Online δ¹³C analysis of volatile fatty acids in sediment/porewater systems by liquid chromatography-isotope ratio mass spectrometry

Abstract

Volatile fatty acids (VFAs) are central intermediates of anaerobic metabolism and present in aquatic environments such as marine sediments. Conceptually, the isotopic composition of volatile fatty acids is presumed to be sensitive to the specific biogeochemical processes involved in their production and consumption. However, due to generally low environmental concentrations, our knowledge on isotopic variability of VFAs is limited. We report the development and application of a new protocol for compound-specific carbon isotopic analysis of VFAs in marine porewaters and other aqueous liquids. This new protocol involves reversed-phase separation of volatile fatty acids with an aqueous mobile phase by high performance liquid chromatography (HPLC) combined with chemical oxidation of the effluents by the Finnigan TM LC IsoLink interface (Krummen et al. 2004) and subsequent online transfer of the resulting CO 2 into an isotope ratio mass spectrometer. We obtained reproducible and accurate results for pure Na-acetate dissolved in artificial seawater at concentrations as low as 2 μM, whereas minimum concentrations in marine porewaters were 10 μM. Our technique extends the previously accessible concentration range and the fully automated online operation allows for systematic analysis of large sample sets. These technical improvements make carbon isotopic analysis of selected VFAs a realistic perspective for many sedimentary environments. Our initial survey of acetate in porewaters and fluids obtained from incubations of marine sediments has revealed an unexpectedly large range of isotopic compositions from -5‰ to -85‰ and provides strong support for process-specific information encoded in the isotopic compositions of VFAs.

Carbon flow from volcanic CO2 into soil microbial communities of a wetland mofette

“…TOC and its stable carbon isotopic composition (d 13 C) was analyzed by continuous-flow elemental analyzer-isotope ratio mass spectrometer (EA-IRMS) as described previously . Pore-water acetate d 13 C was analyzed by isotoperatio-monitoring liquid chromatography/mass spectrometry as described by Heuer et al (2006Heuer et al ( , 2009. Isotope-ratio-monitoring gas chromatography/mass spectrometry was used to determine the d 13 C of pore-water CH 4 as described previously (Ertefai et al, 2010;Hädrich et al, 2012).…”

Archaea in metazoan diets: implications for food webs and biogeochemical cycling

“…Furthermore, a pool of available acetate occurs within the sediment and can also be quite 13 C depleted. Few seep samples have had d 13 C acetate analyzed, but within Black Sea seep sediments this source of acetate was found to be as negative as d 13 C acetate ¼ À85% (Heuer et al, 2006). Thus, the acetate used by Dorvillea sp.…”

Atomic spectrometry update. Atomic mass spectrometry

“…A new method for the carbon CSIA of volatile fatty acids combined aqueous mobile phase RP-HPLC separation, chemical oxidation and on-line SIRMS 222 . The low concentrations of these compounds had made large scale analysis difficult but the high sensitivity of the new method improved the LOD to the previously inaccessible 2 µM level for sodium acetate dissolved in artificial seawater and 10 µM in marine prewaters.…”