The unique chromatographic behaviour of DOM was investigated on three exemplary water samples representing coastal DOM, oceanic surface DOM and oceanic refractory DOM. Weddell Sea surface (30 m depth, oceanic surface DOM) and deep water (1356 m depth, refractory DOM) was sampled with a rosette sampler on RV Polarstern during ANT XXII/2 (station PS67/006-130, latitude -67.5633, longitude -55.3448) and are described elsewhere (El Naggar et al., 2007; Koch et al., 2008). 160 L sea water was filtered with 0.2 µm filter cartridges, acidified to pH 2 and pumped through 60 mL solid phase extraction cartridges (PPL, 5 g). DOM was eluted with 40 mL MeOH and stored at -18 °C. Coastal DOM is routinely extracted from southern North Sea (latitude 54.1447, longitude 7.8711) and used as an in-house laboratory standard. Sea water was filtered over 0.2 µm PTFE (Whatman), acidified to pH 2 and extracted with PPL cartridges. After elution with methanol, extracts are stored at -18 °C until measurement to minimize esterification (Flerus et al., 2011). The molecular composition was obtained by two mass spectrometric platforms with negative electrospray ionisation: 1) Fourier Transform Orbitrap mass spectrometer (FT-Orbitrap-MS; Q-Exactive Plus, Thermo Fisher Scientific, Bremen, Germany) coupled to ultra-high performance liquid chromatography (UPLC, Vanquish, Thermo Fisher Scientific, Bremen, Germany); 2) Fourier-transform ion cyclotron resonance mass spectrometry (FT-ICR-MS; 7 Tesla scimaX MRMS system, Bruker Daltonics GmbH & Co. KG, Bremen, Germany) coupled to UPLC (Elute LC, Bruker Daltonics GmbH & Co. KG, Bremen, Germany). Reversed phase chromatography was done with a C18 column (Waters AQUITY 2 x 100 mm, 1.7 µm) column at 0.3 mL min 1 and a linear gradient: A (ultrapure water, 4 mmol L 1 ammonium formate) 2 min: 99 %, 11 min: 0 %, 14.9 min: 99 %; B (MeOH, 4 mmol L 1 ammonium formate) 2 min: 1 %, 11 min: 100 %, 14.5 min 100 %, 14.9 min 1 %.