Rapid lifetime determination method

Ballew, R. M. and Demas, J. N. (1989). An error analysis of the rapid lifetime determination method for the evaluation of single exponential decays. Anal. Chem. 61, 30-3. 

The best, but also the most laborious, method to determine the lifetime is a longterm test in an integrated reactor. Here the catalyst is operated under intensified conditions (high conversion, high temperature, high concentration, etc.) in so-called stress tests that allow statements about the lifetime to be rapidly made. This method is especially suitable for comparing different variants of a catalyst. 

Insufficient testing is one of the major causes of method failure. The amount of data needed to publish a new procedure in a peer-reviewed journal and the procedural detail supplied therein are often insufficient to allow a different user to validate a method rapidly. The developer should evaluate if the method will work using chemicals, reagents, solid-phase extraction columns, analytical columns, and equipment from various vendors. Separate lots of specific supplies within a vendor should be evaluated to determine if lot-to-lot variation significantly impacts method performance. Sufficient numbers of samples should be assayed to estimate the lifetime of the analytical column and to determine the effects of long-term use on the equipment. 

The laser flash photolysis technique allows the observation of rapid reactions and determines the lifetime of reactive intermediates. Thus, it is not surprising that this method was used extensively to obtain information about the formation, stability, and reactivity of silylene-Lewis base complexes. 

The extension of equilibrium measurements to normally reactive carbocations in solution followed two experimental developments. One was the stoichiometric generation of cations by flash photolysis or radiolysis under conditions that their subsequent reactions could be monitored by rapid recording spectroscopic techniques.3,4,18 20 The second was the identification of nucleophiles reacting with carbocations under diffusion control, which could be used as clocks for competing reactions in analogy with similar measurements of the lifetimes of radicals.21,22 The combination of rate constants for reactions of carbocations determined by these methods with rate constants for their formation in the reverse solvolytic (or other) reactions furnished the desired equilibrium constants. 

An elegant method of obtaining desorption parameters involves the use of molecular beams. Here, the crystal is maintained at the desorption temperature and the requirement for rapid heating is avoided. Two methods are used. If the beam is chopped, the decay rate of the desorbed particles can be directly monitored [217, 218]. Alternatively, the beam may be modulated and the phase shift between the input and output measured [219], These methods were originally used to determine lifetimes for species desorbing as ions, such as Ba+ and Cs+ from tungsten [217, 219, 220] and for metal atoms [221], but CO, 02 and H2 desorption have also been examined in this way [223—225]. 

One advantage of the cyclic voltammetry technique is that, to some extent at least, the lifetime of the experiment may be controlled through the scan rate, i.e. the rate at which the potential range is scanned and then reversed. This means that if one of the components of the couple is not very stable it may still be possible, by increasing the scan rate, to observe reversibility and hence determine °. Thus the n-superoxo/p-peroxo-dicobalt(III) couples in the complexes of the type [(NH3) Co()i-02)Co(NH3)5] + + can be determined readily even though the peroxo complex is not very stable and decomposes rapidly under conditions where the superoxo is stable. However, the scan rate must not be increased too much as at fast scan rates the rate of the electrochemical process becomes slow relative to the scan rate and irreversible behaviour (A p>57/n mV) will be obtained. In fact, this is the basis of a method for obtaining the heterogeneous rate constants of reversible couples. 

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