Identification Techniques II

Infrared (IR) and Raman spectroscopy have a high potential for characterisation of material. Extensive series of wet chemical analysis may be substituted by a single spectroscopic measurement followed by detailed chemometric data evaluation. Topics of this chapter are: (i) basics of IR and Raman spectroscopy, (ii) the registration of “correct” spectra, and (iii) spectra evaluation. Dedicated applications in the area of conservation science are collected in separate chapters. The infrared (IR) spectrum is often called the fingerprint of a substance. An IR spectrum identifies a substance like a human fingerprint. Due to their origin the features of an IR spectrum are bands, not peaks. They indicate vibrations within the molecular framework. Such vibrations are excited by irradiation with infrared light. The infrared spectral range joins the red end of the visible range, it extends from 780 to 1 mm wavelength. Radiation in this spectral range is of low energy, it does not harm material. Commercial IR spectrometers are available since 1945. They dominated the field of structural analysis (identification) of molecular substances until NMR spectrometers became affordable. Nowadays small and easy-to-use IR spectrometers are abundantly available in laboratories [8]. Raman spectroscopy is complementary to IR spectroscopy. It is named after its discoverer, the Indian Nobel Laureate Sir C. V. Raman. Molecular vibrations are excited here by irradiation with intense visible or near IR radiation. The excitation mechanism is different from the excitation of vibrations by IR light, which results in some well-defined differences between IR and Raman spectra of the same sample. For this reason IR and Raman spectra are complementary, not identical. The complementarity of IR and Raman spectra provides additional information about molecular properties of the sample. The need for intense light sources hampered Raman spectroscopy until the advent of lasers. Today a range of miniaturised lasers and convenient fibre-probes are available, which permit the construction of small portable Raman spectrometers for difficult field applications.