Phosphorescence indicators

Differences in the materials employed for the layers can also become evident when chemical reactions are performed on them. Thus, Macherey-Nagel report that the detection of amino acids and peptides by reaction with ninhydrin is less sensitive on layers containing luminescent or phosphorescent indicators compared to adsorbents which do not contain any indicator [7]. 

The mean lifetime of triplet benzene in the glassy matrix is not certain, but all authors agree that it is large and it is probably about 20 sec113. And yet the available data on decay of the triplet state in the gas phase (there is no phosphorescence) indicate a mean lifetime of the order of magnitude of 10 4 to 10"5... 

Figure 4.8 Common optode designs using fiber optics, (a) Chamber or membrane with a single bioluminescent, fluorescent, or phosphorescent indicator dye at the distal end of an optical fiber, (b) Differential or dual-analyte design with two separate indicator dyes. Evanescent waves are produced using (c) step-etched, (d) tapered, or (e) heat-stretched designs.

Phosphorescence indicates a triplet state at ca. 0.7 eV lower in energy than the l Bu state for a wide variety of systems. (See Kohler and Beljonne (2004) for a review of the data.) This triplet state is the 1 B state. 

The same principle is used in indirect absorbance measurement when working with absorbing substances between 250 and 300 nm and TLC plates with phosphorescence indicator. As inorganic indicators absorb in the same range, the residual phosphorescence emission of the indicator is inhibited by the degree the substance absorbs the UV radiation exciting the indicator. 

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