Spectrophotometry fluorescence

A difficulty encountered in measuring the concentration of an unknown absorbing species in solution is deviation from the Beer-Lambert law. For reasons stated earlier in this chapter, some absorbing species do not demonstrate an increase in absorbance that is proportional to an increase in concentration. (In reality, most compounds follow the Beer-Lambert relationship over a relatively small concentration range.) When measuring solution concentration, adherence to the Beer-Lambert law must always be tested in the concentration range under study. 

In our discussion of absorption spectroscopy, we noted that the interaction of photons with molecules resulted in the promotion of valence electrons from ground state orbitals to higher energy level orbitals. The molecules were said to be in an excited state. 

Molecules in the excited state do not remain there long, but spontaneously relax to the more stable ground state. With most molecules, the 

In our discussion above, it was pointed out that a molecule in the excited state can return to lower energy levels by collisional transfer or by light emission. Since these two processes are competitive, the fluorescence intensity of a fluorescing system depends on the relative importance of each process. The fluorescence intensity is often defined in terms of quantum yield, represented by (X This describes the efficiency or probability of the fluorescence process. By definition, XL is the ratio of the number of photons emitted to the number of photons absorbed (Equation 5.6). 

Energy-level diagram outlining the fluorescence process. 

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UV-visible spectrophotometry and fluorescence spectrophotometry are also used for the direct observation of radical species and their reactions in some... 

L. N. M. Duysens and J. Amesz, Fluorescence spectrophotometry of reduced phosphopyridine nucleotide in intact cells in the near-ultraviolet and visible region, Biochem. Biophys. Acta 24, 19-26 (1957). 

Hydrocarbon Anomalies in Soils Soil samples were collected at 200 to 500 m intervals over the Lisbon and Lightning Draw fields and analyzed for thermally desorbed Ci to C12 alkanes by Flame Ionization Detection Gas Chromatography (GC-FID) and solvent-extractable Ce to C36 aromatics by UV-fluorescence spectrophotometry. 

A methanolic extract from ethinyloestradiol tablets is measured using fluorescence spectrophotometry. A standard containing the pure drug is also measured under the same conditions. Calculate the content per tablet of the dmg from the following data ... 

The chimney-sweeper cancer described by P. Pott in England as early as 1775, which is basically related to soot but not to carbon black, was the starting point of intensive research on analytical test methods for polycyclic aromatic hydrocarbons (PAH) in carbon blacks. Increasingly refined test methods have been developed (e.g., column, thin-layer, and paper chromatography, gas chromatography, UV and fluorescence spectrophotometry, and mass spectroscopy) to detect traces of such substances and to investigate the ability of carbon black surfaces to adsorb and to desorb PAHs. 

Atomic absorption spectrophotometry is the method of preference (50, 51). For recent reviews of this procedure as applied to wines, see Refs. 52 and 53. X-ray fluorescence spectrophotometry has also been used (54). 

Since chlorophylls have distinct spectroscopic properties, absorption or fluorescence spectrophotometry can be employed for their identification and quantification (118). Usually wavelengths between 430 and 440 nm and 645 and 660 nm, respectively (93,115), are used for spec-trophotometric detection. With spectrophotometric detectors, detection limits of approximately 80 ng chlorophyll (119) and 1 ng chlorophyll (120), respectively, can be achieved. Fluorescence... 

Vannier and Stanley (31) employed fluorescence spectrophotometry to analyze mixtures of grapefruit and lemon oils. D Amore and Corigliano (32) used fluorescence to characterize mandarin, lemon and bergamot oils and related the characteristics to detect adulteration of mandarin oil with terpenes. 

Dry Oil Oxidation Fluorescence may be measured within or over an oil or within or over an emulsion of lipid in water providing the pH is below 5.5. When plate fluorescence is to be measured, solid sample fluorescence spectrophotometry is necessary (23). A Hitachi Solid Sample Holder Attachment for Model MPF-2A Hitachi-Perkin Elmer Fluorescence Spectrophotometer (Figure 4) was... 

Cold vapor atomic fluorescence spectrophotometry for mercury analysis... 

Exciplexes may be observed by means of fluorescence spectrophotometry. Many exciplexes show a new broad, non-structural emission at longer wavelength, while in other cases no such exciplex emission was observed except for the... 

The quantity of fat from the cocoa shell that ends up in the cocoa butter can be measured by testing with p-dimethyl-aminobenzaldehyde, which reacts with dried fruit pulp adhering to the shell. The reaction gives an intense fluorescence and has been used with TLC and fluorescence spectrophotometry (Kleinert, 1964). Shell fat is softer than nib fat because it contains higher levels of linoleic acid (18 2) and its presence softens the butter (Timms and Stewart, 1999). However, its presence can be usually due to poor separation of shell from nib rather than direct adulteration. 

Another spectroscopic method for the determination of lignin in solution is that utilizing fluorescence spectrophotometry. This technique is reportedly capable of detecting lignin in concentrations two to three orders of magnitude... 

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