Mass difference matching unfolds hidden molecular structures of dissolved organic matter

Abstract

Ultrahigh-resolution Fourier transform mass spectrometry (FTMS) has revealed unprecedented detail of natural complex mixtures such as dissolved organic matter (DOM) on a molecular formula level, but we lack approaches to access the underlying structural complexity. We here explore the hypothesis that every DOM precursor ion is potentially linked with all emerging product ions in FTMS2 experiments. The resulting mass difference (Δm) matrix is deconvoluted to isolate individual precursor ion Δm profiles and matched with structural information, which was derived from 42 Δm features from 14 in-house reference compounds and a global set of 11477 Δm features with assigned structure specificities, using a dataset of ~18000 unique structures. We show that Δm matching is highly sensitive in predicting potential precursor ion identities in terms of molecular and structural composition. Additionally, the approach identified unresolved precursor ions and missing elements in molecular formula annotation (P, Cl, F). Our study provides first results how Δm matching refines structural annotations in Van Krevelen space, but simultaneously demonstrates the wide overlap between potential structural classes. We show that this effect is likely driven by chemodiversity and offers an explanation for the observed ubiquitous presence of molecules in the center of the Van Krevelen space. Our promising first results suggest that Δm matching can both unfold the structural information encrypted in DOM and assess the quality of FTMS-derived molecular formulas of complex mixtures in general.

Deciphering the Energy Use Channels in Soil Organic Matter: Impacts of Long-term Farmyard Manure Addition and Microbial Necromass Revealed by LC-FT-ICR-MS

“…To structurally confine these novel necromass markers, FT-ICR-MS fragmentation experiments could be conducted. Tandem MS can help to elucidate structural motifs within SOM (Simon et al, 2022;DiDonato et al, 2023). This could enable not only the detection of shifts in the molecular composition of SOM during substrate decomposition but also the recognition of alterations in structural motifs associated with specific necromass types (Salas et al, 2024).…”