Use of Diffusion to Determine Inorganic Nitrogen in a Complex Organic Matrix

Abstract

Serious interference can arise in carrying out quantitative or isotopic analyses of inorganic N in samples having a high content of labile organic N. Studies to evaluate existing Mason jar diffusion methods for interference by a high concentration of various amino acids and amino sugars showed the need to minimize the diffusion period. This was accomplished by increasing the volume of H 3 BO 3 ‐indicator solution (to 7 mL), and by carrying out diffusions from a fixed volume (10 mL) with a high salt concentration (4 M KCl). Quantitative recovery (≥98%) of 4 mg of NH 4 ‐N was achieved in the presence of 10 mg of organic N, either in 18 h at 25°C, or in 1.5 h by heating on a hot plate at 45 to 50°C. Under the same conditions, complete recovery of NO 3 ‐N and NO 2 ‐N was achieved in the presence of 4 mg of organic N by increasing the amount of Devarda's alloy from 0.3 to 1.5 g. The modified diffusion methods were much more accurate and precise than conventional steam distillation techniques for inorganic N analysis of animal waste or other complex samples treated with labeled NH 4 ‐N, NO 3 ‐N, or NO 2 ‐N. Recovery of the labeled N by diffusion ranged from 96 to 107%, and the coefficient of variation was usually <2%. The isotopic analyses from these diffusions were usually accurate to within 4%, as determined by isotope‐dilution calculations. The latter finding provides ample evidence that no interference arises from labile organic N.