Characterization of Burned Soil Organic Matter via Sequential Solvent Extractions and 21 T FT-ICR Mass Spectrometry with Electrospray and Atmospheric Pressure Photoionization

Wildfires significantly alter soil organic matter (SOM) composition, generating complex mixtures of polar and nonpolar compounds which are difficult to analyze. This study combines sequential extractions with electrospray ionization (ESI) and atmospheric pressure photoionization (APPI) followed by 21 T Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) to improve the molecular characterization of SOM from burned soils. Soils were extracted first with water and then Soxhlet extracted with organic solvents (acetone/cyclohexane and methanol/toluene/tetrahydrofuran). The compositional range of species identified by positive and negative ESI was expanded by 8-62%, and APPI enabled the detection of hydrocarbons and nitrogen-containing species that were largely undetected by ESI alone. Among the identified compounds, molecular formulas consistent with biomarkers such as stigmasta-3,5-diene, a plant-derived sterol degradation product, were present only in the organic solvents. Stigmasta-3,5-diene and similar compounds have been observed after fire events across various vegetation types, including forest ecosystems. Highly abundant compounds detected at four double bond equivalents (e.g., 25-azacoprostane) may represent sterol interrupters or cholesterol metabolism inhibitors, potentially serving as additional biomarkers for fire conditions. The calculated nominal oxidation state of carbon (NOSC) revealed distinct groupings based on solvent type and ionization mode, emphasizing the selective extraction of less labile organic matter by organic solvents compared to water. These results highlight the critical role of sequential solvent extraction and diverse data analysis strategies in advancing the understanding of fire-impacted SOM chemistry and provide a framework for biomarker discovery in environmental monitoring and biogeochemical studies.