A method for the extraction of ammonium from freshwaters for nitrogen isotope analysis

The measurement of delta N-15 values of inorganic nitrogen species is an im portant analytical tool to trace nitrogen species in order to understand ni trogen cycling in aquatic systems. Nitrogen isotope analysis of freshwater ammonium has, however, been hindered by the lack of a simple and reliable t echnique to measure delta N-15 values at natural abundance levels. We prese nt a simple and rapid method to concentrate ammonium from freshwater sample s for on-line N-isotope ratio determination. Ammonium is collected by adsor ption on N-free cation exchange resins. The dried N-loaded exchange resin i s then directly combusted to produce N-2 gas for subsequent delta N-15 anal ysis. The method was evaluated with simulated freshwater solutions containi ng varying amounts of standard NH4+-N (delta N-15 = 2.1 parts per thousand) and potentially interfering inorganic and organic compounds. In general, t he cation exchange resin method gives accurate and reproducible delta N-15 values (sigma (1) < 0.3 parts per thousand; n = 10). Because of adsorption interference, high concentrations of cations in solution may cause ammonium loss but do not result in measurable isotope fractionation. Replicate extr actions of the ammonium standard added to water collected from four Swiss l akes demonstrate the good performance of this method when applied to low io nic strength natural water samples with modest concentrations of dissolved organic nitrogen.