Determination of the 15N/14N ratio of ammonium and ammonia in aqueous solutions by equilibrium headspace-gas chromatography-combustion-isotope ratio mass spectrometry

Abstract

A method for determination of the ¹⁵N/¹⁴N ratio of total ammoniacal nitrogen (TAN; ammonium and ammonia) in aqueous solutions was developed, primarily intended for use with soil extracts, which have a high TAN level, e.g. from recently fertilised agricultural soils. Ammonium was converted to ammonia by addition of NaOH, followed by nitrogen isotopic analysis of the headspace by gas chromatography-combustion-isotope ratio mass spectrometry (GC-C-IRMS) where complete separation of TAN from the matrix was not necessary. The ammonia concentration in the gas phase was maximised by increasing the temperature and salt concentration and by decreasing the gas ∶ liquid ratio in the headspace vials. Isotopic equilibrium was reached after less than 1 h at 80 °C. The measured isotopic ratio was constant for solutions containing 30–200 mM NH₄-N, corresponding to 950–7000 ng NH₃-N detected with the IRMS. The integrated area response at m/z 28 increased linearly with the ammonium ion concentration in the interval 10–200 mM NH₄-N. The fractionation factor between the liquid and gas phases was 1.0054 ± 0.0007 within the linear range, which is in agreement with values reported in the literature, but with a higher precision. Changes in temperature, gas ∶ liquid ratio or salt concentration did not affect the measured ratio, demonstrating the robustness of the developed method.