Fluorescent additives, analysis

Fluorescence is much more widely used for analysis than phosphorescence. Yet, the use of fluorescent detectors is limited to the restricted set of additives with fluorescent properties. Fluorescence detection is highly recommended for food analysis (e.g. vitamins), bioscience applications, and environmental analysis. As to poly-mer/additive analysis fluorescence and phosphorescence analysis of UV absorbers, optical brighteners, phenolic and aromatic amine antioxidants are most recurrent [25] with an extensive listing for 29 UVAs and AOs in an organic solvent medium at r.t. and 77 K by Kirkbright et al. [149]. 

Stabilisers are usually determined by a time-consuming extraction from the polymer, followed by an IR or UV spectrophotometric measurement on the extract. Most stabilisers are complex aromatic compounds which exhibit intense UV absorption and therefore should show luminescence in many cases. The fluorescence emission spectra of Irgafos 168 and its phosphate degradation product, recorded in hexane at an excitation wavelength of 270 nm, are not spectrally distinct. However, the fluorescence quantum yield of the phosphate greatly exceeds that of the phosphite and this difference may enable quantitation of the phosphate concentration [150]. The application of emission spectroscopy to additive analysis was illustrated for Nonox Cl (/V./V -di-/i-naphthyl-p-phcnylene-diamine) [149] with fluorescence ex/em peaks at 392/490 nm and phosphorescence ex/em at 382/516 nm. Parker and Barnes [151] have reported the use of fluorescence for the determination of V-phenyl-l-naphthylamine and N-phenyl-2-naphthylamine in extracted vulcanised rubber. While pine tar and other additives in the rubber seriously interfered with the absorption spectrophotometric method this was not the case with the fluoromet-ric method. 

Fluorescence in UV radiation is a frequently used method for detection of TLC spots, e.g. of Tinuvin 326 [42]. The fluorescence emitted by optical brighteners under UV light on a thin-layer plate has been utilised as a means of analysing these compounds [42]. On the whole, the use of fluorescence detection in poly-mer/additive analysis of extracts is certainly not overwhelming. Applied fluorescence has been described in a monograph [156]. 

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]. 

Proteins having one chromophore per molecule are the simplest and most convenient in studies of fluorescence decay kinetics as well as in other spectroscopic studies of proteins. These were historically the first proteins for which the tryptophan fluorescence decay was analyzed. It was natural to expect that, for these proteins at least, the decay curves would be singleexponential. However, a more complex time dependence of the emission was observed. To describe the experimental data for almost all of the proteins studied, it was necessary to use a set of two or more exponents.(2) The decay is single-exponential only in the case of apoazurin.(41) Several authors(41,42) explained the biexponentiality of the decay by the existence of two protein conformers in equilibrium. Such an explanation is difficult to accept without additional analysis, since there are many other mechanisms leading to nonexponential decay and in view of the fact that deconvolution into exponential components is no more than a formal procedure for treatment of nonexponential curves. 

Fluorescence is not useful simply for chemical analysis. For example, a fluorescent additive that sticks to textile fibres is added to laundry soap. This compound absorbs solar radiation in the non-visible part of the spectrum and re-emits at longer wavelengths in the blue spectral region, which makes clothing appear whiter. Another application of fluorescence encountered daily is cathode tube lighting. The internal walls of these tubes are covered with mineral salts (luminophores) that emit in the visible region due to excitation by electrons. 

In the present study we investigated energy transfer between the Zn-porphyrin units in a sequence of dendrimers varying in size from 4 to 64 porphyrin units (Fig. 1). Reference measurements were performed on the monomer, P1D1. In order to follow energy transfer within the dendrimers, the fluorescence anisotropy decay were analysed. To determine the lifetime of the dendrimers, additional analysis of the kinetics measured at magic angle was performed. The fluorescence anisotropy is defined by... 

Fluorescence Correlation Spectroscopy and Fluorescence Burst Analysis. Several nanoscopic chemical imaging approaches work very well for measurements of chemical kinetics, interactions, and mobility in solution. Fluorescence correlation spectroscopy (FCS) measures the temporal fluctuations of fluorescent markers as molecules diffuse or flow in solution through a femtoliter focal volume.54 Their average diffusive dwell times reveal their diffusion coefficients, and additional faster fluctuations can reveal chemical reactions and their kinetics if the reaction provides fluorescence modulation. Cross-correlation of the fluorescence of two distinguishable fluorophore types can very effectively reveal chemical binding kinetics and equilibria at nanomolar concentrations. 

In addition to its exquisite sensitivity, other key advantages of ZnO NR platforms include ease of array fabrication, mechanical and chemical robustness, no autofluoroescence, and direct correlation of observed signal to protein concentration. Unlike other commonly used biosupport materials, this unique proper of ZnO NRs exhibiting no spectral overlap with fluorophores can be conveniently used in fluorescence data analysis. Fluorescence signal in the ZnO NR-assisted assays... 

As an adjunct to x-ray fluorescence, we use the laser microprobe (53,54). This, of course, is not an entirely nondestructive technique. However, the hole that is produced has dimensions of the order of 10-20 xm, not visible to the unaided eye. The laser microprobe serves as an important supplement to x-ray fluorescence because analysis is not limited only to the upper surface. The laser beam can carefully excavate to lower and lower layers by controlled repeated laser pulses. Analysis can be performed on each individual layer of interest. In addition, the... 

