Improved diffusion technique for 15N:14N analysis of ammonium and nitrate from aqueous samples by stable isotope spectrometry

Abstract

Nitrogen (N) isotope ratio mass spectrometry (IRMS) by Dumas combustion and continuous flow mass spectrometry has become a wide‐spread tool for the studies of N turnover. The speed and labor efficiency of ¹⁵N determinations from aqueous solutions such as soil solutions or soil extracts are often limited by sample preparation. Several procedures for the conversion of dissolved ammonium (NH₄ ⁺) or nitrate NO₃ ^‐ to gaseous ammonia and its subsequent trapping in acidified traps have been elaborated in the last decades. They are based on the use of acidified filters kept either above the respective solution or in floating PTFE envelopes. 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 in 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 98.8–102% were obtained. By addition of ¹⁵N labeled and non‐labeled NH₄ ⁺ and NO₃ ^‐ it was shown that no cross‐contamination from NH₄ ⁺ to NO₃ ^‐ or vice versa takes place even when one form is labeled to more than 1 at % ¹⁵N while the other form has natural ¹⁵N content. The method requires no intermediate step of ammonia volatilization before NO₃ ^‐ conversion.