Structural Characterization of Dissolved Organic Matter at the Chemical Formula Level Using TIMS-FT-ICR MS/MS.

TIMS-FT-ICR MS is an important alternative to study the isomeric diversity and elemental composition of complex mixtures. While the chemical structure of many compounds in the dissolved organic matter (DOM) remains largely unknown, the high structural diversity has been described at the molecular level using chemical formulas. In this study, we further push the boundaries of TIMS-FT-ICR MS by performing chemical formula-based ion mobility and tandem MS analysis for the structural characterization of DOM. The workflow described is capable to mobility select (R ∼ 100) and isolate molecular ion signals (Δm/z = 0.036) in the ICR cell, using single-shot ejections after broadband ejections and MS/MS based on sustained off-resonance irradiation collision-induced dissociation (SORI-CID). The workflow results are compared to alternative TIMS-q-FT-ICR MS/MS experiments with quadrupole isolation at nominal mass (∼1 Da). The technology is demonstrated with isomeric and isobaric mixtures (e.g., 4-methoxy-1-naphthoic acid, 2-methoxy-1-naphthoic acid, decanedioic acid) and applied to the characterization of DOM. The application of this new methodology to the analysis of a DOM is illustrated by the isolation of the molecular ion [C18H18O10-H]- in the presence of other isobars at nominal mass 393. Five IMS bands were assigned to the heterogeneous ion mobility profile of [C18H18O10-H]-, and candidate structures from the PubChem database were screened based on their ion mobility and the MS/MS matching score. This approach overcomes traditional challenges associated with the similarity of fragmentation patterns of DOM samples (e.g., common neutral losses of H2O, CO2, and CH2-H2O) by narrowing down the isomeric candidate structures using the mobility domain.