Figure 7.2 shows the evolution of the emission spectra of Calix-DANS2 (1.6 X 10 " mol L upon addition of Hg + in CH3CN/H2O (60 40 v/v) at pH 4.0 (in order to observe the strongest photophysical effects). It can be seen that the fluorescence of Calix-DANS2 is almost completely quenched upon Hg + addition. Analysis of the whole emission spectra upon mercury binding reveals the formation of a complex with a 1 1 stoichiometry and an apparent stability constant of... 

Applications of Raman to polymer/additive deformulation are still rather few, especially if compared to IR methods (cfr. Chp. 1.2.1). Hummel [108] has attributed the general lack of applications of RS in the field of plastics additives to poor Raman scattering of certain substance categories, unsatisfactory reproducibility of the spectra and scarcity of specific Raman libraries [385,386]. Polymer/additive analysis by means of Raman spectroscopy is mainly restricted to fillers, pigments and dyes the major usefulness comes from NIR FT-Raman, which greatly overcomes the fluorescence problem. The ion-pair dissociation effect of the 2-keto-4-(2,5,8,11-tetraoxadodecyl)-l,3-dioxolane modified carbonate (MC3) plasticiser in poly(ethylene oxide) (PEO) was studied by means of Raman, FTIR and EX-AFS [387]. Another study established the feasibility of using Raman spectroscopy to quantify levels of melamine and melamine cyanurate in nylons [388]. 

Direct fluorescence, phosphorescence and X-ray fluorescence spectroscopy for polymer/additive analysis have been reported [513]. In commercial polymers, additives having electronic absorption bands in the visible and near-UV wavelength regions may fluoresce and give rise to composite spectra. Some general applications of fluorescence spectroscopic analysis for polymeric materials relate to ... 

Provorov et al. [520] have studied a rather extensive group of elastomer additives (accelerators, stabilisers, softeners, fillers, and other ingredients) for possible analysis by fluorescence techniques. No fluorescence lifetime measurements have been applied for discriminating stabilisers in polymers. UV microscopy is another means of measuring the concentration (and distribution) of UV absorbing or fluorescent additives in plastics (cfr. Chp. 5.3.2). 

Plitt et al. [542] have surveyed the literature eov-ering the fluorescence of fibres, rubber, cellulose, polymers, and plastics long ago. On the whole, fluorescence and phosphorescence techniques find restricted practical application for polymer/additive analysis. There is also little information in the literature on the quantitative aspects of the direct examination of polymer films by luminescence techniques. 

Polymers containing UV stabilisers or fluorescent additives are an obvious target for UV microscopy, but the potential range of applications is much wider, in that UV absorbers or fluorescers can be selectively bound to specific chemical entities in the polymer or will preferentially interact with, or dissolve in, parts of the structure. A variety of applications of the UV microscope to studies of polymers has been reported (Table 5.15). Many applications of UV microscopy require quantitative analysis. 

Determination of the diffusion rates of UV-absorbing or fluorescent additives in solid polymers Polymer oxidation studies Morphological and structural studies Contaminant analysis... 

Fluorescent additives may be studied in the same way as UV absorbers. The results are very similar but slightly more care is required in quantitative interpretation since self-quenching effects can lead to non-linearity in the concentration dependence of fluorescence intensity. UV microscopy has been used to follow the distribution of fluorescent additives (such as Uvitex OB) during isothermal crystallisation and cooling of isotactic PP [64]. Billingham et al. [58] have observed diffusion of Uvitex OB in a PP/rubber blend using UV fluorescence microscopy. UV microscopy can be very useful in the analysis of multilayer films where one layer of polymer is intrinsically fluorescent (e.g. PVDC). 

As an alternative to wet ehemical routes of analysis, this monograph deals mainly with the direct deformulation of solid polymer/additive compounds. In Chapter 1 in-polymer spectroscopic analysis of additives by means of UV/VIS, FTIR, near-IR, Raman, fluorescence spectroseopy, high-resolution solid-state NMR, ESR, Mossbauer and dielectrie resonance spectroscopy is considered with a wide coverage of experimental data. Chapter 2 deals mainly with thermal extraction (as opposed to solvent extraction) of additives and volatiles from polymerie material by means of (hyphenated) thermal analysis, pyrolysis and thermal desorption techniques. Use and applieations of various laser-based techniques (ablation, spectroscopy, desorption/ionisation and pyrolysis) to polymer/additive analysis are described in Chapter 3 and are critically evaluated. Chapter 4 gives particular emphasis to the determination of additives on polymeric surfaces. The classical methods of... 

Acronyms abound in phofoelecfron and relafed specfroscopies buf we shall use only XPS, UPS and, in Sections 8.2 and 8.3, AES (Auger elecfron specfroscopy), XRF (X-ray fluorescence) and EXAFS (exfended X-ray absorption fine sfmcfure). In addition, ESCA is worth mentioning, briefly. If sfands for elecfron specfroscopy for chemical analysis in which elecfron specfroscopy refers fo fhe various branches of specfroscopy which involve fhe ejection of an elecfron from an atom or molecule. Flowever, because ESCA was an acronym infroduced by workers in fhe field of XPS if is mosf often used to refer to XPS rather than to electron spectroscopy in general. 

In addition to the spark emission methods, quantitative analysis directly on soHds can be accompHshed using x-ray fluorescence, or, after sample dissolution, accurate analyses can be made using plasma emission or atomic absorption spectroscopy (37). 

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