography) is that the applied potential is varied over the course of the measurement. The voltammogram, which is a current-applied potential curve, / = /( ), corresponds to a voltage scan over a range that induces oxidation or reduction of the analytes. This plot allows identification and measurement of the concentration of each species. Several metals can be determined. The limiting currents in the redox processes can be used for quantitative analysis this is the basis of voltammetric analysis [489]. The methods are based on the direct proportionality between the current and the concentration of the electroactive species, and exploit the ease and precision of measuring electric currents. Voltammetry is suitable for concentrations at or above ppm level. The sensitivity is often much higher than can be obtained with classical titrations. The sensitivity of voltammetric... 

Determination of trace metals in seawater represents one of the most challenging tasks in chemical analysis because the parts per billion (ppb) or sub-ppb levels of analyte are very susceptible to matrix interference from alkali or alkaline-earth metals and their associated counterions. For instance, the alkali metals tend to affect the atomisation and the ionisation equilibrium process in atomic spectroscopy, and the associated counterions such as the chloride ions might be preferentially adsorbed onto the electrode surface to give some undesirable electrochemical side reactions in voltammetric analysis. Thus, most current methods for seawater analysis employ some kind of analyte preconcentration along with matrix rejection techniques. These preconcentration techniques include coprecipitation, solvent extraction, column adsorption, electrodeposition, and Donnan dialysis. 

Radi [41] used an anodic voltammetric assay method for the analysis of omeprazole and lansoprazole on a carbon paste electrode. The electrochemical oxidations of the drugs have been studied at a carbon paste electrode by cyclic and differential-pulse voltammetry in Britton-Robin-son buffer solutions (0.04 M, pH 6-10). The drug produced a single oxidation step. By differential-pulse voltammetry, a linear response was obtained in Britton-Robinson buffer pH 6 in a concentration range from 2 x 10-7to 5 x 10 5 M for lansoprazole or omeprazole. The detection limits were 1 x 10 8 and 2.5 x 10 8 M for lansoprazole and omeprazole, respectively. The method was applied for the analysis of omeprazole in capsules. The results were comparable to those obtained by spectrophotometry. 

The determination of hydroperoxide number is significant because of the adverse effect of hydroperoxides on certain elastomers in the fuel systems. This method (ASTM D-6447) measures the same peroxide species, primarily the hydroperoxides in diesel fuel. This test method does not use the ozone-depleting substance l,l,2-trichloro-l,2,2-trifluoroethane (ASTM D-3703) and is applicable to any water-insoluble, organic fluid, particularly gasoline, kerosene, and diesel fuel. In this method, a quantity of sample is contacted with aqueous potassium iodide (KI) solution in the presence of acid. The hydroperoxides present are reduced by potassium iodide, liberating an equivalent amount of iodine, which is quantified by voltammetric analysis. 

The example given on Figure 20.12 corresponds to anodic voltammetric analysis. When combined with the technique of DPP (Section 20.5.2), stripping voltammetry constitutes the most universally applied voltammetric method. Current instruments for which control of all the parameters is established, allow reproducible conditions and offer an alternative to spectroscopic methods such as atomic emission. 

Several related bulk electrolysis techniques should be mentioned. In thin-layer electrochemical methods (Section 11.7) large AIV ratios are attained by trapping only a very small volume of solution in a thin (20-100 fxm) layer against the working electrode. The current level and time scale in these techniques are similar to those in voltammetric methods. Flow electrolysis (Section 11.6), in which a solution is exhaustively electrolyzed as it flows through a cell, can also be classified as a bulk electrolysis method. Finally there is stripping analysis (Section 11.8), where bulk electrolysis is used to preconcentrate a material in a small volume or on the surface of an electrode, before a voltammetric analysis. We also deal in this chapter with detector cells for liquid chromatography and other flow techniques. While these cells do not usually operate in a bulk electrolysis mode, they are often thin-layer flow cells that are related to the other cells described. 

In trace analysis the voltammetric stripping methods are popular because of their sensitivity - ranging down to sub ppb concentration. These methods are accurate and precise and the instrumentation is of low cost. The stripping methods are based on previous accumulation of the ion or compound to be determined on the working electrode. In addition to the electrolytical accumulation - used in anodic or cathodic stripping voltammetry (ASV or CSV resp.) also adsorptive accumulation of the species on the working electrode can be exploited, as many organic compounds exhibit surface active properties that are manifested by their adsorption from solution onto the surface of the solid phase. 

