Comparison of the information gained by pyrolytic techniques and NMR spectroscopy on the structural features of aquatic humic substances

Abstract

The molecular composition of fulvic acid (FA) and humic acid (HA) fractions from a German natural lake have been characterized by solid state 13C-NMR spectroscopy and on-line Curie-point pyrolysis at 510°C both in the presence and the absence of tetramethylammonium hydroxide (TMAH) followed by the separation and identification of the compounds released by gas chromatography-mass spectrometry (GC-MS). The two humic fractions released similar assemblages of aliphatic and aromatic pyrolysis compounds, arising most probably from carbohydrate, polyaromatic, and aliphatic structural subunits. After thermochemolysis with TMAH, most of the products released were found as methyl esters. These products also included typical lignin markers. The same structural components responsible for the release of pyrolysis products were apparent in the NMR spectra of the FA and HA, showing a similar distribution in both fractions. The reliable quantitative distribution of C atoms pertaining to alkyl and aromatic structures obtained from NMR data was not in total agreement with the relative quantification of pyrolysis products, which seems to be biased towards an 'enhancement' of aliphatic moieties. However, the two techniques give similar information on the overall structural make-up of the humic fractions, and thus agree that the FA fraction does contain less aromatics and more aliphatics than the HA fraction, whereas the content in O-alkyl structures is similar in the two fractions.