Organic molecules, fluorescence analysis

Fluorescence detectors can be more sensitive, but have a much narrower applicability. Only a small proportion of organic molecules exhibit natural fluorescence. One may choose to derivatise samples with a fluorescent or fluoro-genic agent, but this adds to the complexity of the analysis and the validation required. 

The fluorescence of organic molecules and ions in solution is a photoluminescent process that decreases extremely rapidly when the excitation ceases, in contrast to phosphorescence, which latter has a much slower decrease and is seldom used for analysis. Finally, chemiluminescence, the emission of light during a chemical reaction, is involved in some analytical applications. 

Laser-Induced Fluorescence. Laser-induced fluorescence (Lif) provides, much as does ir spectroscopy, fingerprints of different organic molecules, which can be quantified by measuring fluorescence intensities. Selectivity is excellent, as both pump and fluorescence frequencies can be individually chosen for optimum performance, and it can be improved with measurements of fluorescence lifetimes and polarization behavior. The enhanced null-background sensitivity can achieve single-atom or single-molecule detection (256—258). Lif has important applications in gas analysis (259) and combustion and plasma diagnostics (260). 

The electronic excited state is inherently unstable and can decay back to the ground state in various ways, some of which involve (re-)emission of a photon, which leads to luminescence phenomena (fluorescence, phosphorescence, and chemiluminescence) (22). Some biologic molecules are naturally fluorescent, and phosphorescence is a common property of many marine and other organisms. (Fluorescence is photon emission caused by an electronic transition to ground state from an excited singlet state and is usually quite rapid. Phosphorescence is a much longer-lived process that involves formally forbidden transitions from electronic triplet states of a molecule.) Fluorescence measurement techniques can be extremely sensitive, and the use of fluorescent probes or dyes is now widespread in biomolecular analysis. For example, the large increase in fluorescence... 

Labelled antibodies with a fluorescent derivative obtained from fluorescein, rhodamine or luminarin, are well suited for the measurement of a number of human and animal immunoglobulins. However, the transposition of this principle to immunochemical testing for simple organic molecules remains, as yet, non-existent. There may be discrimination between the labelled and non-labelled species at the isolation stage for small organic molecules. This explains that only a few examples of analysis followed by fluorescence measurement. 

Chemical imaging is described, including confocal Raman imaging. UV and visible spectroscopy includes innovations such as flow-through sample holders and fiber-optic probes, as well as instruments for analysis of submicroliter volumes and nondestructive analysis for nucleic acid and protein determinations. UV absorption spectral interpretation for organic molecules is covered in depth. Applications described include nucleic acid and protein measurements, spectrophotometric titrations, and new applications in forensic chemistry. Nephelometry, turbidimetry, fluorescence, and phosphorescence are described in detail, including instrumentation and applications. The measurement of color using the CIE system is described with examples. 

Fluorescence Analysis of Organic Molecules in Solid Solutions. 748... 

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