engleski

Molecular-level structure analysis of highly complex aquatic natural organic matter and sea- surface microlayers using FT-ICR-MS/MS

FT-ICR-MS analyses resulted in over 2500 assigned molecular formulas for each of the samples in the mass range up to m/z 600. The intensity-weighted average molecular O/C ratio was substantially lower and H/C ratios much higher in the sea-surface microlayer than in the underlying water column, reflecting the lipophilic character of the microlayer. The apolar characteristics were also reflected by the low number of double-bond equivalents (DBE), i.e. the sum of rings and double bonds in a molecule. In addition, the surface film analysis showed compounds with up to 2 nitrogen atoms and abundant signals for organic sulphur compounds which were characterized by high O/C and low H/C ratios and which probably were derived from anthropogenic origin. FT-ICR-MS delivered first extensive molecular insights in the molecular composition of the marine surface microlayer. 367 lipophilic compounds uniquely occurred in the surface film probably reflecting the aggregation mechanism which forms the microlayer during calm sea conditions. Structural features of compounds in complex mixtures like NOM, which cannot be separated by chromatographic methods, were obtained by a fragmentation study. Fragmentation pathways of single isolated mass peaks of the aquatic Suwannee River Fulvic Acid Standard were studied for the first time using collision induced dissociation FT-ICR-MS [5]. Mainly stepwise losses of CO2 and H2O were observed for all precursor masses even if they differed in DBE. Therefore, it can be concluded that similar structures (carboxyl and hydroxyl groups) are ubiquitously present in this NOM standard. In recent studies of NOM, it has been shown that the diversity of molecular formulas in NOM covers a large proportion of the chemical feasible formulas [2]. Our new results imply that the structural diversity might be lower than expected. Coupling chromatographic separation and fragmentation experiments will allow further insights into the structural features of these compounds.

Molecular-level structure ; natural organic matter ; sea-surface microlayer

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