Understanding the elemental and structural composition of mercury-dissolved organic matter (Hg-DOM) complexes is crucial for comprehending Hg speciation, bioavailability, and transformations in aquatic ecosystems. However, low concentrations of these organo-metal complexes in natural waters and extraction at acidic pH constrain their characterization. Here, we used solid phase extraction (SPE) methodology to extract Hg-DOM complexes at ambient pH and validated their preconcentration by preserving the composition for identification using ultra-high-resolution Fourier transform ion cyclotron resonance mass spectrometry (FT-ICR-MS). While the dissolved organic carbon (DOC) extraction efficiency was higher with cartridges containing styrene divinylbenzene copolymer (PPL) than silica structure bonded with hydrocarbon chains (C18), Hg in both extracts showed no significant difference. FT-ICR-MS analysis revealed that Hg-DOM complexes extracted by C18 cartridges were aliphatic with smaller carbon chains (16–18), whereas complexes extracted with PPL exhibited both aliphatic and aromatic characteristics with a wide distribution of carbon chains ranging from 17 to 25. The C18 cartridge appeared to be selective in extracting and preserving the nonpolar complexes, as evidenced by the identification of two molecular formulae, C16H31HgNO3 and C16H35HgNO2S, with m/z ratios of 487.2 and 507.21, across triplicate extractions. This study addresses the challenge of the spectroscopic limitation of Hg-DOM identification by extracting these complexes at circumneutral pH and presumably preserving them from dissociation during extraction. Graphical abstract

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