Continuous lifetime analysis

The analysis of the positron annihilation lifetime spectra is a very important aspect of using the PAL techniques to analyze polymers. Without proper data analysis interpretation of data might be misleading and important scientific information will be lost. In PAL studies of polymers the PAL spectrum can be analyzed in two ways (1) a finite lifetime analysis or (2) continuous lifetime analysis. In the finite lifetime analysis the PAL spectra is resolved into a finite number of negative exponentials decays. The experimental data y(t) is expressed as a convoluted expression (by a symbol ) of the instalment resolution function R(t) and a finite number (n) of negative exponentials ... 

In continuous lifetime analysis, a PAL spectrum is expressed in a continuous decay form [20] ... 

In practice the lifetime distributions are usually obtained using a computer program such as the MELT [21] or CONTIN [22, 23] programs. The reliablity of these programs for measurring the o-PS lifetime distribution in polymers was shown by Cao et al [24]. A detailed description of these methods of data analysis is presented in Chapter 4. The advantage of the continuous lifetime analysis is that one can obtain free volume hole distributions rather that the average values obtained in the finite analysis. 

Until recently, most of the PAL data were analyzed in a finite-term lifetime approach. A computer program PATFIT, which represents annihilation lifetime distribution in a discrete manner, i.e. as a sum of several exponents, is en )loyed for this purpose. As an alternative, Gregory and Jean (10,11) proposed using a continuous lifetime analysis. In this approach the Laplace inversion program CONTIN, originally developed by Provencher (12) for analysis of fluorescence spectra, is used to obtain a continuous probability density function of annihilation lifetimes from PAL spectra. In this way one can obtain size distributions of FV in polymers. 

The complexes of 2,3 (sodium salt) and 4 (potassium salt) with P-CD and (2,3,6- tri-0-methyl)-/ -CD were studi using steady-state fluorescence and time-correlated, single-photon counting techniques [52]. The formation of both 1 1 and 2 1 complexes between p-CD and 2,3 was confirmed. Trimethyl- -CD gave evidence only of 1 1 complexes. The fluorescence decay of systems giving exclusively 1 1 complexes was collected at CD concentrations that ensure more than 90% complexation. The analysis performed using a continuous lifetime distribution model... 

When using routine LT9.0, a partial improvement occurs by assuming that the o-Ps lifetime xs shows a distribution. The artifacts are removed more or less completely when the distribution in the e+ lifetime X2 is also taken into account. As mentioned, we have confirmed these conclusions by LT9.0 analysis of simulated spectra. The allowance of a distribution in x uncouples the t2 analyzed from T3, and the allowance of a distribution in X2 uncouples ri from t2. The reason that LT9.0 avoids artifacts also observed in continuous Melt and Contin analysis lies in reduction in the degree of freedom by assuming a number of different lifetime channels in LT9.0. Moreover, because the lifetime analysis is less sensitive to the particular shape of the distributions, the assumed lognormal k function usually seems to describe the real situation sufficiently well. These are the reasons that we prefer to use the routine LT9.0 for the analysis of positron lifetime spectra. 

FIGURE 10.2 Performance of a stack during continuous operation. (Reprinted from /. Power Sources, 106, S. Y. Ahn, S.-J. Shin, H. Y. Ha. 2002. Performance and lifetime analysis of the kW-class PEMFC stack, 295-303, with permission from Elsevier.)... 

To answer the question as to whether the fluorescence decay consists of a few distinct exponentials or should be interpreted in terms of a continuous distribution, it is advantageous to use an approach without a priori assumption of the shape of the distribution. In particular, the maximum entropy method (MEM) is capable of handling both continuous and discrete lifetime distributions in a single analysis of data obtained from pulse fluorometry or phase-modulation fluorometry (Brochon, 1994) (see Box 6.1). 

One can expect that the analysis of continuous distributions of electronic excited-state lifetimes will not only provide a higher level of description of fluorescence decay kinetics in proteins but also will allow the physical mechanisms determining the interactions of fluorophores with their environment in protein molecules to be elucidated. Two physical causes for such distributions of lifetimes may be considered ... 

Most literature on enzyme kinetics is devoted to initial rate data and the analysis of reversible effects on enzyme activity. In many applications and process settings, however, the rate at which the enzyme activity declines is of critical importance. This is especially true when considering its long-term use in continuous reactors. In such situations the economic feasibility of the process may hinge on the useful lifetime of the enzyme biocatalyst. The focus of this section is on the mechanisms and kinetics of loss of enzyme activity. It should also be recognised that the alteration of protein structure is central to the practical manipulation of proteins (e.g. precipitation, affinity and other forms of protein chromatography, and purification in general). 

A major focus of researchers creating planar CE devices is speed of analysis, in part so that the devices can be used as chemical sensors and circumvent the severe selectivity and lifetime requirements of conventional chemical sensors. To increase the speed of analysis, shorter capillaries should be used, in combination with higher electric field strengths. Optimum efficiency depends on minimization of all unavoidable sources of band broadening, in addition to the elimination of nonideal effects such as Joule heating and adsorption on capillary walls. Therefore, work to understand the contributions which limit the efficiency of the separation is continuing. 

Fig. 3.27. Erosion lifetime (in number of ELMs or corresponding ITER full power pulses) of a CFC target (20 mm thick) as a function of the ELM energy loss for various assumptions on the value of Continuous curves refer to analysis done...

For positrons and Ps in polymeric surfaces, one needs to consider three additional important effects in addition to the bulk (1) the diffusion of the positron and Ps back to the surface, (2) the formation of Ps from the positrons by abstracting the surface electron, and (3) the Ps emission to the vacuum from the surface or the sticking of Ps on the polymeric surfaces. The dynamic behavior of the positron and Ps near the surface is schematically shown in Figure 11.2 below. The lifetimes of the positron and of Ps are different among those three types in addition to that of the bulk. If each has one distinct lifetime, a typical PAL lifetime spectrum could contain eight lifetimes for a complete analysis. This is beyond the current resolving power of the PAL data analysis method, either discrete or continuous. A practical approach is to invoke some good theoretical models before one applies the conventional data analysis method to a PAL spectrum near the surface for polymeric materials. 

In collaboration with Magura et al., we examined the utility of hair analysis for determining the prevalence of cocaine use among criminally involved youth. Personal interviews and scalp hair specimens were obtained from 121 male youths (median age = 19) who had been in jail in New York City and were followed up in the community after their release. Of these hair specimens, 67% were positive for cocaine at a mean concentration of 68 ng/10 mg of hair. Only 23% of the youths reported any use of cocaine or crack during the previous 3 months, and 36% reported any lifetime use. Associations were found between cocaine in hair and several behavioral variables prior number of arrests, rearrest after release from jail, not continuing education, and no legal employment. 

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