MEASURING N-15-NH4-WATERS - AN ADAPTATION OF THE AMMONIA DIFFUSION METHOD FOR SAMPLES WITH LOW AMMONIUM CONCENTRATIONS( IN MARINE, ESTUARINE AND FRESH)

We present a method for measuring N-15-NH4+ in marine, estuarine and f resh waters. The advantage of this method is that it is broadly applic able to all types of water and it allows measurements in samples with lower ammonium concentrations than has previously been possible. The p rocedure is a modification of the ammonia diffusion method and uses la rge sample volumes (often 41) to obtain sufficient N for isotope ratio mass spectrometric analysis. Large volume samples have not previously been used with the diffusion procedure because isotopic fractionation occurs due to incomplete recovery of ammonium. However, the method we present accounts for this fractionation and allows precise correction of measured delta(15)N values. (C) 1998 Elsevier Science B.V.