Phosphorescence method

Sensitivity From equations 10.32 and 10.33 we can see that the sensitivity of a fluorescent or phosphorescent method is influenced by a number of parameters. The importance of quantum yield and the effect of temperature and solution composition on f and p already have been considered. Besides quantum yield, the sensitivity of an analysis can be improved by using an excitation source that has a greater... 

Time, Cost, and Equipment As with other optical spectroscopic methods, fluorescent and phosphorescent methods provide a rapid means of analysis and are capable of automation. Fluorometers are relatively inexpensive, ranging from several... 

H H Willard, L L Merritt, J R Dean and F A Settle, Instrumental methods of analysis, Molecular Fluorescence and Phosphorescence Methods, 6th edn, Van Nostrand Reinhold, New York, 1981, Chapter 5... 

Samples in highly rigid or viscous media (e.gglass) is examined frequently in phosphorescence methods and also in some fluorescence methods. 

The H(D) atom abstraction rate constants in durene crystals by the impurity molecules quinoline, isoquinoline, quinoxaline, and quinozaline in their excited triplet state were measured by Hoshi et al. [1990] using the phosphorescence method described above. The transfer occurs in the fragment CH N formed by a methyl group of durene and a nitrogen atom of the impurity molecule. In the interval 300-100 K the activation energy drops from 3.5 kcal/mol to 1.6 kcal/mol. Deuteration reduces the... 

The linear portion of the curve has an activation energy of 1.8 kcal mol and is believed to be associated with the activated process involving small motions of the phenyl ring on the ester group. The positions of the transitions were determined by the phosphorescence method (23) and are shown in the figure. The activation energy for the ortho product is 1.2 kcal mol which presumably reflects the smaller amount of motion required to move to the ortho than to the para position. The small value of the activation energy is presumably associated with the very small volume required for the rotation of the phenoxy radical before it recombines to form the hydroxy ketone. 

Analysis of solvent extracts of plastics. Acetone, carbon disulfide, chloroform, cyclohexane, diethyl ether, ethanol, hexane, toluene, and water are used as solvents, extracts are further analyzed by, e.g., UV-visible spectrophotometry, fluorescence and phosphorescence methods, GC, LC, electrochemical methods, etc. 

Aromatic amines and phenols are among the few classes of compounds in which a large proportion of them exhibit useful fluorescence. Parker and Barnes [21] found that in solvent extracts of rubbers the strong absorption by pine tar and other constituents masks the absorption spectra of phenylnaphthylamines, whereas the fluorescence spectra of these amines are sufficiently unaffected for them to be determined directly in the unmodified extract by the fluorescence method. In a later paper Parker [22] discussed the possibility of using phosphorescence techniques for determining phenylnaphthylamines. Drushel and Sommers [7] have discussed the determination of Age Rite D (polymeric dihydroxy quinone) and phenyl-2-naphthylamine in polymer films by fluorescence methods and Santonox R and phenyl-2-naphthylamine by phosphorescence methods. 

Fluorescence and phosphorescence methods have inherently lower limits of detection than absorption-based spectrophotometric measurements. They are among the most sensitive analytical techniques available to the scientist. The enhanced sensitivity arises from the concentration-related parameter for fluo-rometry and phosphorimetry F being directly proportional to the source radiant power Fo The luminescence intensity can be measured independently of In contrast. an absorbance measurement requires evaluation... 

Phosphorescence methods have fewer applications than fluorescence. Since sample molecules may show both fluorescence and phosphorescence, it is necessary to measure the slower phosphorescence by introducing a finite delay between excitation and measurement. This is done using a shutter system. 

Rotational relaxation times measured for several membrane proteins, mostly by the triplet phosphorescence method, are given in Table 1.5. Clearly some integral proteins rotate rather rapidly in membranes. The rotational relaxation time is defined by = 1/D where D is the rotational diffusion coefficient referred to... 

Jovin T. M., W. L. C. Vaz. Rotational and Translational Diffusion in Membranes Measured by Fluorescence and Phosphorescence Methods, Meth. Enzym., 172, 471-513 (1989). 

