The pulse technique

The continuous wave technique has a definite advantage over the other techniques a very narrow band of frequencies is needed to transmit the information. The pulse techniques, on the contrary, use a large band of frequencies, and the various noises, pump noises in particular, are more difficult to eliminate. 

In order to minimize this problem, Ryan (57, 58) combined the pulse techniques of Tal roze (61) with a small continuous repeller field. In this operation, a cluster of ions is formed by a short ionizing pulse and is allowed to react under the influence of a small d.c. field for a certain time. The reaction is then quenched by applying a large (80 volts/cm.)... 

When this is done, the dependence of k(Ee) upon Ee is even greater than predicted by the dipole-alignment model, and the thermal rate constant predicted from this variation, extrapolated to thermal energies, is more than twice the thermal rate constant found experimentally by using the pulsing technique. 

Table I. Comparison between Rate Constants Measured by the Pulsing Technique and the Pressure Method...

Condensation reactions are somewhat more difficult to study by the pulsing technique since the secondary ion usually has a considerably higher mass than the primary ion. However, by restricting the total reaction time to 1 nsec, or less, we have found it possible to study this type of reaction under thermal conditions. Preliminary results are presented in Table IV. Many of the product ions in these systems can be formed in more than one reaction, and the details of reaction identification will be presented elsewhere. 

For method K3 and K4 where again Eltart is chosen to be less negative than E , we must consider the pulse technique in relation to the polarographic wave in the same ways as in Figs. 3.41(a) and 3.47 however, the measurement result is different as we sample the current either at the pulse top (K4) versus its base or at the pulse base (K3) versus its top (see Fig. 3.51). 

In the built-in pulse technique for DC, the stripping step of SV can also use the pulse technique. 

The pulse technique may also be conveniently extended to include stages of reactant preparation. Figure 9 shows a schematic representation of a pulse reactor system recently used by Gault et al. (81), which includes stages for alcohol (the reactant precursor) dehydration and subsequent olefin hydrogenation, the resulting saturated hydrocarbon being the material of catalytic interest. A method has been described (82) which allows the use of a pulse reactor at above atmospheric pressure. 

Pulsed FT-NMR has facilitated the study of nuclei other than H where the sensitivity obtainable from a CW instrument is totally inadequate. In particular, 13C NMR, the sensitivity of which is nearly 10-4 less than that of the proton (Table 9.9), is now a well-established technique that yields information on the skeletal structure of complex molecules. The pulsed technique also enables proton spectra to be obtained from samples as small as a few micrograms. 

After tq is passed, the second step starts by scanning the potential from Ed to a potential when all the deposited metals are re-oxidized (the reverse of reaction 25). The oxidation current recorded as a function of potential is the anodic stripping voltammogram (ASV). A typical ASY of three metals (Cd, Pb, and Cu) deposited on a mercury film electrode is shown in Fig. 18b.12b. The sensitivity of ASY can be improved by increasing the deposition time and by using the pulse technique to record the oxidation current. ASV in Fig. 18b. 12b was obtained by using the square wave voltammetry. In most cases a simple linear or step ramp is sufficient to measure sub-ppm level of metals in aqueous solution. The peak current of a linear scan ASV performed on a thin mercury film electrode is given by... 

All the pulsed techniques have as a target to exalt the faradaic currents while minimizing the interfering capacitive currents. These techniques exploit the different decay rate of the faradaic currents (which, decaying with the square root of the time, decay rather slowly) with respect to the decay rate of the capacitive currents (which, decaying exponentially with time, decay quickly). This means that, after a short time (from yts to ms) from the application of a potential value to the working electrode proper to trigger the electron transfer, the current is purely faradaic. 

Verma and Kaliaguine [339] measured the reaction rate using the pulse technique. They found good agreement with the data of Table 2, obtained for stationary conditions. The authors gave a clear presentation of advantages and disadvantages of the pulse method. 

The pulse technique is in many ways similar to the flow technique except that the adsorbate is introduced by adding pulses (e.g. from a gas sample valve) into the carrier gas. The pulse volume is chosen so that the first few pulses will be completely adsorbed. Further pulses are introduced until no more gas is adsorbed. The quantity of gas adsorbed is calculated by summing up the amounts adsorbed in the successive pulses. This technique is only applicable for strongly retained adsorbates. 

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... 

Long range dipolar interactions between an unpaired electron and nuclear spins on adjacent atoms will not normally be resolved in conventional powder EPR spectra.The pulse technique of electron spin echo modulation (ESEM) is in favourable cases able to detect very weak hyperfine interactions not seen in CW EPR. The method measures modulation of the electron spin echo signal by dipolar hyperfine coupling in the time domain at fixed magnetic field. Until recently,... 

The principal difficulties with the pulse technique lie in the problems connected witlh obtaining a reasonable pulse at a reactor s entrance. The injection must take place over a period which is very short compared with residence times in various segments of the reactor or reactor system, and there must be a negligible amount of dispersion between the point of injection and the entrance to the reactor system, ff these conditions can be fulfilled, this technique represents a simple and direct way of obtaining the RTD. 

Fig. 2,9. Gas chromatographic peaks showing the extent of hydrogen chemisorption as determined by the pulse technique.

The pulse technique provides very brief contacts of the catalyst with hydrocarbon charge and allows measurements to be made at the very high fresh platinum activities. 

The advantages of the pulse technique are that only small quantities of reactant are required and the results are obtained rapidly. A major difficulty in obtaining kinetic measurements is the absence of a steady state, causing the pulse to change shape as the products are formed and as it passes through the reactor. Some... 

Figure 15 shows the sonoelectrochemical enhancement of the regenerative procedure for Ru(bpy)3 ECL. The noise in the trace is due to the pulsing technique. 

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