CHARACTERIZATION OF HUMIC ACIDS, COMPOSTS, AND PEAT BY DIFFUSE REFLECTANCE FOURIER-TRANSFORM INFRARED-SPECTROSCOPY

Transmission infrared (IR) spectroscopy, either dispersive or Fourier transformed (FTIR), has been used extensively in studies on humic substances. A variety of bands characteristic of molecular structures and functional groups have been identified for these substances. The development of an attachment mounted onto FTIR spectrophotometers has allowed the determination of diffuse reflectance Fourier-transformed infrared (DRIFT) spectra. The purpose of this work was to study the applicability of DRIFT to soil organic-matter research and to compare the use of this instrumentation to dispersive and Fourier-transformed transmission IR spectroscopy. The DRIFT spectra were determined for humic acids, peat samples, and composts. In addition, the possibility of using DRIFT spectra to quantitatively measure sample concentration and measuring the relative concentration of functional groups was assessed. Sample preparation for DRIFT is much simpler than for transmission IR spectroscopy, interferences due to water adsorption are reduced, and resolution is improved. The spectra obtained using DRIFT had a higher degree of resolution as compared with dispersive and Fourier-transform transmission IR spectroscopy. Bands indicative of aliphatic C-H, carboxyl and corboxylate functional groups, aromatic C = C, and C-O stretch of polysaccharides were prominent and very well resolved. The DRIFT spectra obtained can also be used to fingerprint organic matter acquired from various sources. Spectra obtained at various concentrations of humic acid indicated that DRIFT cannot be used to estimate concentrations of organic matter in a given mixture. Relative concentrations of functional groups, however, were found to be fairly constant regardless of sample concentration. Therefore, changes in the relative concentration of functional groups can be measured during the humification process. It is expected that the application of DRIFT to organic-matter research will prove especially useful for characterizing bulky heterogeneous samples such as peat and composts.