Current discrimination

Zoski, C. G. Charging Current Discrimination in Analytical Voltammetry, /. Chem. Educ. 1986, 63, 910—914. 

Potential step methods have emerged as valuable electrochemical methods due to the highly sensitive nature of the technique. The waveform employed in potential step methods, also referred to as pulsed methods, have some advantages over potential sweep methods. The main advantage is that the steplike waveform can discriminate and separate the capacitive current versus the faradaic current, the current due to the reduction or oxidation undergone by the analyte, increasing signal to noise. Capacitive versus faradaic current discrimination is the basis for all of the pulsed techniques. The rate of decay of the capacitive current and the faradaic current is not the same. The capacitive current has an exponential decay whereas the faradaic current decays as a function of t Since the rate of decay of the capacitive current is much... 

Residual Current Even in the absence of analyte, a small current inevitably flows through an electrochemical cell. This current, which is called the residual current, consists of two components a faradaic current due to the oxidation or reduction of trace impurities, and the charging current. Methods for discriminating between the faradaic current due to the analyte and the residual current are discussed later in this chapter. 

The ground fault current may be detected through three or four CTs, one in each phase and the fourth in the neutral circuit (Figures 21.5(a) and (b)). Through the neutral to discriminate the fault, as discussed later. 

For the amplifier pulse to be recognized in the ADC, it must exceed the lower level set by a discriminator, which is used to prevent noise pulses from jamming the converter. Once the pulse is accepted it is used to charge a capacitor that is discharged through a constant current source attached to an address clock typically... 

Fig. 12A, has a 10° libration. This gives a channel size which would be optimal for an ionic radius between that of Rb+ and Cs+. Therefore enhanced discrimination is not expected between Rb+ and Cs+, but the energy required to librate further inward to make contact with smaller ions in the series can be expected to enhance selectivity between these ions. Work is currently in progress to calculate the change in channel energy as a function of libration angle or of the equivalent, the effective channel radius S6). The implications of a peptide libration mechanism for enhancing ion selectivity can also be pursued experimentally as outlined below. 

However, a maximum is reached at about 15 volts, and a decrease is observed at higher voltages. With increasing voltage, the kinetic energy of the primary ions is also increased. The increase in primary ion current apparently is counterbalanced and finally exceeded by the decrease in the cross-section of the ion molecule reaction. In addition, discrimination of CH5 + ions which are formed via a complex becomes more effective. At a potential of 20 volts between trap and chamber, most of the primary ions have kinetic energies around 20 e.v., and a secondary CH5+ ion... 

Certain occluders also discriminate among Na channels from neuronal and skeletal muscle. But in this case the blocking ligands are small peptides, the x-conotoxins from the mollusc Conus geo aphus. This molecule binds tightly to muscle Na channels, effectively reducing Na current (55 see Figure 6A), and also can displace bound... 

As with in vivo voltammetry, a variety of electrochemical techniques have been used for the stripping step. Because of its simplicity, linear sweep voltammetry has enjoyed widespread use however, the detection limit of this technique is limited by charging current. Differential pulse has become popular because it discriminates against the charging current to provide considerably lower detection limits. 

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