The analysis of ¹⁵N in aqueous samples requires the concentration of dissolved nitrogen (N) into a small volume that can be analysed by mass spectrometry. This is conveniently achieved by the NH₃ diffusion technique, where NH₄⁺ is captured on small acidified filters enclosed in PTFE. NO₃^- can be analysed the same way by reducing it to NH₄+ with Devarda's alloy. H₂SO₄ is commonly used for the acidification of the filters. During combustion, however, this acid leads to the production of SO₂ and elemental sulphur, which both have detrimental effects on the mass spectrometer. We propose here to replace H₂SO₄ with citric acid because it is combusted completely to CO₂ and H₂O in the elemental analyser before entering the mass spectrometer. Citric acid was found to give comparable results in terms of N recovery and ¹⁵N values, both for NH₄⁺ and for NO₃^- samples. Blank samples revealed that N contamination was slightly lower using citric instead of sulphuric acid as acidifier of the glass filters. NH₄⁺ samples first concentrated over cation-exchange columns were strongly acidic and several methods were tested to raise the pH for the subsequent diffusion. These samples gave incomplete N recoveries, but this problem was independent of the acid used on the filters and of the final pH of the sample. Complete recovery was achieved only by increasing the volume of the eluate from the columns. Citric acid can thus generally be recommended instead of H₂SO₄ for ammonia diffusion.