On-Line Technique To Determine the Isotopic Composition of Total Dissolved Nitrogen

A new on-line analytical setup for 15N measurements of total dissolved nitrogen (TDN) has been developed through the coupling of a high-temperature catalytic (Ce(IV)O2) oxidation furnace, a Cu reduction furnace, and an isotope ratio mass spectrometer. The detection limit for accurate δ15N measurements is 20 mg of N L-1. For N-containing compounds dissolved in water, a standard deviation on N concentration measurements of 0.2 mg of N L-1, independent of N concentration, has been found. Reproducibility on δ15N increased with increasing N concentration, with standard deviations varying from 0.8 to 0.1‰ in the range of 20−100 mg of N L-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 δ15N and N concentration measurements of the TDN pool. Potential applications are large and are believed to result in an increase...