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.