Annals of Environmental Science
Volume 1, February 2007, Pages 57-68
Progression from Model Structures to Molecular Structures of Natural Organic Matter Components
J.A. Leenheer, U.S. Geological Survey, Bldg 95, MS 408, Federal Center, Denver, CO 80225, USA.
Received August 31, 2006; in final form November 29, 2006; Accepted January 2, 2007
Natural organic matter (NOM) found in soil, sediment and water has diverse mixture and molecular structural complexity. This complexity has been simplified by number-average molecular models, but the documental validity of these models decreases as the heterogeneity of the mixture increases, coupled with chemical and physical alterations that occur during NOM extraction and analysis. NOM models based on assumptions of extensive condensation and oxidative coupling reactions in humification processes portrayed NOM as macromolecular random-coil shapes, but more recent NOM models based on analytical evidence indicate that NOM is an aggregate of small molecules. Various organic matter fractionation approaches have been developed that separate NOM molecules into relatively homogeneous compound classes. Compound-class model structures developed from data synthesis of elemental, titrimetric, and nuclear magnetic resonance, infrared, and mass spectral analyses are better approximations of chemical structures than obtained previously on unfractionated NOM. The progression from model structures to molecular structures of NOM requires improved purification and fractionation methods, application of complementary and quantitative spectral characterizations, and ultra-high resolution mass separations combined with tandem spectral analyses. Syntheses of standards are required for final confirmation of NOM molecular components.