Stable Isotope Measurement of Ammonium Using HPLC-RTS (high performance liquid chromatography-retention time shift)

Abstract

Despite the usefulness of nitrogen isotope tracer experiments in nitrogen cycling studies, there are not such many measurement data mainly due to the difficulties in analytical methods. Although Gardner et al. (1996) developed a relatively simple and accurate method that can measure ammonium isotope using HPLC and used it widely in various N dynamics studies, the technique was not adopted to other laboratories. An HPLC-RTS system using updated HPLC pumps that can perform the same measurements as that of Gardner et al. (1996) was built. The result of standard sample showed linear increase of RTS with the 15 N proportions. Centroid retention times calculated with Matlab ® program enhanced the linearity of the response. In a sea water incubation experiment spiked with 15 NH 4 + , the uptake and regeneration of ammonium could be separately estimated using the temporal change of 15 N/ 14 N.

High ammonium recycling in an anthropogenically altered Yeongsan River Estuary, South Korea

“…Each filtrate (<200 µm and<0.7 µm) and unfiltered water sample was then treated according to the procedure described above. Ammonium isotope ratios and concentrations were measured by high-performance liquid chromatography (HPLC) using a cation-exchange column as described in Gardner et al (1995), but with a modified procedure using updated HPLC pumps as described in An et al (2013). Ammonium regeneration and potential uptake rates were calculated from changes in ammonium concentration and isotope ratios over time, using the Blackburn-Caperon model (Blackburn, 1979;Caperon et al, 1979).…”

“…This study aims to estimate the ammonium regeneration and potential uptake rates in the YRE by measuring changes in ammonium isotope ratios (Gardner et al, 1995;An et al, 2013) during incubation experiments. An additional goal is to improve the understanding of N recycling in the YRE, where a combination of artificial (high anthropogenic N loads, dam construction) and natural (macrotidal regime, high suspended sediment loads) perturbations co-occur, and to examine how dam construction may alter N cycling in an estuarine system.…”