Complementary molecular windows revealed by direct NanoESI and SPE-ESI FT-ICR-MS for dissolved organic matter characterization

Nanoelectrospray ionization (nanoESI) coupled with Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS) has emerged as a powerful approach for the direct molecular characterization of dissolved organic matter (DOM) without solid-phase extraction (SPE). However, the differences in molecular coverage and compositional selectivity between direct nanoESI and conventional SPE-ESI for DOM analysis remain unclear. In this study, two standard reference materials, Suwannee River natural organic matter (SRNOM) and Elliott soil fulvic acid (ESFA), were used to systematically compare two ionization sources (nanoESI and ESI) and two sample introduction modes (direct injection and SPE pretreatment). Compared with conventional ESI, nanoESI increased total ion intensity by 4.5-fold and 3.9-fold for SRNOM and ESFA, respectively, and expanded assigned molecular formulas by 1.5-1.7 times. ESI yielded a greater number of aromatic and highly unsaturated compounds, whereas nanoESI enabled broader molecular coverage, including aliphatic compounds, oxygen-rich aromatics, and heteroatom-containing species. SPE pretreatment enriched lignins and condensed aromatics in both samples, while direct nanoESI retained more lipid- and peptide-like compounds in SRNOM and abundant aminosugar- and carbohydrate-like compounds in ESFA. Combing the ionization and pretreatment effects, SPE-ESI favors the detection of aromatic fractions, whereas direct nanoESI better captures aliphatic- and tannin-like components. The two methods yield complementary information and enabled more comprehensive and unbiased molecular profiling of environmental DOM.