Abstract: This paper presents a simple and rapid method to concentrate ammonium and nitrate from freshwater samples for an on-line isotope analysis using an elemental analyzer coupled to a mass spectrometer. Ammonium and nitrate are collected by passing the water-sample through pre-filled, disposable, ion exchanging resin columns. The ammonium and nitrate in sample solutions are separated as ammonium into diluted H₂SO₄ by sequential distillation techniques. Then, the excrescent SO₄^2- in the distillate is precipitated directly as an insoluble salt of BaSO₄. (NH₄)₂SO₄ is collected by freezing-drying filtrate, which is subsequently combusted in an elemental analyzer for δ¹⁵N determination. The sorption efficiency of 001×7 cationic resin and 732 cationic resin for 2 L sample solutions with ammonium and nitrate concentration of 0.5, 1.0, 5.0 mg/L is >95.1%, elution recovery is >95%, showing insignificant isotope fractionation during precipitation. Analyses of the solutions prepared from (NH₄)₂SO₄ and KNO₃ distributed, and also the rain, river, lake and well samples showed that the method can produce good quality of isotope measurements.