In this work, laser desorption ionization coupled with Fourier transform ion cyclotron resonance mass spectrometry (LDI-FTICRMS) was used to investigate the molecular composition of a peat fire and laboratory heated soil organic matter (SOM). SOM isolated from soils obtained from unburned and burned sites at Central Kalimantan, Indonesia, were analyzed with LDI-FTICRMS. About 7500 peaks were found and assigned with molecular formulas for each mass spectrum. SOM isolated from fire-affected soil sites are relatively more abundant in low oxygenated classes (e.g., O1-O5) and thermally stable compounds, including condensed hydrocarbon and nitrogen heterocyclic compounds. Abundances of highly condensed hydrocarbon compounds with carbon number > 30 were increased for the fire-affected SOM. In vivo heating experiments were conducted for SOM extracted from unburned sites, and the prepared SOMs were analyzed with LDI-FTICRMS. Overall, the same trend of change at the molecular level was observed from both the laboratory heated and the peat fire-affected SOM samples. In addition, it was observed that heat caused the degradation of SOM, generating lignin and tannin-type molecules. It was hypothesized that they were formed by thermal degradation of high molecular weight SOM. All the information presented in this study was obtained by consuming ~ 5 μg of sample. Therefore, this study shows that LDI-FTICRMS is a sensitive analytical technique that is effective in obtaining molecular level information of SOM. Graphical abstract.
Keywords: LDI–FTICRMS; Molecular transformation; Peat fire; Soil organic matter.