Apollo 2

The ESI source (Apollo II, Bruker Daltonics) was used in both negative- and positive-ion modes. The methanolic solutions were infused with a flow rate of 120 µL/h. The drying gas temperature and the flow rate were kept at 180 °C and 4 L/min, respectively, and the pressure of the nebulizer gas was 2.2 bar. In negative detection mode, the capillary voltage was set at 4 kV and while, in positive-ion mode, it was maintained at 3.6 kV. The negative-ion mass spectrum ranges from m/z 147 to 2000 and results from the accumulation of 5000 scans. For positive-ion mode experiment, 3000 scans were accumulated and the mass spectrum ranges from m/z 92 to 1500.

1D NMR and 2D NMR spectra were recorded on a Bruker DRX-400 instrument. HRESIMS was carried out on Bruker Daltonics Apex ultra 7.0 T Fourier transform mass spectrometer with an electrospray ionization source (Apollo II, Bruker Daltonics, Bremen, Germany). Optical rotations were measured with a P-1020 digital polarimeter (JASCO Corporation, Tokyo, Japan). CD spectra were measured on a J-715 spectropolarimeter (JASCO Corporation). The UV spectra were recorded on a UV-1800 spectrophotometer (Shimadzu, Japan). Thin-layer chromatography (TLC) plates (5 × 10 cm) were performed on GF254 (Branch of Qingdao Marine Chemical Co. Ltd., Qingdao, China) plates. For column chromatography (CC), RP-C18 (ODS-A, 50 µm, YMC, Kyoto, Japan), silica gel (200–300 mesh, 300–400 mesh, Branch of Qingdao Marine Chemical Co. Ltd., Qingdao, China), and Sephadex LH-20 (GE Healthcare Bio-Science AB, Pittsburgh, PA, USA) were used. The high performance liquid chromatography (HPLC) analysis was performed on a Waters 2695–2998 system (Waters, Milford, CT, USA). Semi-preparative HPLC was run with a P3000 pump (CXTH, Beijing, China) and a UV3000 ultraviolet-visible detector (CXTH, Beijing, China), using a preparative RP-C18 column (5 µm, 20 × 250 mm, YMC, Kyoto, Japan).

Mass spectra were obtained
with an FT-ICR mass spectrometer equipped with a dynamically harmonized
analyzer cell (solariX XR, Bruker Daltonics, Billerica, U.S.A.) and
a 12 T refrigerated, actively shielded superconducting magnet (Bruker
Biospin, Wissembourg, France). The data were acquired in negative
ion mode with an ESI source (Apollo II, Bruker Daltonics, Billerica,
U.S.A., capillary voltage: 4.3 kV) in full profile magnitude mode
with a transient size of 4 MWord (∼1.6 s free induction decay,
FID). The ion accumulation time (IAT) was set to 1.6 s, and the mass
range was set to m/z 147–1000.
The mass resolving power (m/Δm, full width half-maximum) at m/z 400 was approximately 500,000 ±
40,000, which is sufficient to resolve all major DOM ions in the considered
mass range. SRFA spiked with model compounds was measured with 0.5
s IAT (cf. SI Text: Instrument Quality Control).
As reference to state-of-the art analysis, the PPL extracted
seawater sample (AO_high^SPE) was diluted to 0.8
mmol DOC L^–1 (10 mg DOC L^–1) in
ultrapure water and MeOH (50/50, v/v) and measured with the standard
direct infusion (DI-) FT-ICR MS method (256 scans, 4 MWord, 8 ms IAT,
ESI(−), 4 μL min^–1).