ABSTRACT

A simple and rapid method was developed for the δ¹⁵N determination of ammonium and nitrate in water samples. This method is based on an on-line isotope analysis using an elemental analyzer coupled to a mass spectrometer. The ammonium and nitrate in sample solutions are separated as ammonium into diluted H₂SO₄ from each other by sequential distillation techniques. Then, the ammonium in the distillate is precipitated directly as an insoluble salt of (C₆H₅)₄BNH₄, which is subsequently combusted in an elemental analyzer for δ¹⁵N determination. This precipitation step allows us to eliminate the evaporation step used for sample preparation in the previous on-line methods, which contributes to simple and rapid determination. The δ¹⁵N values of precipitates did not depend on the fraction (F) of NH₄⁺ precipitated in the investigated range of F = 0.64–0.96, showing insignificant isotope fractionation during precipitation. Analyses of the solutions prepared from Reference Materials, (NH₄)₂SO₄ and KNO₃ distributed by the National Institute of Standards and Technology (NIST) and also the rain samples showed that this method can produce good quality isotope measurements.