4 - MOLECULAR ANALYSIS BY ABSORPTION AND RAMAN SPECTROSCOPY

Summary

This chapter discusses those spectroscopic techniques which use the ultraviolet, visible, and infrared part of the electromagnetic spectrum to give information about molecules and compounds, rather than about elements as discussed in Chapter 3. Three techniques are presented, each of which uses that relatively narrow part of the electromagnetic spectrum in or close to the visible region. Colorimetry is the use of the absorption of visible or UV radiation by solutions to quantitatively measure the concentration of the absorbing species. An important archaeological example is the determination of soil phosphorus by the “molybdenum blue” method. Infrared radiation (heat) is emitted and absorbed by the rotation, stretching, or vibration of molecular bonds, and thus gives details of the molecular species present. It is an important tool for the analysis of the bonds present in organic samples, although it rarely gives a unique identification. An important development for IR analysis is the use of Fourier transform (FT) techniques to increase the rate of collection of the data, which in turn has allowed the development of the infrared microscope. A related technique is Raman spectroscopy, which also employs FT techniques, and can be used in a microscope. The value of Raman spectroscopy is that those bonds which are infrared inactive (because of the lack of a change in dipole during the vibration) are Raman active, making the two techniques extremely complementary. These last two techniques are often referred to as vibrational spectroscopies, since they measure the frequency of vibration of molecular bonds.