Diffusion Methods for Automated Nitrogen-15 Analysis using Acidified Disks
- S. A. Khan,
- R. L. Mulvaney* and
- P. D. Brooks
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 NH3. 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 NH4-N, or with MgO plus Devarda's alloy to recover (NH4 + NO3)-N or (NH4 + NO3 + NO2)-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 NH4-N, whereas serious error occurred in analysis of (NH4 + 15NO3)-N.
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