Fluorescent reagents, inorganic

Molecular fluorescence and, to a lesser extent, phosphorescence have been used for the direct or indirect quantitative analysis of analytes in a variety of matrices. A direct quantitative analysis is feasible when the analyte s quantum yield for fluorescence or phosphorescence is favorable. When the analyte is not fluorescent or phosphorescent or when the quantum yield for fluorescence or phosphorescence is unfavorable, an indirect analysis may be feasible. One approach to an indirect analysis is to react the analyte with a reagent, forming a product with fluorescent properties. Another approach is to measure a decrease in fluorescence when the analyte is added to a solution containing a fluorescent molecule. A decrease in fluorescence is observed when the reaction between the analyte and the fluorescent species enhances radiationless deactivation, or produces a nonfluorescent product. The application of fluorescence and phosphorescence to inorganic and organic analytes is considered in this section. 

Zirconium is often deterniined gravimetrically. The most common procedure utilizes mandelic acid (81) which is fairly specific for zirconium plus hafnium. Other precipitants, including nine inorganic and 42 organic reagents, are Hsted in Reference 82. Volumetric procedures for zirconium, which also include hafnium as zirconium, are limited to either EDTA titrations (83) or indirect procedures (84). X-ray fluorescence spectroscopy gives quantitative results for zirconium, without including hafnium, for concentrations from 0.1 to 50% (85). Atomic absorption determines zirconium in aluminum in the presence of hafnium at concentrations of 0.1—3% (86). 

Factors such as dissociation, association, or solvation, which result in deviation from the Beer-Lambert law, can be expected to have a similar effect in fluorescence. Any material that causes the intensity of fluorescence to be less than the expected value given by equation (2) is known as a quencher, and the effect is termed quenching it is normally caused by the presence of foreign ions or molecules. Fluorescence is affected by the pH of the solution, by the nature of the solvent, the concentration of the reagent which is added in the determination of inorganic ions, and, in some cases, by temperature. The time taken to reach the maximum intensity of fluorescence varies considerably with the reaction. 

However, these compounds and the fragments are not without their intrinsic problems and should not be used as is. Some examples of potentially problematic compounds include those with chemically reactive groups, dyes, and fluorescent compounds which interfere with assays, frequent hitters/promiscuous binders, and inorganic complexes (55). It is important, then, to a priori filter out such compounds or reagents which are practically useless from a drug discovery point of view. 

There are various alternatives to obtain fluorescence with a pesticide. For instance, fluorescence may be derived from the pesticide by treatment with heat, acid, base, inorganic salts or a combination of these. Another approach is to prepare a derivative Cfluorogenic labelling) in solution the derivative is then extracted and applied directly on a chromatogram for separation. A fluorogenic reagent that will become fluorescent on contact with the pesticide may also be used. These alternatives are summarized in Figure 5. 

Inorganic Reagents. Another way to obtain fluorescence from pesticides is with inorganic salts. On the one hand, pesticides may act as ligands capable of combining with metal ions to yield fluorescence,... 

Sensitized luminescence in inorganic analysis will be discussed below in the section on lanthanides. Fluorescence, phosphorescence and sensitized luminescence processes are independent of the electronic structure of the organic reagent and the metal ion alone. Of importance are the composition of the complex, the nature, strength, and spatial orientation of metal-ligand bonds, and conditions under which the luminescence reaction proceeds (such as pH and the nature of solvent). All these factors significantly influence the detection limit, sensitivity and selectivity of determination. 

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