Validation of a dynamic ammonium extraction technique for the determination of 15N at enriched abundances

A diffusion method for extracting ammonium from marine, estuarine and fresh waters for 15N/14N isotopic ratio determinations at enriched level was developed and validated. The method is based on the conversion of NH4+ to NH3 gas under alkaline conditions, diffusion of NH3 out of the solution to the headspace, NH3 trapping on an acidified GF/D glass fiber filter, and subsequent 15N/14N isotope ratio determination with mass spectrometry. The diffusion period necessary to extract sufficient N in order to accurately measure the atom% 15N was reduced to less than 15 h by bubbling the sample with a carrier gas (Air) at room temperature. The technique uses 250 mL sample volume and enables accurate atom% 15N measurements in NH4+ pools as small as 1.25 μM. A standard operating procedure for ammonium extraction is given involving method performance criteria, such as accuracy, precision, detection limit, quantification limit and robustness. The efficiency of the NH4+ extraction ranged from 40 to 100%. The quantification limit of the method was estimated at around 0.26 μmol% N, for an initial 15N abundance of ∼1%. The within-laboratory reproducibility amounted to 0.03 atom% 15N. It was shown that the recovery rate obtained after extraction of the certified reference material (CRM: IAEA-311) solution falls within the 95% confidence interval of the certified values. By applying the developed method to fortified natural water samples of different conductivities, the atom% 15N determinations were precise and accurate for α = 1-5%