Analytical pyrolysis and computational chemistry of aquatic humic substances and dissolved organic matter

Humic acids (HA), fulvic acids (FA), non-humic substances (NHS) and dissolv ed organic matter (DOM) in a bog lake water are investigated by analytical pyrolysis. The applied thermal methods are direct, in-source pyrolysis-fiel d ionization mass spectrometry in the high electric field (Py-FIMS), and Cu rie-point pyrolysis-gas chromatography/mass spectrometry (Py-GC/MS) in comb ination with library searches. Based on the identified building blocks and together with complimentary analytical data, proposals for a general concep t of the basic molecular structures of humic macromolecules in water are pu t forward. Computational chemistry is utilized for structural modeling and geometry optimization of DOM. Molecular mechanics calculations are performe d to evaluate the conformation of structural, three-dimensional models and to determine the total energy and the partial contributions from bond-, ang le-, dihedral-, van der Waals-, stretch-bend-, and electrostatic energies. Quantitative structure-activity relationship (QSAR) properties are calculat ed and allow the correlation of molecular structures with properties such a s mass, surface area, volume, partial charges (electronegativity), polariza bility, refractivity, hydrophobicity, and hydration energy. The principal a im and long-term strategy are to develop step by step improvements of the p resented model structures of organic matter in water which explain the mole cular composition as well as their ecological meaning, dynamic character, a nd structure-property relationships in natural and contaminated aquatic and terrestrial systems. In a first integrated approach, the dissociation and association processes of humic substances are simulated at nanochemistry le vel and are proposed as concepts for future collaboration incorporating res ults of additional chemical, biological, spectroscopic and microscopic meth ods. (C) 1999 Elsevier Science B.V. All rights reserved.