Dried samples were accurately weighed and dissolved/diluted to 50 mg DOM L -1 with 50% methanol (LCMS grade, LiChroSolv, VWR) in ultrapure water (Milli-Q, Millipore). These solutions were infused at 10 µL min -1 into a high-resolution mass spectrometer (Q-Exactive Orbitrap, Thermo Fisher) equipped with a heated electrospray ionisation source (HESI) operating in negative ion mode, at 100 ℃ with a setting of 25 units of sheath gas and 3 kV. The S-lens was set to 60 and capillary temperature was 250 degrees ℃. The MS resolution was set to 140,000 at 200 Da, and transients were measured between m/z 150 -1000, with 369.11911 and 525.19775 set as lock masses. 500 transients were collected and averaged, and a wash solution of 50% methanol:water (LCMS grade) was infused between samples until the blank signal stabilised.