Adsorptive transfer stripping voltammetry (AdTSV) was introduced in 1986 as a new anal5 ical procedure based on the adsorptive pre-concentration of biomacromolecules on an electrode, the transfer of the adsorbed layer into a background electrolj e and subsequent voltammetric analysis [35]. The advantages of AdTSV were summarized as follows (i) the method utilizes differences in adsorbability of substances to their separation, (ii) due to their strong adsorption, analytes (oligonucleotides) can be separated from complex media, which are not suitable for voltammetric analysis of the conventional type, (iii) the interaction of biomacromolecules immobilized on the surface of the electrode with substances contained in the solution is possible, and (iv) all mentioned points can be affected by electrode potential [35]. 

Escherichia coli heat-shock gene, and in rRNA genes. Except spectral and thermodynamic analysis (CD, NMR, and calorimetry), this heptamer was studied electrochemically (CV, LSV, EVLS) [46]. On mercury electrodes the hairpin d(GCGAAGC) provides voltammetric reduction signals of A and C, and oxidation signals of G. Both signals have been studied in dependence on pH, accumulation time, scan rate, and loop sequences. The AdS EVLS was employed for the determination of the detection limit (2 nM), which was verified by multidimensional voltammetric analysis using Fourier transform in combination with the confidence ellipse statistic method. Our results showed the difference in electrochemical behavior of DNA and RNA heptamers (Fig. 11.5). 

One advantage of being able to perform the voltammetric analysis of en2ymes is that it is possible to obtain information about the kinetics of electron transfer by varying the scan rate. It is crucial, however, that proper control experiments be performed to demonstrate that interactions between the modified electrode and en2yme are not perturbing the electrochemical properties of the en2yme. In the likely event that voltammetric analysis is not feasible for a particular protein system, indirect electrochemical methods are often successful. 

Semiintegration is a mathematical operation signified by the operator applied to some function of the variable x. When applied to transient voltammetry, x is invariably time, while the function to which it is applied may be current, flux density, or concentration [21]. Semiintegration of I-t data resulting from the voltammetric analysis of dissolved species produces curves (M(t) A s )) that mimic those obtained by steady-state voltammetry. Analysis of the M(f) data can be undertaken using the methods developed predominantly by Saveant et alf [22] and Oldham and coworkers [23-26]. The... 

The approach used in the analysis of voltammetric data, and which has evolved over the past 50 years, is briefly surveyed in schematic form in Figure 2.16. At this point, it is deemed necessary to emphasize the importance of varying the concentration in any experiment aimed at studying the kinetics - a simple experimental rule which has often been overlooked in many years. Furthermore, as shown in the final step of the diagram in Figure 2.16, an experimentalist should always seek to reconcile conclusions from voltammetric analysis with the results obtained from independent methods. 

Voltammetric/polarographic methods are not very important for anion analysis. Only bromate, iodate, and periodate can be determined by means of polarographic reduction currents. Indirect determinations are possible for those anions that form compounds of low solubility or stable complexes with the Hg ions formed in the anodic oxidation of the electrode mercury. The current caused by the dissolution of the mercury is proportional to the concentration of these anions in the sample... 

While the redox chemistry of metal- and flavin-based cofactors may be readily detected by voltammetry, that associated with thiol-disulfides and amino acids is not. It is also important to be aware that voltammetry by itself provides no direct insight into the chemical identity of the redox couple. This can be overcome by using electrodes that allow for simultaneous spectroscopic and voltammetric analysis of adsorbed proteins. Examples include Ag electrodes that allow for surface-enhanced resonance Raman spectroscopy (SERRS) and mesoporous nanocrystalline Sn02 electrodes that allow for electronic absorption or magnetic circular dichro-ism spectroscopies [3]. Another consideration is the need for a redox center to be positioned within ca. 14 A of the electrode, and so the surface of the protein, for facile interfacial electron exchange. As a consequence, proteins with only buried redox centers are not routinely addressed by direct electrochemical methods. 

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