On-line technique to determine the isotopic composition of total dissolved nitrogen

A new on-line analytical setup for [15.sub.N] measurements of total dissolved nitrogen (TDN) has been developed through the coupling of a high-temperature catalytic (Ce(IV)[O.sub.2]) oxidation furnace, a Cu reduction furnace, and an isotope ratio mass spectrometer. The detection limit for accurate [sigma] [sub.15]N measurements is 20 mg of N [L.sup.-1]. For N-containing compounds dissolved in water, a standard deviation on N concentration measurements of 0.2 mg of N [L.sup.-1], independent of N concentration, has been found. Reproducibility on [sigma] [sup.15]N increased with increasing N concentration, with standard deviations varying from 0.8 to 0.1%o in the range of 20-100 mg of N [L.sup.-1]. Salt matrixes, in which the N species might be dissolved, could influence the analysis capacity and continuity, mainly at concentrations above 0.1 M. To our knowledge, this system is the first successful on-line setup capable of performing routine [sigma] [sup.15]N and N concentration measurements of the TDN pool. Potential applications are large and are believed to result in an increased insight in N cycling and dissolved organic nitrogen behavior in terrestrial and aquatic ecosystems.