Pyrolysis field ionization mass spectrometry of lignins, soil humic compounds and whole soil

Abstract

Pyrolysis, followed by field ionization mass spectrometry (FI-MS) of lignins, revealed a regular pattern of phenolic products related to the building blocks of these compounds. The signals were found to be related to dimers of the respective lignin alcohols in addition to their methylated derivatives. However, overlapping was observed in the signals from ionized subunits having the same mass numbers but different structures. Although lignins play an important role in the formation of terrestrial humic compounds, their thermal fragments, analysed by FI-MS, are largely different from those of lignins. Correlation with the chemical structure of lignins is only indicated by some phenol-related signals in the lower mass range ( m/z 90–160). These also became more obvious in hydrolysed humic acids. The higher mass range from m/z 300 to 350 showed a regular pattern of signals apparently indicating complex aliphatic hydrocarbon structures. Pyrolysates from typical soil humic acids also showed additional thermal fragments resulting from polysaccharides, which were even more prominent in pyrolysates from whole soil samples. The analyses of humic compounds by pyrolysis and FI-MS led to the conclusion that the compact phenolic framework of lignin becomes substantially altered and dispersed during its transformation into humic compounds. These results are discussed together with recently obtained solid-state 13C NMR measurements on soil organic matter fractions. They are further compared with the results of studies of microbial attack on lignin, which were carried out by 14C NMR measurements of specifically 13C-enriched lignins after incubation with several ligninolytic organisms. A first direct isotopic determination of the subunits in the biomacromolecule using pyrolysis in combination with soft ionization mass spectrometry confirmed the label content in the dehydropolymer-lignin.