Ramp, voltage

The basic instrumentation required for controlled-potential experiments is relatively inexpensive and readily available commercially. The basic necessities include a cell (with a three-electrode system), a voltammetric analyzer (consisting of a potentiostatic circuitry and a voltage ramp generator), and an X-Y-t recorder (or plotter). Modem voltammetric analyzers are versatile enough to perform many modes of operation. Depending upon the specific experiment, other components may be required. For example, a faradaic cage is desired for work with ultramicroelectrodes. The system should be located in a room free from major electrical interferences, vibrations, and drastic fluctuations in temperature. 

A partial solution to this problem can be obtained by so-called interrupted LSV, which means that on passing the first peak [see Fig. 3.35(a)] the voltage ramp is held sufficiently long at the value concerned before being continued, and so on with the same procedure for subsequent peaks the peak splitting in the interrupted technique is impressive [see Fig. 3.35(b)] in contrast to the normal technique. 

Linear-svreep voltammogram A normal voltammogram, in which the voltage ramp stops at the switch potential (in contrast to a cyclic voltammogram, in which it does not). 

Ramp A jargon term, meaning to smoothly increase at a constant rate of d(variable)/dr. A voltage ramp is therefore dE/dt. 

Switch potential, Ex, In cyclic voltammetry, the potential at which the voltage ramp changes direction. 

For calculating effective mobilities for identification purpose, voltage ramping needs to be corrected. ... 

Xuan, X., Hu, G., and Li, D. (2006). Joule heating effects on separation efficiency in capillary zone electrophoresis with an initial voltage ramp. Electrophoresis 27, 3171 — 3180. 

In this mode of operation, the standard voltage ramp is replaced by a series of brief pulses (50 to 100 ms) of increasing potential. The pulses are delivered at the... 

Figure 19.4—Pulse polarography. The techniques of NPP and DPP are shown with two examples of measurements. Arrows on the voltage ramp diagram indicate the instant at which the current is measured.

In cyclic voltammetry, we apply the triangular waveform in Figure 17-22 to the working electrode. After the application of a linear voltage ramp between times t0 and f, (typically a few seconds), the ramp is reversed to bring the potential back to its initial value at time t2. The cycle may be repeated many times. 

Square wave voltammetry achieves increased sensitivity and a derivative peak shape by applying a square wave superimposed on a staircase voltage ramp. With each cathodic pulse, there is a rush of analyte to be reduced at the electrode surface. During the anodic pulse, reduced analyte is reoxidized. The voltammogram is the difference between the cathodic and the anodic currents. Square wave voltammetry permits fast, real-time measurements not possible with other electrochemical methods. 

Figure 7.34 Principle of kinetic single-photon counting. L, pulsed light source S, sample P, photodiode F, interference filter or monochromator D, photomultiplier R, voltage ramp generator (1 to start the ramp, 0 to stop it). The voltage V is fed into a multichannel analyser M. Inset voltage ramp V(t)...

Using two pulsed tunable dye lasers, Na atoms in a beam are excited to an optically accessible ns or ml state as they pass between two parallel plates. Subsequent to laser excitation the atoms are exposed to millimeter wave radiation from a backward wave oscillator for 2-5 [is, after which a high voltage ramp is applied to the lower plate to ionize selectively the initial and final states of the microwave transition. For example, if state A is optically excited and the microwaves induce the transition to the higher lying state B, atoms in B will ionize earlier in the field ramp, as shown in Fig. 16.5. The A-B resonance is observed by monitoring the field ionization signal from state B at fB of Fig. 16.5 as the microwave frequency is swept. 

A further standard method for electrochemical analysis is cyclic voltammetry. A voltage ramp is increased and decreased between two potential limits and the curent is monitored. In the resulting curve, electrochemical reactions in the equilibrium state can be detected. At platinum electrodes, the formation of Pt-H complexes and the oxidation and reduction at the metal surface can clearly be observed (Fig. 23). 

Column is flushed for 20 minutes with mobile phase (AP = 150 bar) followed by Electrokinetic conditioning min. at 10 kV with a 25 min. voltage - ramp... 

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