Fluorescence excitation spectroscop

Spectroscopic methods such as uv and fluorescence have rehed on the polyene chromophore of vitamin A as a basis for analysis. Indirectly, the classical Carr-Price colorimetric test also exploits this feature and measures the amount of a transient blue complex at 620 nm which is formed when vitamin A is dehydrated in the presence of Lewis acids. For uv measurements of retinol, retinyl acetate, and retinyl palmitate, analysis is done at 325 nm. More sensitive measurements can be obtained by fluorescence. Excitation is done at 325 nm and emission at 470 nm. Although useful, all of these methods suffer from the fact that the method is not specific and any compound which has spectral characteristics similar to vitamin A will assay like the vitamin... 

With the aim of elucidating molecular dynamics in a small domain, we have constmcted several microspectroscopic systems, that is, (i) the confocal microscope with the excitation light source being a femtosecond NIR laser emitting a 35 fs pulse, and (ii) the fluorescence correlation spectroscopic system with optical tweezers. 

FBAs can also be estimated quantitatively by fluorescence spectroscopy, which is much more sensitive than the ultraviolet method but tends to be prone to error and is less convenient to use. Small quantities of impurities may lead to serious distortions of both emission and excitation spectra. Indeed, a comparison of ultraviolet absorption and fluorescence excitation spectra can yield useful information on the purity of an FBA. Different samples of an analytically pure FBA will show identical absorption and excitation spectra. Nevertheless, an on-line fluorescence spectroscopic method of analysis has been developed for the quantitative estimation of FBAs and other fluorescent additives present on a textile substrate. The procedure was demonstrated by measuring the fluorescence intensity at various excitation wavelengths of moving nylon woven fabrics treated with various concentrations of an FBA and an anionic sizing agent. It is possible to detect remarkably small differences in concentrations of the absorbed materials present [67]. 

We believe that detailed analysis of absorption, fluorescence and fluorescence excitation spectra permits us to investigate the main spectroscopic features of individual NH-tautomers in porphyrins with non-symmetrical substitution. Then, obtained information... 

A few years ago, a powerful version of molecular optical spectroscopy with supersonic beams and jets was developed by Smalley, Wharton and Levy . Supersonic expansion of molecules in an inert carrier gas yields an ideal spectroscopic sample. As a result of the expansion, the translational temperature of the carrier gas decreases to extremely low values (below O.I K). The flow is collisionless so that even extremely unstable species survive. Special attention was paid to fluorescence excitation spectroscopy but the technique is by no means limited to this type of spectroscopy. (Because of fundamental difficulties, however, direct measurement of light absorption in molecular beams is not easy.) Cooled molecules in the beam are electronically excited with a tunable dye laser. The emitted fluorescence is detected and plotted against the wavenumber of the exciting radiation. The obtained fluorescence excitation spectrum is generally very similar to the corresponding absorption spectrum. The technique was used for analysis of the spectra of interesting vdW molecules He. .. NOj, He... Ij, X. .. tetrazine and Xj. .. tetrazine (X = He, Ar, H ) complexes . 

Fig. 6. Spectroscopic characteristics of iPCP from Dinoflagellates. a) absorbance spectrum, b) circular dichroism spectrum, c) fluorescence excitation spectra monitored at 676 nm, d) fluorescence emission spectra. In c the solid line is the fluorescence spectrum and the dashed line the normalized absorbance spectrum. (Hiller et ah, 1993, Iglesias-Prieto et ah, 1993)...

In addition to laser fluorescence excitation, several other laser spectroscopic methods have been found to be useful for the state-selective and sensitive detection of products of reactive collisions resonance-enhanced multiphoton ionization [58], coherent anti-Stokes Raman scattering [M], bolometric detection with laser excitation [30], and direct infrared absorption [7]. Several additional laser techniques have been developed for use in spectroscopic studies or for diagnostics in reacting systems. Of these, four-wave mixing [ ] is applicable to studies of reaction dynamics although it does have a somewhat lower sensitivity than the techniques mentioned above. 

In excited-state spectroscopies, including fluorescence spectroscopy, spectroscopic intensity is usually linear in functions of each of three or more independent variables, so that a three-way array of data can be fit with a trilinear model. The presence of three or more linear relationships makes algebraic methods for resolving the spectra and other properties of individual components substantially more powerful than in the case of two linear relationships. The use of a general trilinear model is sometimes known as three-way factor analysis, three-mode factor analysis, or threemode principal component analysis. For a review of the mathematics and application to spectroscopy, see our survey article. ... 

It is generally believed that among the pyrimidine bases, uracil and thymine mainly exist in the keto form. However, the existence of a small amount of the enol form of these compounds has also been indicated in some experiments. Hauswirth and Daniels have explained the observed deviation in the excitation spectrum of thymine firom the corresponding absorption spectrum in terms of the possibility of emission firom the enol tautomer. On the other hand, Vigny and Duquesne observed that both absorption and fluorescence excitation spectra of thymine are in resemblance with each other. Suwaiyan et al. have suggested the existence of a small amount of the enol tautomer in aqueous solutions of 5-chlorouracil at room temperature. The existence of keto-enol tautomerism in uracil, thymine and their derivatives has also been suggested in supersonic jet-cooled spectroscopic studies. ... 

The absence of a contribution from the B777 form in the fluorescence excitation spectra of the B820 particle clearly demonstrates that the different forms are behaving spectroscopically as distinct particles. This confirms experiments showing that the three different forms are exhibiting different hydrodynamical properties [2,4,5]. 

Spectroscopic properties of reconstituted complexes, such as absorption spectra, fluorescence excitation and emission spectra and energy transfer from Chi a to Chi b, are very similar to those of LHCll isolated from thylakoid membranes (not shown). The resemblance between the CD spectra also suggests a very similar arrangement of pigments in the reconstituted complex as compared to native LHCll (Fig. 3). 

Laser-induced fluorescence (LIF) A spectroscopic technique usually employing visible or UV laser light, in which the fluorescence emission from a gaseous, liquid, or solid sample is monitored. A fluorescence excitation spectrum is a plot of the total emitted fluorescence vs. excitation wavelength and provides information similar to an absorption spectrum. 

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