Triple-potential-step technique

Fig. 9. Variation of potential, current and photon flux vs. time in an ECL experiment performed using the triple-potential-step technique.

Theoretical specific capacity 307 Triple-potential-step technique 23 Triplet-triplet annihilation 4, 20,25, 26, 30 33... 

This elaborate triple potential step method was employed to determine k ff and values at various monolayer densities. As the mean area occupied by each lipid molecule decreased from 50 to 30 A, k ff increased from 0.035 to 0.06 cm/s. These rate constants, however, are several orders of magnitude lower than expected from the Marcus theory for bimolecular ET reactions either in solution or at a liqnid-liquid interface. The slow ET kinetics was ascribed to highly constrained local environment in the compressed monolayer at the air-water interface. Moreover, decreased from l.Ox 10 to <1.0 x 10" cm /s at the more compressed monolayer. These diffusion coefficients agree with values determined previously by the microline electrode technique [62], thereby validating the SECM method. 

In its initial application, a triple potential step was applied at a submarine UME placed in the aqueous subphase of a Langmuir trough, close (1-2 pm) to the monolayer. The technique involves generating an electroactive species (Ox) at the UME by diffusion-controlled electrolysis of a precursor (Red) in an initial potential step. Ox diffuses to, and reacts with, the redox-active amphiphile at the water-air interface resulting in the conversion of the solution redox species to its initial form (Red), which then undergoes diffusional feedback to the UME. In this first step, the rate constant for electron transfer between the solution mediator and the surface-confined species can be measured from the UME current-time transient. In the second period, the potential step is reversed to convert the electrogenerated species (Ox) to its initial form (Red). Lateral diffusion of electroactive amphiphile into the interfacial zone probed by the UME occurs simultaneously in this recovery period. 

The technique employs a triple-step potential waveform, typically as illustrated schematically in Fig. 21(a), although others are possible [81]. The potential Ex is such that the platinum electrode surface is rapidly oxidized, any contaminants being desorbed. The electrode is then stepped to a potential E2, which is large and negative enough to reduce the oxidized surface and allow adsorption of the analyte. Stepping the potential to E3 oxidises the species on the electrode surface. The analyte is detected in the chronoam-perometric response of the electrode to this last step. Figure 21(b) illustrates schematically the chronoamperometric behaviour in the absence of adsor-... 

From a synthetic point of view, mass spectrometrists use triple-quadrupoles as a useful tool for further refined mechanistic studies (Figure 5.7). Triple-quadrupoles, Q-traps, and other devices certainly afford one of the most complete laboratories for the studies of gas-phase ion/molecule reactions. The remarkable success of this concept has inspired many scientists to devote more attention to this potentially advantageous strategy. Thus, usingtriple-quadrupoles, ions can be obtained from the ion-source (ESI, APCI), purified via mass-selection in Q1 and reacted in q2 (collision cell) under controlled conditions, and further mass analysis of the reaction products can be performed in Q3. The main advantage is that all these steps are carried out on line in very short time intervals under conditions that can be maintained for long periods and can be easily described and reproduced. Another important method in the study of well-known reactions is to compare the data obtained by ESI-MS with other spectroscopic techniques. 

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