The mass spectra were obtained on a 15 T Solarix FT-ICR-MS (Bruker Daltonics, USA) equipped with an electrospray ionization source (Bruker Apollo II) applied in negative mode. DOM extracts were diluted to a final DOC concentration of 10 mg C L− 1 in a 1:1 mixture of ultrapure water and methanol (HPLC-grade, Sigma-Aldrich, USA). A total of 500 scans were accumulated per run, the mass window was set to 150–2000 Da. The spectra were calibrated with an internal calibration list using the Bruker Daltonics Data Analysis software package. The mass to charge,
SB-334867 and intensity were then exported and processed using in-house Matlab routines. Molecular formulae were assigned to peaks with a
respiration minimum signal-to-noise ratio of 4 following the rules published in Koch et al. (2007). For each molecular formula, the double bond equivalents (DBE = 1 + ½(2C–H + N + P)) as a measure for the degree of unsaturation and the Aromaticity Index (AI = (1 + C–O–S–½H) / C–O–S–N–P)) as well as the modified Aromaticity Index (AImod = (1 + C–½O–S–½H) / (C–½O–S–N–P)) to assess the presence and extent of aromatic structures (Koch and Dittmar, 2006) were calculated.
