Analytical pyrolysis of humic substances and dissolved organic matter in aquatic systems: structure and origin

Samples of freeze-dried surface-lake brownwater (HO original), derived fulvic (FA) and humic acids (HA) and ultrafiltrate (K) which represent reference materials of a priority program of the German Research Council on “Refractory Organic Substances in Waters” were investigated using pyrolysis–field ionization mass spectrometry (Py–FIMS), Curie-point pyrolysis–gas chromatography/mass spectrometry (Py–GC/MS) and, for the first time, Curie-point pyrolysis–gas chromatography–combustion–isotope ratio mass spectrometry (Py–GC–C–IRMS). Py–FIMS of dissolved organic matter (DOM), FA, and HA gave highly reliable fingerprint mass spectra, thermograms describing the thermal properties and qualitative and quantitative data of 10 classes of characteristic molecular building blocks. Carbohydrates, phenols and lignin monomers, lignin dimers, lipids, alkylaromatics, aromatic nitrogen compounds, sterols, peptides, suberins and loosely bound fatty acids were determined. Complementary molecular structure information was obtained by Py–GC/MS which allowed the identification of a wide range of humic building blocks as main pyrolysis products. Our first results using the novel method of combined Py–GC/MS and Py–GC–C–IRMS indicate that the δ13C-values of the pyrolysis products are in agreement with generally accepted data for carbohydrates, lignin and benzenes from biological sources. This implies that the pyrolytical step does not have an isotopic effect. In contrast, some of the thermal products, e.g. 5-methyl-2-furancarboxaldehyde and benzenemethyl, had stronger depletions in 13C in comparison to the mean δ13C-values of furans and benzenes, respectively. This result contradicts the enrichment of 13C due to trophic effects and therefore suggests the incorporation of light carbon sources, such as respiratory CO2, methane, or anthropogenic pollution by fossil fuels, in the humification processes.