Analytical spectroscopy quantitative aspects

Frequently, during organic synthesis or drug metabolism studies, unknown compounds are isolated and their structure must be confirmed or, if they are totally unknown, solved using a combination of analytical techniques. Exercises of this type use qualitative spectroscopy rather than the quantitative aspects discussed so far. Whole university courses and many textbooks and online tutorials have been produced to cover elucidation of chemical structure, and a detailed treatment of the subject is beyond the scope of this book. However, a brief overview of the major techniques and their limitations may be useful. 

Since the determination of inhibitors in polymers by Drushel et al. [13] little information has been added to the literature on the quantitative aspects of the direct examination of polymer films by phosphorescence spectroscopy. These authors examined phosphorescence (at liquid-nitrogen temperature) of thin EPR films containing Santonox R (2,2 -di-methyl-5,5 -di-r -butyl-4,4 -dihydroxydiphenyl sulfide) and V-phenyl-2-naphthyl-amine (PBN). The rather intense phosphorescence of PBN may be used to advantage when other additives interfere in the UV absorption method. As to quantitative phosphorescence analysis, several factors, e.g. film thickness, concentration quenching, and background absorption, etc., affect the linearity of the analytical working curves and precision of the measurements [13]. The reliability of a correlation between stabiliser concentration in the film and phosphorescence intensity at 77 K is also influenced by the degree of crystallinity [544]. 

Many analytical techniques are available for the determination of the primary molecular structure of a block copolymer and its average chemical composition. Among them spectroscopic techniques used for low molecular weight compounds are the most powerful and most widely employed. NMR can provide both qualitative and quantitative information with respect to comonomer composition and stereochemical configuration of polymeric molecules (136,137). The IR technique provides information on chemical, structural, and conformational aspects of polymeric chains (138). Because of the inherently high sensitivity of UV spectroscopy the technique is often utilized for the identification and quantitative determination of comonomers in block copolymers. 

The principle of optical spectroscopy involves the measurement of the amount of light (radiation) that is absorbed by the sample when the radiation interacts with the sample. The most basic method involves the determination of the fraction of the radiation that is actually transmitted through a sample. The aspects of the measurement, and their relationship to the actual absorption of radiation are illustrated in Fig. 56. In this example, 7o is the power of the incident radiation from the infrared light source, and I is the actual amount of radiation transmitted through the sample. The fundamental relationships are provided with Fig. 56, and these form the basis of a fundamental expression that is used to correlate the analytical spectrum with the amount(s) of material(s) present in a sample. This fundamental expression is a simple rendering of the Beer-Lambert-Bouguer law, which is used in one form or another in the quantitative determination of material composition. 

The pioneering works in chemometrics are summarized in some books. In a later phase chemometrics has been presented more fundamentally. Nowadays a series of books for the different aspects of chemometrics are available, such as in analytical chemistry, spectroscopy, calibration, quantitative structure-activity relationships, experimental design, chemical structures, organic synthesis, and neural networks. Research papers in chemometrics are widespread but some concentration occurs in journals of analytical chemistry, chromatography, spectroscopy, and computer chemistry. Two journals have the word chemometrics in their name Chemometrics and Intelligent Laboratory Systems and Journal of Chemometrics. Comprehensive reviews about chemometrics in analytical chemistry have appeared since 1980 every two years 1982, 1984, 1986, 1988, 1990, 1992, 1994, and 1996. ... 

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