Diffusion methods for automated nitrogen-15 analysis using acidified disks

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 mag be incomplete. Simple diffusion methods were developed that use two acidified disks in a 473-mL (1-pint) wide-month Mason jar. Diffusions with MgO to recover N-4-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 recavery of 150 mu g of N was achieved in I to 7 d at 20 to 25 degrees C, and in 2 to 14 h by heating on a hot plate at 45 to 50 degrees C. Isotopic analyses of labeled soil extracts were accurate to within 5%, as determined by isotope-dilution calculations, Incomplete diffusion fed to <2% error in analysis of NH4-N, whereas serious error occurred in analysis of(NH4 + (NO3)-N-15)-N.