Quantitative analysis via near-infrared spectroscopy

2-7 and 6.7-23). In addition, reasonable optical path lengths of millimeter or centimeter size may be used in NIR spectroscopy, as compared to the layers of micrometer size which are required in infrared analysis of liquid or compressed gaseous materials. Since almost all substances which are practically relevant have characteristic NIR absorption bands, quantitative analysis via near-infrared spectroscopy is generally applicable to on-line concentration measurements in connection with chemical reactions, chemical equilibria, and phase equilibria. In.spection of the reference NIR spectral data for the relevant chemical species shows which NIR range provides special advantages for the quantitative analysis of a particular system. 

The scope of this method is by far too broad to be presented in detail. The on-line NIR technique is especially valuable for studies under unusual or extreme conditions. The fluid phase equilibrium in the ethene-polyethylene system has been studied by Nees (1978) up to 300 °C and 3000 bar the solubility of adamantane, of octacosane, and of squalane in fluid CO2 up to 1000 bar has been determined NIR-spectroscopically by Swaid et al. (1985). 

The potential of the NIR method for the investigation of chemical transformations is demonstrated by a few examples of polymerization reactions. Fig. 6.2-24 shows a series of NIR spectra, measured in the second overtone region of the C -H stretching modes 

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