Waveform, voltammetric

The difference between the various pulse voltammetric techniques is the excitation waveform and the current sampling regime. With both normal-pulse and differential-pulse voltammetry, one potential pulse is applied for each drop of mercury when the DME is used. (Both techniques can also be used at solid electrodes.) By controlling the drop time (with a mechanical knocker), the pulse is synchronized with the maximum growth of the mercury drop. At this point, near the end of the drop lifetime, the faradaic current reaches its maximum value, while the contribution of the charging current is minimal (based on the time dependence of the components). 

To overcome these problems, most voltammetric detectors have used pulsed waveforms such as staircase , squarewaveand differential pulseThe current is sampled at the end of the pulse after the charging current has decayed. In addition, because the charging current is typically the major current source, iR problems are not as severe. Last has described a coulostatic detector based on charge pulses instead of potential pulses which eliminates iR and charging current... 

The most commonly used waveform for in vivo voltammetric measurements is square-wave. This involves the application of a potential pulse to the working electrode for a fixed time at fixed intervals. The current is measurai at the end of the potential pulse to minimize capacitive charging current contributions. This waveform is shown in Fig. 15 A. 

Differential pulse voltammetry provides greater voltammetric resolution than simple linear sweep voltammetry. However, again, a longer analysis time results from the more sophisticated potential waveform. At scan rates faster than 50 mV/sec the improved resolution is lost. Because it takes longer to scan the same potential window than by linear sweep, an even longer relaxation time between scans is required for differential pulse voltammetry. 

The problem of selectivity is the most serious drawback to in vivo electrochemical analysis. Many compounds of neurochemical interest oxidize at very similar potentials. While this problem can be overcome somewhat by use of differential waveforms (see Sect. 3.2), many important compounds cannot be resolvai voltammetrically. It is generally not possible to distinguish between dopamine and its metabolite 3,4-dihydroxyphenylacetic acid (DOPAC) or l tween 5-hydroxytryptamine (5-HT) and 5-hydroxyindolacetic acid (5-HIAA). Of even more serious concern, ascorbic acid oxidizes at the same potential as dopamine and uric acid oxidizes at the same potential as 5-HT, both of these interferences are present in millimolar concentrations... 

The realization that current sampling on a step pulse can increase the detection sensitivity by increasing the faradaic/charging ratio is the basis for the development of various pulse voltammetric (or polarographic) techniques. Also, the pulses can be applied when it is necessary and can reduce the effect of diffusion on the analyte. Figure 18b. 11 shows the waveform and response for three commonly used pulse voltammetric techniques normal pulse voltammetry (NPY), differential pulse voltammetry (DPV), and square-wave voltammetry (SWV). 

Figure 5.2 Small-amplitude voltammetric techniques (a) various small-amplitude waveforms are imposed on a dc ramp (normally only one waveform is used in a given experiment) (b) the sigmoidal dc response is typical of dc polarography and hydrodynamic voltammetry. The greatest amplitudes for the small-amplitude current (Aiac) are achieved on the rising part of the dc current, where the small-amplitude voltage signal causes the greatest change in the surface concentrations (c) small-amplitude current response versus applied dc potential.

Various voltammetric waveforms can be employed during the stripping step, including linear scan, differential pulse, square-wave, staircase, or alternating-current operations. The differential pulse and square-wave modes are usually performed at the hanging mercury drop electrode, while linear scan stripping is usually performed in connection with the mercury film electrode. 

P. Ivarsson, S. Holmin, N.-E. Hojer, C. Krantz-Rulcker and F. Winquist, Discrimination of tea by means of a voltammetric electronic tongue and different applied waveforms, Sens. Actuators B, 76 (2001) 449-454. 

Equation (6.96) can be applied to any sequence of constant potential pulses and so to any voltammetric technique. In the particular case of cyclic voltammetry, the waveform is given by Eq. (5.1) and the current takes the form... 

Note that in this case (Q,JQ ) is only dependent of Ep, i.e., it has an stationary character being independent of the potential-time waveform applied [48], and has an analogous dependence on the potential to that shown by the normalized voltammetric current (///d,c) obtained for a reversible charge transfer reaction under diffusion control (see Eq. (2.36)). Equation (6.132) can be written as... 

A sensitivity increase and lower detection limit can be achieved in various ways with the use of voltammetric detectors rather than amperometry at fixed potential or with slow sweep. The principle of some of these methods was already mentioned application of a pulse waveform (Chapter 10) and a.c. voltammetry (Chapter 11). There is, nevertheless, another possibility—the utilization of a pre-concentration step that accumulates the electroactive species on the electrode surface before its quantitative determination, a determination that can be carried out by control of applied current, of applied potential or at open circuit. These pre-concentration (or stripping) techniques24"26 have been used for cations and some anions and complexing neutral species, the detection limit being of the order of 10-10m. They are thus excellent techniques for the determination of chemical species at trace levels, and also for speciation studies. At these levels the purity of the water and of the... 

The projected curve on the plane records the change of the imposed working electrode overpotential waveform during the scan. An important parameter in cyclic voltammetric studies is the sweep rate t). The linear sweep rate for Figure 45 was 30 mV/s. 

Perhaps the simplest to discuss is the rectangular excitation waveform. In fact, this method can even be combined with a slow voltammetric scan upon which the chopped irradiation response appears superimposed. This photovoltammetry experiment is schematized in Figure 25. Two types of transient responses are apparent. 

In voltammetric methods, the is varying via some waveform, to alter the working electrode potential as a function of time and the resulting current measured. The current change occurs at the decomposition potential range, which is hopefully specific for a given analyte. However, the location of the current response as a function of Egppi provides... 

Finally, transient potential/current waveforms may be used for polymerization. Cyclic voltammetric growth has mostly been used to carry out mechanistic studies. The use of pulsed current or potential is not a common practice. Recently, however, pulsed-current methods28 29 have been used by Mitchell and coworkers to produce more ordered anisotropic films. The use of transient waveforms adds another dimension to electropolymer growth, because the oxidation/reduction of the polymer according to Equation 2.2 will occur during growth, and the effect of this on the polymerization process must be considered. 

Stripping voltammetry involves the pre-concentration of the analyte species at the electrode surface prior to the voltammetric scan. The pre-concentration step is carried out under fixed potential control for a predetermined time, where the species of interest is accumulated at the surface of the working electrode at a rate dependent on the applied potential. The determination step leads to a current peak, the height and area of which is proportional to the concentration of the accumulated species and hence to the concentration in the bulk solution. The stripping step can involve a variety of potential waveforms, from linear-potential scan to differential pulse or square-wave scan. Different types of stripping voltammetries exist, all of which commonly use mercury electrodes (dropping mercury electrodes (DMEs) or mercury film electrodes) [7, 17). 

In voltammetry, a variable potential excitatioti signal is impressed on a working electrode in an electrochemical cell. I his excitation signal produces a characlerisiic current response, which is the measurable quantity in this method. The waveforms of four of the most common excitation signals used in voltammetry are shown in Figure 23-1. The classical voltammetric excitation... 

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