Annals of Environmental Science
Volume 3, March 2009, Pages 1-130
www.aes.northeastern.edu, ISSN 1939-2621
Systematic Approaches to Comprehensive Analyses of Natural Organic Matter
Jerry A. Leenheer, U.S. Geological Survey, PO Box 25046, MS 408, Denver Federal Center, Denver, CO 80225, USA
Received April 30, 2008; in final form December 11, 2008; accepted March 18, 2009.
Interest in the composition of natural organic matter (NOM) in various environments is increasing because of its significance in soil fertility, water quality, the global carbon cycle, and the maintenance of life itself. NOM composition cannot yet be described at the molecular level of characterization because of NOM complexity and its interactions with itself and environmental matrixes, but composition descriptions have progressed from the humus “black box” to compound-class descriptions that can be related to biochemical precursors modified by various diagenetic decay processes. The objective of this account is to present a number of comprehensive, systematic approaches to NOM analyses in a variety of soil, sediment, water, atmosphere, and rock environments. The focus of comprehensive NOM analyses is primarily based on analytical organic chemistry, but relevant information from biochemistry, plant science, soil science, hydrology, inorganic chemistry, and limnology is also included where needed. Comprehensive analysis is inclusive; its objective is to include all of NOM in its various fractions. Systematic analysis of NOM is based upon a hierarchical approach designed to quantitatively separate NOM into various fractions based on molecular size; polarity; acid, base, and neutral characteristics; specific gravity; extractability in various solvents; and volatility. The hierarchy level, fraction homogeneity, instrumental requirements, time, and cost of analyses are directly related to the number of NOM properties applied in the fractionation process. Given the fact that each NOM analysis is a research endeavor, the intended audiences for this book are researchers. These researchers should have a fundamental understanding of analytical chemistry, organic chemistry, and inorganic chemistry to successfully apply and modify comprehensive NOM analyses for studies with specific objectives, particularly as applied to environmental samples with unique properties that require procedural modifications. Comprehensive NOM analyses are designed for the discovery of unknown components that comprise NOM. After their discovery, analytical methods for these previously unknown components can usually be developed that are more rapid, sensitive, and inexpensive. Thus, comprehensive NOM analysis is not intended for environmental monitoring studies. The analytical approach of comprehensive NOM analysis is reductionist, with the ultimate intent of separating pure NOM compounds that can be identified in a systematic manner similar to that described in The Systematic Identification of Organic Compounds by Shriner, Fuson, and Curtin. Obtaining pure NOM compounds that can be identified by conventional analyses is not yet possible, and the most homogeneous of NOM fractions still contains hundreds to thousands of compounds, as shown by high-resolution mass spectrometry. Consequently, NOM structures derived from analytical data are models of average data sets, and these models are only approximations. Finally, there has been a revolution in the development of multidimensional spectra characterization methods and in ultrahigh resolution chromatography and mass spectrometry. Electrospray ionization/mass spectrometry has been especially useful in its application to NOM analyses. Fractions produced by comprehensive NOM analyses have been purified of inorganic components and have sufficient compound-class homogeneity to render these fractions especially suitable for advanced chromatographic and spectral analyses.