Molecular Characterization of Biochar-derived Dissolved Organic Matter Based on A Nanoelectrospray Ultra-high Resolution Mass Spectrometer

Accurate molecular characterization of biochar-derived dissolved organic matter (DOM) is essential for assessing its environmental transport, transformation, and ecological effects. In this study, a direct injection analytical method was developed based on nano-electrospray ionization Fourier transform ion cyclotron resonance mass spectrometry (Nano-ESI-FT-ICR-MS) for high-resolution molecular profiling of rice straw biochar-derived DOM. The analytical performance of this method was systematically evaluated and compared with that of the conventional solid-phase extraction electrospray ionization FT-ICR-MS (SPE-ESI-FT-ICR-MS) approach. The results showed that under optimized Nano-ESI conditions (Water-methanol volume ratio of 1:4, injection carbon concentration of 5–10 mg/L), the number of molecular formulas identified for rice straw biochar-derived DOM increased significantly from 6736 (obtained by the conventional SPE-ESI method) to 10731, with a notable rise in heteroatom-containing compounds, particularly those incorporating sulfur and phosphorus. The detected DOM molecules exhibited overall higher average mass-to-charge ratio (m/zwa) and hydrogen-to-carbon ratio (H/Cwa), along with lower average oxygen-to-carbon ratio (O/Cwa), nominal oxidation state of carbon (NOSCwa), double bond equivalent (DBEwa), and modified aromaticity index (AImod wa). These molecular signatures indicated that the method offered enhanced detection capability for highly polar, long alkyl-chain, low-aromaticity aliphatic compounds as well as sulfur- and phosphorus-containing macromolecules. The established protocol required no complex pretreatment, effectively avoided the selective loss and enrichment bias inherent in SPE, and thus provided a reliable analytical technique for comprehensive and highly sensitive molecular characterization of biochar-derived DOM.