IMPROVED DIFFUSION TECHNIQUE FOR N-15-N-14 ANALYSIS OF AMMONIUM AND NITRATE FROM AQUEOUS SAMPLES BY STABLE-ISOTOPE SPECTROMETRY

Nitrogen (N) isotope ratio mass spectrometry (IRMS) by Dumas combustio n and continuous flow mass spectrometry has become a wide-spread tool for the studies of N turnover. The speed and labor efficiency of N-15 determinations from aqueous solutions such as soil solutions or soil e xtracts are often limited by sample preparation. Several procedures fo r the conversion of dissolved ammonium (NH4+) or nitrate NO3- to gaseo us ammonia and its subsequent trapping in acidified traps have been el aborated in the last decades. They are based on the use of acidified f ilters kept either above the respective solution or in floating PTFE e nvelopes. in this paper, we present an improved diffusion method with a fixed PTFE trap. The diffusion containers are continuously kept in a vertical rotary shaker. Quantitative diffusion can thus be achieved i n only three days. For solutions with NH4+ levels of only 1 mg N kg(-1 ) and NO3- concentrations of 12 mg N kg(-1), recovery rates of 98.8-10 2% were obtained. By addition of N-15 labeled and non-labeled NH4+ and NO3- it was shown that no cross-contamination from NH4+ to NO3- or vi ce versa takes place even when one form is labeled to more than 1 at % N-15 while the other form has natural N-15 content. The method require s no intermediate step of ammonia volatilization before NO3- conversio n.