Standardizing the Method Equations 10.32 and 10.33 show that the intensity of fluorescent or phosphorescent emission is proportional to the concentration of the photoluminescent species, provided that the absorbance of radiation from the excitation source (A = ebC) is less than approximately 0.01. Quantitative methods are usually standardized using a set of external standards. Calibration curves are linear over as much as four to six orders of magnitude for fluorescence and two to four orders of magnitude for phosphorescence. Calibration curves become nonlinear for high concentrations of the photoluminescent species at which the intensity of emission is given by equation 10.31. Nonlinearity also may be observed at low concentrations due to the presence of fluorescent or phosphorescent contaminants. As discussed earlier, the quantum efficiency for emission is sensitive to temperature and sample matrix, both of which must be controlled if external standards are to be used. In addition, emission intensity depends on the molar absorptivity of the photoluminescent species, which is sensitive to the sample matrix. 

Accuracy The accuracy of a fluorescence method is generally 1-5% when spectral and chemical interferences are insignificant. Accuracy is limited by the same types of problems affecting other spectroscopic methods. In addition, accuracy is affected by interferences influencing the fluorescent quantum yield. The accuracy of phosphorescence is somewhat greater than that for fluorescence. 

Physical methods Physical methods include photometric absorption and fluorescence and phosphorescence inhibition, which is wrongly referred to as fluorescence quenching [1], and the detection of radioactively labelled substances by means of autoradiographic techniques, scintillation procedures or other radiometric methods. These methods are nondestructive (Chapt. 2). 

After the laser flash, one then monitors the progress of events by some rapidly responding method. Conductivity, absorption spectroscopy, and fluorescence spectroscopy are the methods most commonly used. If a reaction product has a characteristic absorption band of sufficient intensity, one can monitor its buildup with time. This might be a UV, visible, or IR band. The need for a band with a high molar absorptivity arises because the reactive transient is usually present at a relatively low concentration, KT6-lCr5 M being typical. If the species of interest is phosphorescent, then the timed decay of its phosphorescence intensity can be recorded. 

In 1944, Lewis and Kasha (52) identified phosphorescence as a forbidden" transition from an excited triplet state to the ground singlet state and suggested the use of phosphorescence spectra to identify molecules. Since then, phosphorimetry has developed into a popular method of analysis that, when compared with fluorometry, is more sensitive for some organic molecules and often provides complimentary information about structure, reactivity, and environmental conditions (53). 

The active state of luminescence spectrometry today may be judged ly an examination of the 1988 issue of Fundamental Reviews of Analytical Chemistry (78), which divides its report titled Molecular Fluorescence, Phosphorescence, and Chemiluminescence Spectrometry into about 27 specialized topical areas, depending on how you choose to count all the subdivisions. This profusion of luminescence topics in Fundamental Reviews is just the tip of the iceberg, because it omits all publications not primarily concerned with analytical applications. Fundamental Reviews does, however, represent a good cross-section of the available techniques because nearly every method for using luminescence in scientific studies eventually finds a use in some form of chemical analysis. Since it would be impossible to mention here all of the current important applications and developments in the entire universe of luminescence, this report continues with a look at progress in a few current areas that seem significant to the author for their potential impact on future work. 

Interactions in Solid-Surface Luminescence Temperature Variation. Solid-surface luminescence analysis, especially solid-surface RTF, is being used more extensively in organic trace analysis than in the past because of its simplicity, selectivity, and sensitivity (,1,2). However, the interactions needed for strong luminescence signals are not well understood. In order to understand some of the interactions in solid-surface luminescence we recently developed a method for the determination of room-temperature fluorescence and phosphorescence quantum yields for compounds adsorbed on solid surfaces (27). In addition, we have been investigating the RTF and RTF properties of the anion of p-aminobenzoic acid adsorbed on sodium acetate as a model system. Sodium acetate and the anion of p-aminobenzoic acid have essentially no luminescence impurities. Also, the overall system is somewhat easier to study than compounds adsorbed on other surfaces, such as filter paper, because sodium acetate is more simple chemically. 

UV/VIS absorption spectroscopy, pioneered by Beckman (1941), is one of the oldest and most widely used instrumental techniques, despite being regarded by some analysts as obsolete. Recently there has been a renaissance in UV spectroscopy with many new techniques, instruments and data processing methods [8]. Modem highest specification UV/VIS absorption and fluores-cence/phosphorescence spectrometer instruments extend their wavelength region from the far UV (175 nm) into the NIR region (1100 nm). Small footprint UV/VIS spectrometers (200-1100 nm) are now available. Paul [9] has traced the history of UV/VIS instrumental developments. 

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. 

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