Improved Data Interpretation of FT-ICR MS Spectra by Intensity-Weighted Molecular Parameters at Nominal Mass Level for Dissolved Organic Matter

The molecular properties of dissolved organic matter (DOM) by Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR MS) are often characterized by intensity-weighted average bulk descriptors. Due to the differential ionization efficiencies of individual peaks and the vast number of molecular formulas, the traditional intensity-weighted method cannot resolve the imperceptible compositional and property variations across different nominal mass ranges. In this study, we proposed a nominal mass- and intensity-weighted strategy for characterizing DOM across different physical fractionations and redox reactions to improve the interpretation of DOM molecular variations. The nominal mass- and intensity-weighted strategy revealed differential DOM fractionation by traditional and sequential solid-phase extraction (SPE) methods with distinct mass-specific enrichment patterns. The hydroxylamine–HCl reaction with tap water exhibited strong reactivity in the mass windows of 450–550 Da. The nominal- and intensity-weighted strategy revealed that chlorination of surface water induced differential responses across nominal mass ranges, with compounds in the nominal mass ranges of 150–250 and 550–650 Da preferentially reacting or being degraded, while chlorinated disinfection byproducts were predominantly formed in the 250–550 Da range. Furthermore, the novel analysis strategy facilitated the visual identification of nominal masses with pronounced deviations from traditional intensity-weighted parameters, effectively revealing the differential reactivity. The findings of this study have provided a strategy for improved data interpretation of DOM molecular transformations, complementing traditional intensity-weighted molecular parameters and enabling a more comprehensive molecular-level characterization.