Deciphering Dissolved Organic Matter: Ionization, Dopant, and Fragmentation Insights via Fourier Transform-Ion Cyclotron Resonance Mass Spectrometry

Abstract

Fourier transform-ion cyclotron resonance mass spectrometry (FT-ICR MS) has been increasingly employed to characterize dissolved organic matter (DOM) across a range of aquatic environments highlighting the role of DOM in global carbon cycling. DOM analysis commonly utilizes electrospray ionization (ESI), while some have implemented other techniques, including dopant-assisted atmospheric pressure photoionization (APPI). We compared various extracted DOM compositions analyzed by negative ESI and positive APPI doped with both toluene and tetrahydrofuran (THF), including a fragmentation study of THF-doped riverine DOM using infrared multiple photon dissociation (IRMPD). DOM compositions followed the same trends in ESI and dopant-assisted APPI with the latter presenting saturated, less oxygenated, and more N-containing compounds than ESI. Between the APPI dopants, THF-doping yielded spectra with more aliphatic-like and N-containing compounds than toluene-doping. We further demonstrate how fragmentation of THF-doped DOM in APPI resolved subtle differences between riverine DOM that was absent from ESI. In both ionization methods, we describe a linear relationship between atomic and formulaic N-compositions from a range of DOM extracts. This study highlights that THF-doped APPI is useful for uncovering low-intensity aliphatic and peptide-like components in autochthonous DOM, which could aid environmental assessments of DOM across biolability gradients.

Limited Presence of Permafrost Dissolved Organic Matter in the Kolyma River, Siberia Revealed by Ramped Oxidation

“…FT‐ICR MS offers unmatched insight into complex mixtures of DOM because of its high mass accuracy and capability to distinguish molecular formula (Hendrickson et al., 2015; Kurek et al., 2020). In short, samples were first extracted by concentration and purification through a 100 mg PPL resin (Agilent Bond Elute) cartridge (Dittmar et al., 2008).…”

“…We paired RPO with Fourier transform ion cyclotron resonance mass spectrometry (FT‐ICR MS) because of its unparalleled insights into the molecular composition of complex mixtures such as DOM (Kujawinski, 2002; Kurek et al., 2020; Stubbins et al., 2010). Additionally, for RPO analyses, this study utilized E as opposed to temperature as it is a more objective proxy for chemical composition (Hemingway, Rothman, et al., 2017).…”

Mass difference matching unfolds hidden molecular structures of dissolved organic matter

“…Here, we only considered negative ESI mode data which is commonly employed for DOM analysis, and thus, adding positive ESI or other ionizations would extend the range of Δm features and structural classes covered and likely decrease bias. 14,16,23,84 The same applies to other fragmentation techniques than CID.…”

“…84 Taken together, our results show a wide potential diversity of N and S compounds in DOM that contradicts with earlier reports of mainly aromatic N and sulfonic S. 34,89,90 As most of these studies analyzed marine DOM, the detection of potentially more diverse sets of CHOS and CHNO precursors could relate to the terrestrial, less degraded DOM analyzed here. 16,[91][92][93] Further tests with N-and S-containing reference compounds and DOM samples are warranted to reveal the hidden diversity and identity of dissolved organic nitrogen and sulfur and confirm potential structures, e.g., by NMR.…”

“…5,6 DOM is a central intermediate of ecosystem metabolism and mirrors molecular imprints of interactions with its abiotic and biotic environment [7][8][9] , which form the basis for processes such as carbon sequestration and nutrient recycling. 10,11 Despite significant advances in ultrahigh-resolution mass spectrometry (FTMS) 2,4 and nuclear magnetic resonance spectroscopy 12 , scientists still struggle to decode this information on the molecular level [13][14][15][16][17] , and novel approaches to identify distinct structures are required to translate molecular-level information into improved process understanding.…”