Fluorescence factors contributing

Several different factors contribute to the depolarization of emitted fluorescence relative to the polarization of the excitation light. Most of these can be controlled by experimental parameters, but two factors are intrinsic to the method and must be evaluated ... 

Fluorescein is probably one of the most widely used dyes for ophthalmic use. Several factors contribute to its utility, including its hydrophilicity low toxicity, and excellent fluorescent properties in the visible spectrum, even in very dilute concentration. Early ocular applications were used in detection of corneal ulcers and aqueous flow, followed shortly thereafter by retinal diagnostic application. 

Spectroscopic studies (fluorescence and circular dichroism) indicate that the variation in lipases stability is related to their hydrophobicity and therefore to the different degrees of interactions between the enzymes and the micellar interface [88]. Two important factors contribute to inducing conformational changes on the enzyme molecules in reverse micelles the unusual properties of the encapsulated water and the electrostatic interactions between ionic head groups of the surfactant and the polypeptide chains of the enzyme. Spectroscopic data on the structure of enzymes entrapped in reverse micelles are controversial, as regards the changes in the protein structure upon incorporation into reverse micelles (Table 13.3). 

The fluorescence emission spectrum of a molecule usually is approximately a mirror image of the absorption spectmm, as illustrated in Fig. 5.3. Several factors contribute to this synuneby. First, if the Bom-Oppenheimer approximation holds, and if the vibrational modes are harmonic and have the same frequencies in the ground and excited electronic states (all significant approximations), then the... 

As in the adults, skin auto-fluorescence increases age-dependently already in early childhood. In adults, body mass index (BMI) was also an independent factor contributing to skin auto fluorescence. In healthy toddlers body weight or BMI can not be considered as factors independent fi om age, thus we considered only age as independent factor. 

Though we and others (27-29) have demonstrated the utility and the improved sensitivity of the peroxyoxalate chemiluminescence method for analyte detection in RP-HPLC separations for appropriate substrates, a substantial area for Improvement and refinement of the technique remains. We have shown that the reactions of hydrogen peroxide and oxalate esters yield a very complex array of reactive intermediates, some of which activate the fluorophor to its fluorescent state. The mechanism for the ester reaction as well as the process for conversion of the chemical potential energy into electronic (excited state) energy remain to be detailed. Finally, the refinement of the technique for routine application of this sensitive method, including the optimization of the effi-ciencies for each of the contributing factors, is currently a major effort in the Center for Bioanalytical Research. 

Both emission and absorption spectra are affected in a complex way by variations in atomisation temperature. The means of excitation contributes to the complexity of the spectra. Thermal excitation by flames (1500-3000 K) only results in a limited number of lines and simple spectra. Higher temperatures increase the total atom population of the flame, and thus the sensitivity. With certain elements, however, the increase in atom population is more than offset by the loss of atoms as a result of ionisation. Temperature also determines the relative number of excited and unexcited atoms in a source. The number of unexcited atoms in a typical flame exceeds the number of excited ones by a factor of 103 to 1010 or more. At higher temperatures (up to 10 000 K), in plasmas and electrical discharges, more complex spectra result, owing to the excitation to more and higher levels, and contributions of ionised species. On the other hand, atomic absorption and atomic fluorescence spectrometry, which require excitation by absorption of UV/VIS radiation, mainly involve resonance transitions, and result in very simple spectra. 

This equation shows that, at time t, each anisotropy term is weighted by a factor that depends on the relative contribution to the total fluorescence intensity at that time. This is surprising at first sight, but simply results from the definition used for the emission anisotropy, which is based on the practical measurement of the overall ly and I components. A noticeable consequence is that the emission anisotropy of a mixture may not decay monotonously, depending of the values of r, and Ti for each species. Thus, r(t) should be viewed as an apparent or a technical anisotropy because it does not reflect the overall orientation relaxation after photoselection, as in the case of a single population of fluorophores. 

Fluorescence is also a powerful tool for investigating the structure and dynamics of matter or living systems at a molecular or supramolecular level. Polymers, solutions of surfactants, solid surfaces, biological membranes, proteins, nucleic acids and living cells are well-known examples of systems in which estimates of local parameters such as polarity, fluidity, order, molecular mobility and electrical potential is possible by means of fluorescent molecules playing the role of probes. The latter can be intrinsic or introduced on purpose. The high sensitivity of fluo-rimetric methods in conjunction with the specificity of the response of probes to their microenvironment contribute towards the success of this approach. Another factor is the ability of probes to provide information on dynamics of fast phenomena and/or the structural parameters of the system under study. 

The above show that both the quantum yield and fluorescence lifetime can be modified by any factor that affects the relative contributions of the nonradiative (k) and radiative (F) decay processes. As described in Section 2.2, these factors include environmental factors such as solvent polarity, ionization and... 

A major contributing factor to the increased sensitivity of the improved HPLC system over that originally described (5 ) is the detector. The original method utilized a fluorescence spectrophotometer adapted for HPLC detection by use of a fabricated 40 ul flow cell. The present system utilizes a highly sensitive HPLC fluorescence detector and this contributes greatly to the improved detection limits. 

In a search for sources of alkaline materials in rural air and rain, we have sampled and performed multi-element analyses on ambient particulate matter and potential source materials. Ambient aerosols were sampled daily using single Nuclepore filters or Florida State University "streakers." Samples of soil and unpaved road materials were also collected and analyzed. The samples were analyzed by various multi-element methods, including ion-and proton-induced X-ray emission and X-ray fluorescence, as well as by atomic absorption spectrophotometry. Visual observations, as well as airborne elemental concentration distributions with wind direction and elemental abundances in aerosols and source materials, suggested that soil and road dust both contribute to airborne Ca. Factor analysis was able to identify only a "crustal" source, but a simple mass balance suggested that roads are the major source of Ca in rural central Illinois in summer. 

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