Infrared and Laser Raman Microspectroscopy

Computerized IR microspectrophotometers such as the Nanospec (Table 1) can scan areas as small as 20 x 20 pm over the infrared wavelength range of 2.5 to 14.5 pm (4000-690 cm-1). Samples to be measured are placed on the microscope stage. Selection of small areas is possible by direct viewing at 150 x magnification. 

Several reviews are available on the infrared spectroscopy of fossil organic materials (Speight, 1971 20), Robin et al., 197721), Rouxhet et al., 198022)), but no detailed objective review exists on the application of such spectroscopic techniques on individual particles of microscopic size. 

Comparable infrared (IR) spectra of complex organic solids such as coals (Fig. 7) 23), cherts, kerogens, humic substances and some natural polymers have been presented in various publications. They show a limited number of rather broad bands which are due to well defined chemical groups and can often be interpreted by comparison to less complicated spectra. The signals commonly observed in fossil organic matter are as follows (Robin et al., 1977 21), Tissot Welte, 1978 24), Rouxhet et al., 1980 22 , Friedel Carlson, 1972 25))  

A limited number of parameters calculated from the absorption coefficients has been proven useful. Although they are empirical to some extent, they have a rather clear chemical meaning. In fact, the spectra differ from each other by the intensity of the various bands. For instance, the spectra of kerogens and coals vary in regular fashion according to rank and reflect the main chemical modifications occuring as rock metamorphism proceeds (Rouxhet et al., 1980) 22) (Fig. 15, p. 23). 

The band intensities arising from the aliphatic C—H bonds are of particular interest, since they depend on the atomic weights of the atoms to which the other three valences of the carbon are linked. The peaks around 2930 and 2860 cm-1 are due mainly to the asymmetric and symmetric stretching of alkyl CH2 groups, expected at 2926 and 2853 + 10 cm-1. The CH3 groups are expected to give asymmetric and symmetric stretching bands at 2962 and 2872 cm-1. 

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