Humic substances infrared spectra

Figure 1 gives a typical infrared spectrum of lake humic acid. The interpretation of infrared spectra of humic substances is discussed in depth by -MacCarthy and Rice in Chapter 21. The similarity of infrared spectra of humic acids from different lakes suggests a similarity of the aspects of chemical structure that are related to their infrared absorptions. However, infrared spectroscopy is not sensitive enough to uncover minor structural differences among humic acids. In fact, humic acids were separated by organic solvents (chloroform, methylethylketone, methanol, dimethylformamide) into various fractions (Ishiwatari, 1969b, 1973). Infrared spectra of two of these fractions, the chloroform-soluble fraction and the methylethylketone-... 

The applicability of spectroscopic methods (other than NMR) for determining functionality in humic substances is reviewed. Spectroscopic methods, like all other investigational techniques, are severely limited when applied to humic substances. This is because humic substances are comprised of complicated, ill-defined mixtures of polyelectrolytic molecules, and their spectra represent the summation of the responses of many different species. In some cases only a small fraction of the total number of molecules contributes to the measured spectrum, further complicating the interpretation of spectra. The applicability and limitations of infrared spectroscopy, Raman spectroscopy, UV-visible spectroscopy, spectrofiuorimetry, and electron spin resonance spectroscopy to the study of humic substances are considered in this chapter. Infrared spectroscopy, while still very limited when applied to humic substances, is by far the most useful of the methods listed above for determining functionality in these materials. Very little information on the functionality of humic substances has been obtained by any of the other spectroscopic methods. 

With a few exceptions, all infrared spectra of humic substances have been measured in dry solid samples, and the pressed-pellet method has been used almost exclusively. The spectrum of humic acid consists of relatively few bands that are very broad. The broadness of the bands reveals that humic substances are complex mixtures where a particular type of functional group can exist in a wide variety of chemical environments, each characterized by slightly different force constants for its bonds. Fourier-transform infrared (FTIR) spectroscopy allows the samples to be observed in their native wet state and avoids the shifts in chemical equilibrium which must necessarily accompany the drying process. 

Figure 14 shows the improvement in Raman spectra via near-infrared excitation due to the elimination of fluorescence of humic substances. The top spectrum was taken with 514.5 -nm excitation, and the Raman signal is completely obscured by fluorescence. However, the bottom spectrum that was taken with 1064-nm excitation with an FT-Raman system provided two clearly observable Raman bands. The peak in the 1300-cm range (referred to as the D band) proves to be extremely valuable because it indicates the amount of disorder of carbon networks. Because of the presence of this band and the absence of second-order Raman bands of humic substances (2000-3000-cm range), Yang and Wang [99] have concluded that the backbones of humic substances are structurally disordered carbons (i.e., graphitelike) for most cases. 

Figure 14 Improvement in the Raman spectrum of peat humic acid in neutralized form by use of near-infrared excitation. The top spectrum was obtained with 514.5-nm excitation, which results in a large fluorescence background that obscures the signal. However, as shown in the bottom spectrum, with 1064-nm excitation, and after subtraction of thermal emission background, spectral features of peat humic are clearly discernible. (Reprinted with permission from YH Yang, HA Chase. Applications of Raman and surface enhanced Raman scattering techniques to humic substances. Spectr Lett 31 821-848, 1998. Copyright 1998 Marcel Dekker, Inc.)...

---