Fourier transform infrared spectroscopy (FT-IR), surface-enhanced Raman spectroscopy (SERS), and nuclear magnetic resonance (NMR) (<sup>1</sup>H, <sup>13</sup>C, and <sup>31</sup>P) have been applied to the study of fractions of humic acids in Irish peat. Owing to the high fluorescence of these materials, no Raman spectra have been obtained, up to now, for the characterization of humic acids. The SERS technique was employed for the first time in the study of these complex substances, demonstrating that very valuable information about the aromatic groups and the spacial conformation of these macromolecules in water solution can be obtained. The combined use of these techniques has evidenced an increase of oxygenated groups in those fractions containing humic acids with low molecular weight. The presence of a greater number of carboxylate and phenolic groups can explain the high concentration of metals and phosphate ions found in these fractions.
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