Term fluorescence

The intensity and colour of the fluorescence of many substances depend upon the pH of the solution indeed, some substances are so sensitive to pH that they can be used as pH indicators. These are termed fluorescent or luminescent indicators. Those substances which fluoresce in ultraviolet light and change in colour or have their fluorescence quenched with change in pH can be used as fluorescent indicators in acid-base titrations. The merit of such indicators is that they can be employed in the titration of coloured (and sometimes of intensely coloured) solutions in which the colour changes of the usual indicators would... 

The term fluorescence is used for a transformation of absorbed light into a light (7p) of lower energy. The extent of this transformation is described by the quantum yield factor q. Light intensity absorbed by the sample can be calculated from the light intensity reflected from the clean plate snrfaee (Jq) minus the light intensity (J) reflected from a sample spot [13]. 

Descriptive terms such as fluorescent whitening agents , optical brighteners and optical bleaches have all been used for the products described in this chapter as FBAs. Many of these terms have validity and the term fluorescent brightening agents is preferred here only because it has been adopted in the indexes of Chemical Abstracts. 

By absorption of light a molecule is promoted to a higher electronic state. The monomolecular physical processes for the dissipation of the excess energy are outlined in Fig. 5 in a so called Jablonski diagramm. In principle one has to differentiate between radiative and non-radiative deactivation on the one side and on the other side one has to consider if the multiplicity of the system is conserved or not. Radiative deactivation, i.e. deactivation accompanied by emission of light, is termed fluorescence if the transition occurs with spin conservation and phosphorescence, if spin inversion occurs. 

Class 2 fluorophores that can reversibly bind an analyte. If the analyte is a proton, the term fluorescent pH indicator is often used. If the analyte is an ion, the term fluorescent chelating agent is appropriate. Fluorescence can be either quenched upon binding (CEQ type Chelation Enhancement of Quenching), or enhanced (CEF type Chelation Enhancement of Fluorescence). In the latter case, the compound is said to be fluorogenic [e.g. 8-hydroxyquinoline (oxine)]. 

It is a remarkable fact that the contemporary history of absorption and emission spectroscopy began simultaneously, from the simultaneous discoveries that Bunsen and Kirchhoff made in the middle of the 19th century. They observed atomic emission and absorption lines whose wavelengths exactly coincided. Stokes and Kirchhoff applied this discovery to the explanation of the Fraunhofer spectra. Nearly at the same time approximately 150 years ago, Stokes explained the conversion of absorbed ultraviolet light into emitted blue light and introduced the term fluorescence. Apparently, the discovery of the Stokes shift marked the birth of luminescence as a science. 

Define the following terms fluorescence, phosphorescence, luminescence, photolumines-cence. and chemiluminescence. 

The term fluorescence may also be applied to Ihe radiation emitted, as well as to the emission process. 

In X-ray terminology, the term fluorescence may be used in the mure specific sense (than given in the general definition above) to denote the characteristic X-rays emitted as a result of Ihe absorption of X-rays of higher frequency. 

Two additional xanthene analogues are termed fluorescent brighteneis. Fluorescent Brightener 74 (Cl 45550) and Fluorescent Brightener 155 (Cl 45555) are used in the formulation of solid dielectric compositions for application in high voltage cables to prevent conductive treeing. 

Edmond Becquerel (1820-1891) was the nineteenth-century scientist who studied the phosphorescence phenomenon most intensely. Continuing Stokes s research, he determined the excitation and emission spectra of diverse phosphors, determined the influence of temperature and other parameters, and measured the time between excitation and emission of phosphorescence and the duration time of this same phenomenon. For this purpose he constructed in 1858 the first phosphoroscope, with which he was capable of measuring lifetimes as short as 10-4 s. It was known that lifetimes considerably varied from one compound to the other, and he demonstrated in this sense that the phosphorescence of Iceland spar stayed visible for some seconds after irradiation, while that of the potassium platinum cyanide ended after 3.10 4 s. In 1861 Becquerel established an exponential law for the decay of phosphorescence, and postulated two different types of decay kinetics, i.e., exponential and hyperbolic, attributing them to monomolecular or bimolecular decay mechanisms. Becquerel criticized the use of the term fluorescence, a term introduced by Stokes, instead of employing the term phosphorescence, already assigned for this use [17, 19, 20], His son, Henri Becquerel (1852-1908), is assigned a special position in history because of his accidental discovery of radioactivity in 1896, when studying the luminescence of some uranium salts [17]. 

Fluorescence is a special type of emission of radiation (Section 2.1.5). In emission, a molecule is moving from an excited state to a lower state, and the frequency of emission is a manifestation of the energy change between the two states. When the excited molecules have been created by absorption of radiation shone on the molecules, the resultant return to lower states is termed fluorescence. The initial exciting radiation can either be a conventional source or a laser (Section 2.1.4). 

In LC, derivatization is almost solely used to improve the detection limits of a specific substances, e.g., peptides, proteins, etc. Occasionally, derivatives are made to improve the selectivity of a particular stationary phase for certain materials, but the use of such reagents in this manner is not germane to the subject of this book. There are various classes of reagents that link specific chemical groups to the solute molecule, e.g., UV chromophores or fluorophores, and thus would be termed fluorescence reagents, UV chromaphore... 

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