Analytical Chem istry - DePauw University

620 Analytical Chemistry 2.0by immobilizing the sample on a solid substrate, making possible roomtemperature measurements. One approach is to place a drop of the solutioncontaining the analyte on a small disc of filter paper. After drying thesample under a heat lamp, the sample is placed in the spectrofluorimeter foranalysis. Other solid surfaces that have been used include silica gel, alumina,sodium acetate, and sucrose. This approach is particularly useful for theanalysis of thin layer chromatography plates.10F.3 Quantitative ApplicationsMolecular fluorescence and, to a lesser extent, phosphorescence have beenused for the direct or indirect quantitative analysis of analytes in a variety ofmatrices. A direct quantitative analysis is possible when the analyte’s fluorescentor phosphorescent quantum yield is favorable. When the analyte isnot fluorescent or phosphorescent, or if the quantum yield is unfavorable,then an indirect analysis may be feasible. One approach is to react theanalyte with a reagent to form a product with fluorescent or phosphorescentproperties. Another approach is to measure a decrease in fluorescenceor phosphorescence when the analyte is added to a solution containing afluorescent or phosphorescent probe molecule. A decrease in emission isobserved when the reaction between the analyte and the probe molecule enhancesradiationless deactivation, or produces a nonemittng product. Theapplication of fluorescence and phosphorescence to inorganic and organicanalytes are considered in this section.In o r g a n i c An a l y t e sHOHOOHNFigure 10.55 Structure of alizarin garnetR and its metal–ligand complexwith Al 3+ .NHOalizarin garnet RONAl 3+NOfluorescent complexOHOHExcept for a few metal ions, most notably UO 2 + , most inorganic ions arenot sufficiently fluorescent for a direct analysis. Many metal ions may be determinedindirectly by reacting with an organic ligand to form a fluorescent,or less commonly, a phosphorescent metal–ligand complex. One example isthe reaction of Al 3+ with the sodium salt of 2, 4, 3′-trihydroxyazobenzene-5′-sulfonic acid—also known as alizarin garnet R—which forms a fluorescentmetal–ligand complex (Figure 10.55). The analysis is carried outusing an excitation wavelength of 470 nm, monitoring fluorescence at 500nm. Table 10.12 provides additional examples of chelating reagents thatform fluorescent metal–ligand complexes with metal ions. A few inorganicnonmetals are determined by their ability to decrease, or quench, the fluorescenceof another species. One example is the analysis for F – based on itsability to quench the fluorescence of the Al 3+ –alizarin garnet R complex.Or g a n i c An a l y t e sAs noted earlier, organic compounds containing aromatic rings generally arefluorescent and aromatic heterocycles are often phosphorescent. As shownin Table 10.13, several important biochemical, pharmaceutical, and environmentalcompounds may be analyzed quantitatively by fluorimetry or