TEMPERATURE-RESOLVED IN-SOURCE PYROLYSIS SOFT IONIZATION MASS-SPECTROMETRY OF SOIL HUMIC ACIDS

Two humic acids (HAs) extracted from the Bainsville soil, a Haplaquoll, and the Armadale soil, a Haplaquod, were investigated by in-source pyrolysis (Py) mass spectrometry (MS) with direct chemical ionization (DCI) and fast atom bombardment (FAB) in the positive and negative ion modes, and positive field ionization mass spectrometry (FIMS). The aim was to evaluate the efficacy of these soft ionization methods for structural investigations on HAs and to compare in-source pyrolysis data with results of Curie-point Py-GC/MS. With temperature-resolved in-source Py FIMS, it is possible to produce thermograms for prominent classes of compounds in HAs such as fatty acids, acetamide, carbohydrates, dihydroxybenzenes, alkylphenols, alkylbenzenes, and nitrogen-containing compounds. The devolatilization of these constituents is characteristic for each HA and allows for a clear differentiation between Bainsville and Armadale HAs, with the former being richer in alkyl-N-heterocyclics and other alkyl-N compounds than the latter. In addition, first hints were obtained of the identities of so far unidentified high-mass nitrogen-containing compounds in seven HAs with wide ranges in elemental composition and from widely different pedologic and geographic environments.