Abstract: When the δ¹⁵N-NH₄⁺ in water samples is pretreated by using the ion exchange and diffusion method with NH₄Cl and KCl, the key problem is how to reduce the blank, avoid isotopic fractionation and yield accurate isotope results. In this study, a closed ion exchange system was set up to prevent the small amount of NH₄⁺ contamination to the water samples. Furthermore, the sample and elution flow rates of 1.2 L/h and 0.2 L/h respectively were achieved by adjusting the speed of a peristaltic pump. Thus, both the experiment time and the contamination were reduced. In addition, all of the NH₄⁺ in the sample is absorbed on the ion exchange resin and the N recoveries were from 93.5% to 102.8%. Therefore, the whole process will not cause isotopic fractionation. The blank introduced by KCl was reduced by being heated at 450℃ for 24 h. As a result, the blank ion exchange was reduced to below the detection limit of 0.02 mg/L by using the closed ion exchange system. The problem of contamination for water sample pretreatment in air was solved. The process was accelerated and efficiency was improved