Variability-lifetime methods

Although satisfactory criteria for deciding whether data are better analyzed by distributions or multiexponential sums have yet to established, several methods for determining distributions have been developed. For pulse fluorometry, James and Ware(n) have introduced an exponential series method. Here, data are first analyzed as a sum of up to four exponential terms with variable lifetimes and preexponential weights. This analysis serves to establish estimates for the range of the preexponential and lifetime parameters used in the next step. Next, a probe function is developed with fixed lifetime values and equal preexponential factors. An iterative Marquardt(18) least-squares analysis is undertaken with the lifetimes remaining fixed and the preexponential constrained to remain positive. When the preexponential... 

When using fluorophores of known lifetime, it is important to validate the lifetime used. Fluorescence lifetimes can be sensitive to concentration, temperature, pH, and other environmental variables. Fluorophores from different suppliers can have variable purity. As a result, one should not assume that a value reported in the literature will be exactly transferable to other labs and conditions. Users of the method should be particularly careful to use low concentrations of fluorophore (<10 /iM) to avoid a variety of processes which can perturb lifetimes in solution. There are a limited number of well characterized fluorophores. If one is not available for a particular wavelength this will require a change of filters leaving the method with nothing to recommend it over reflection and scatter. 

Estimates of exposure levels posing minimal risk to humans (MRLs) have been made, where data were believed reliable, for the most sensitive noncancer end point for each exposure duration. MRLs include adjustments to reflect human variability and, where appropriate, the uncertainty of extrapolating from laboratory animal data to humans. Although methods have been established to derive these levels (Barnes et al. 1987 EPA 1989a), uncertainties are associated with the techniques. Furthermore, ATSDR acknowledges additional uncertainties inherent in the application of these procedures to derive less than lifetime MRLs. As an example, acute inhalation MRLs may not be protective for health effects that are delayed in development or are acquired following repeated acute insults, such as hypersensitivity reactions, asthma, or chronic bronchitis. As these kinds of health effects data become available and methods to assess levels of significant human exposure improve, these MRLs will be revised. 

Fatigue testing can be adapted to the application, by using special loading sequences derived from studies of the service conditions. Examples are in aircraft, trucks and prostheses. The fatigue of composite materials for use in aircraft has been studied extensively and methods proposed for the calculation of lifetime under variable or random loads. 

Reactions of c -[Ru(bpy)2Cl2] with ligands (86) or (87) (X = CH2) in EtOH(aq) lead to [Ru(bpy)2(86)] + and [Ru(bpy)2(87, X = CH2)] respectively. When X = 0 in ligand (87), the product is the pyridine carboxylate complex [Ru(bpy)2(pyC02)], the structure of which is confirmed by X-ray crystallography. Complexes of the type [Ru(bpy)2L] " in which L represents a series of mono- and dihydrazones have been prepared and characterized by spectroscopic methods (including variable temperature H NMR) and a structure determination for L = biacetyl di(phenylhydrazone). When L is 2-acetylpyridine hydrazone or 2-acetylpyridine phenylhydrazone, [Ru(bpy)2L] + shows an emission, but none is observed for the dihydrazone complexes. The pyrazoline complex [Ru(bpy)2L] (L = 5-(4-nitrophenyl)-l-phenyl-3-(2-pyridyl)-2-pyrazoline) can be isolated in two diastereoisomeric forms. At 298 K, these exhibit similar MLCT absorptions, but at 77 K, their emission maxima and lifetimes are significantly different. ... 

A simplified instrument for the measurement of fluorescent lifetimes using the stroboscopic method has been described by Brown (67). The major virtue of this system is that it makes use of a Tektronix oscilloscope to obtain all the necessary trigger pulses, including a trigger of continuously variable delay. Since most laboratories are equipped with a good oscilloscope, the need to purchase expensive trigger-delay apparatus is thus eliminated. 

Suzuki, R., Kobayashi, Y., Awazu, K., Mikado, T. et al. (1994) Positron lifetime study on ion-implanted amorphous Si02 with a variable-energy pulsed positron beam Nucl. Instrum. Methods Phys. Res., Sect. B 91,410. 

Fatigue resistance of polymers in a stress cracking environment is a complex topic where molecular variables have a strong influence. FCP experiments are a fast and effective method for determining the resistance to FCP in stress cracking environments and hence to predict polymer lifetime. Other mechanical testing methods have also been cited as they are somewhat more common. 

However, there are indications that the commonly used lifetime prediction methods can lead to inaccurate results for the composite blade s lifetime under the strongly varying loads to which they are subjected. The present paper presents the preliminaries to an extensive study into the fatigue behaviour of wind turbine rotor blade materials under variable amplitude loading. This paper will discuss some of the problems and drawbacks of the currently prescribed lifetime prediction methods, and describes possible ways of tackling these. 

As has been indicated in the previous chapter, quite sophisticated lifetime prediction algorithms are continuously being developed. Unfortunately, no appropriate validation has been performed for any universally applicable theory. Therefore, current guidelines for lifetime prediction methods for variable amplitude loading of composites lag the intricate modeling efforts described in the literature, and resort to the classical and well-known method. This is referred to as the baseline method. 

In the first reported measurements made with picosecond pulses, an optical beam splitter was used to pick off a portion of the pulse train and a variable optical delay path was introduced between the two beams [7]. The main beam was used to excite (pump) a dye sample, and the weak (probe) beam was used to monitor the recovery of dye transmission as a function of delay. Over the past two decades, this pump-probe method has been extended to a variety of measurement geometries and used to measure electronic polarization dephasing times as well as population lifetimes. 

In photochromic systems, the number, nature, and kinetic and spectral properties of the transient species formed under irradiation cannot be established without a good knowledge of the reaction mechanism. Flash photolysis is often used for analysis of this type of problem.1-9 However, fast methods are not well suited to the study of slow photochromic systems such as those used in variable-transmission glasses. In practice, the fast time scales of observation (10-14-10-3 s) in flash photolysis are completely different from those of the slower reactions (10 -10+6s) characterizing the accumulation of a slow photoisomer under a low-powered photon flux.10 Among the different photoisomers formed, only those with the longest lifetimes (> 10 s) will be observed by techniques using continuous irradiation (see Appendix 1 of this chapter). 

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