Novel tool for Simultaneous Carbon and Nitrogen Stable Isotope Analyses in Aqueous Samples

The quantitative and isotopic analysis of dissolved matter (e.g. dissolved organic carbon, total dissolved nitrogen, etc.) is of particular importance since this pool is a prime conduit in the cycling of N and C. Studying the two elemental pools simultaneously is of importance, as the transformation and transport processes of N and C are inextricably linked in all biologically mediated systems. Dissolved Carbon concentration and isotopic composition can now be determined routinely through coupling of high temperature combustion (HTC) systems to isotope ratio mass spectrometry (IRMS). However the analysis of δ15N of Total Dissolved Nitrogen is fraught with limitations: low concentration makes lyophilisation followed by EA/IRMS laborious and subject to contamination; wet chemical oxidation-IRMS runs the risk of incomplete conversion and cannot distinguish dissolved N2 from Total Dissolved Nitrogen. Further development of our HTC system lead to the implementation of the δ15N determination which is now coupled into a novel total organic carbon (TOC) analyzing system. An integrated, innovative purge and trap technique (peak focusing) for nitrogen with aluminosilicate adsorber and thermoelectric cooling element based system, in combination with high injection volume (up to 3 mL) significantly improves sensitivity. Down to 1 ppm and less total nitrogen can be measured with precision of ≤ 0.5‰. To decrease the background caused by physically dissolved nitrogen a new, membrane-based, degasser was designed for online separation of physically dissolved nitrogen. This novel HTC system, “iso TOC cube”, provides an innovative tool with large potential in investigation of biogeochemical carbon and nitrogen cycles.