Volatile fatty acids (VFA) such as formate, acetate, propionate and butyrate represent important intermediates in the anaerobic degradation of organic matter by microorganisms (Capone and Kiene, 1988). Knowledge on the concentrations and fluxes of these substrates, which are both end products and energy substrates of microbial metabolism is a key factor to determining energetic limits of sub-surface life and constraining the spatial extent of the so-called deep biosphere (Hoehler, 2007). Over the past 3 decades, numerous studies have quantified VFAs in marine pore water (e.g. Sansone and Martens, 1982; Shaw et al., 1984). The main problem hampering these analyses until today is that VFAs are only present in minor concentrations within a matrix containing large amounts of inorganic anions (mainly chloride). These inorganic anions interfere with analytical methods to determine VFAs and result in low detection sensitivity. Several attempts have been made to overcome this problem by separating VFAs from the background matrix, e.g. by distillation procedures (Parkes and Taylor, 1983) or derivatization techniques (Albert and Martens, 1997). These methods were able to lower the detection limits. However, they require time-consuming sample pre-treatment that substantially lower the sample throughput. As a result, a number of important questions, such as the potential occurrence of different pools of acetate of different bio-availability in sediments (Parkes et al., 1984) have never been clearly answered. We now use a novel combination of 2-dimensional ion chromatography (ICS 3000, Thermo Scientific) with mass spectrometry (MSQ PLUS, Thermo Scientific) (2D IC-MS) that enables the qualification and quantification of several VFAs directly within marine pore water samples without sample pre-treatment. Hereby the 1st chromatographic dimension is used to separate the organic compounds from inorganic background ions (mainly chloride). A window in the retention time of the bulk organic acids is cut-out of the 1st dimension and trapped onto a 2nd column. This column is used to separate the VFAs by use of a different eluent concentration. The separation of ions on the individual column is monitored by a conductivity detector for each column. Quantification of VFAs is then achieved by a mass spectrometer coupled to the second-dimension-column using individual single ion monitoring (SIM) channels, to achieve maximal sensitivity. Remains of inorganic ions on the 2nd dimension are excluded from the mass spectrometer. Detection limits are generally in the lower ppb range depending on individual sample salinity. For example, in marine pore waters with up to 35 g/L salt the detection limit for acetate is around 1 µM. Analysis time is about 36 min per sample resulting in a sample throughput of more than 35 samples per day. In a first case study, we applied our novel procedure to pore water samples obtained from surface and sub-surface sediments of Aarhus Bay (Denmark). REFERENCES Albert, D.B., Martens, C.S., 1997. Determination of low-molecular-weight organic acid concentrations in seawater and pore-water samples via HPLC. Marine Chemistry 56, 27-37. Capone, D.G., Kiene, R.P., 1988. Comparison of microbial dynamics in marine and freshwater sediments: Contrasts in anaerobic carbon catabolism. Limnology and Oceanography, 725-749. Hoehler, T.M., 2007. An energy balance concept for habitability. Astrobiology 7, 824-838. Parkes, R., Taylor, J., 1983. Analysis of volatile fatty acids by ion-exclusion chromatography, with special reference to marine pore water. Marine Biology 77, 113-118. Parkes, R., Taylor, J., Joerck-Ramberg, D., 1984. Demonstration, using Desulfobacter sp., of two pools of acetate with different biological availabilities in marine pore water. Marine Biology 83, 271-276. Sansone, F.J., Martens, C.S., 1982. Volatile fatty acid cycling in organic-rich marine sediments. Geochimica et Cosmochimica Acta 46, 1575-1589. Shaw, D.G., Alperin, M.J., Reeburgh, W.S., McIntosh, D.J., 1984. Biogeochemistry of acetate in anoxic sediments of Skan Bay, Alaska. Geochimica et Cosmochimica Acta 48, 1819-1825.
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International Meeting on Organic Geochemistry, 2013