Enhanced Speciation of Pyrogenic Organic Matter from Wildfires Enabled by 21 T FT-ICR Mass Spectrometry

Wildfires
affect soils through the formation of pyrogenic organic
matter (pyOM) (e.g., char and soot). While many studies examine the
connection between pyOM persistence and carbon (C) composition, nitrogen
(N) transformation in wildfire-impacted systems remains poorly understood.
Thermal reactions in wildfires transform biomass into a highly complex,
polyfunctional, and polydisperse organic mixture that challenges most
mass analyzers. High-field Fourier transform ion cyclotron resonance
mass spectrometry (FT-ICR MS) is the only mass analyzer that achieves
resolving powers sufficient to separate species that differ in mass
by the mass of an electron across a wide molecular weight range (m/z 150–1500). We report enhanced
speciation of organic N by positive-ion electrospray ionization (ESI)
that leverages ultrahigh resolving power (m/Δm50% = 1 800 000 at m/z 400) and mass accuracy (<10–100 ppb)
achieved by FT-ICR MS at 21 T. Isobaric overlaps, roughly the mass
of an electron (Me– =
548 μDa), are resolved across a wide molecular weight range
and are more prevalent in positive ESI than negative ESI. The custom-built
21 T FT-ICR MS instrument identifies previously unresolved mass differences
in CcHhNnOoSs formulas and assigns more than 30 000 peaks
in a pyOM sample. This is the first molecular catalogue of pyOM by
positive-ion ESI 21 T FT-ICR MS and presents a method to provide new
insight into terrestrial cycling of organic carbon and nitrogen in
wildfire impacted ecosystems.