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Molecular characterization of sea-surface microlayers in the Adriatic using Fourier Transform Ion Cyclotron Resonance Mass Spectrometry

Novel Aspect: Molecular composition of marine surface microlayers (films) can be resolved with high resolution mass spectrometry and linked to environmental processes. Introduction Surface microlayers represent the boundary interface between ocean and atmosphere and have intensely been studied. During calm weather conditions the lipophilic surface films reduce and control the air-sea gas exchange. Despite decades of research the surface microlayers are almost uncharacterized on the molecular level. However, the chemical composition is an important information and urgently needed to be able to explain, e.g., formation mechanisms. Fourier Transform Ion Cyclotron Resonance Mass Spectrometry (FT-ICR-MS) has been used in recent studies for the characterization of highly complex dissolved organic matter (DOM). In our study, we apply this technique to obtain insights in the molecular differences between dissolved organic matter in the water column and organic matter which aggregates and forms surface microlayers. Methods All samples were derived from an estuary in the Adriatic Sea near the Croatian City of Sibenik. One liter of surface microlayer was collected using a stainless steel mesh. The surface microlayer attached to the pores of the mesh and was transferred into a glass bottle. The sample from the water column (4 L) was taken with a Niskin sampler. All samples were GF/F filtered and acidified to pH 3 prior to extraction by solid phase extraction using a styrene divinyl benzene polymer type sorbents (PPL, Varian). DOM samples were analyzed by electrospray ionization in negative ion mode using the new solariX 12T FTMS platform (Bruker Daltonics). Sulphur compounds were verified by the isotope signals of 13C and 34S. Preliminary results FT-ICR-MS analyses resulted in over 2500 assigned molecular formulas for each of the two samples in the mass range up to m/z 600. Weighted average molecular element ratios of O/C and H/C were calculated from the peak intensity in the FT-ICR-MS data and visualized in van Krevelen plots. The weighted average O/C was substantially lower and H/C much higher in the surface microlayer than in the underlying water column, reflecting the lipophilic character of the surface microlayer. This is also shown in the low number of double-bond equivalents (DBE), i.e. the sum of rings and double bonds in a molecule. Apart from various compounds with up to 2 nitrogen atoms, the surface film showed especially abundant signals for organic sulphur compounds which are also characterized by high O/C and low H/C ratios. The origin of these sulphur compounds is not yet clear. They might either be derived by allochthonous input or contamination or by metabolic processes. FT-ICR-MS delivered first extensive molecular insights in the molecular composition of the marine surface microlayer in which 367 compounds uniquely occurred with clear lipophilic characteristics. Chromatographic separation and fragmentation experiments in the ICR cell will allow further insights into the structural features of these compounds.

sea surface microlayer; FT-ICR-MS; dissolver organic matter

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