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Diffusion Methods for Automated Nitrogen‐15 Analysis using Acidified Disks
Author(s) -
Khan S. A.,
Mulvaney R. L.,
Brooks P. D.
Publication year - 1998
Publication title -
soil science society of america journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.836
H-Index - 168
eISSN - 1435-0661
pISSN - 0361-5995
DOI - 10.2136/sssaj1998.03615995006200020017x
Subject(s) - diffusion , chemistry , analytical chemistry (journal) , stable isotope ratio , nitrogen , isotope dilution , filter paper , isotope , dilution , mass spectrometry , chromatography , thermodynamics , physics , organic chemistry , quantum mechanics
Diffusion methods are often employed to speciate inorganic N in soil extracts or water for N‐isotope analysis with automated mass spectrometers. Diffusion is usually carried out in a plastic specimen container with an acidified filter disk to collect the diffused NH 3 . Owing to the small size of the container and the limited surface area of the acidified disk, a diffusion period of 6 to 14 d is generally required, and even then recovery may be incomplete. Simple diffusion methods were developed that use two acidified disks in a 473‐mL (1‐pint) wide‐mouth Mason jar. Diffusions with MgO to recover NH 4 ‐N, or with MgO plus Devarda's alloy to recover (NH 4 + NO 3 )‐N or (NH 4 + NO 3 + NO 2 )‐N, were performed from 5 to 100 mL of either 2 or 4 M KCl. Quantitative recovery of 150 µg of N was achieved in 1 to 7 d at 20 to 25°C, and in 2 to 14 h by heating on a hot plate at 45 to 50°C. Isotopic analyses of labeled soil extracts were accurate to within 5%, as determined by isotope‐dilution calculations. Incomplete diffusion led to <2% error in analysis of NH 4 ‐N, whereas serious error occurred in analysis of (NH 4 + 15 NO 3 )‐N.