Dear Gosh

We are using a microdiffusion method to measure N-15 from NH4+ and NO3- in
soil extracts (KCl) or soil solutions (porous caps).
In short:
add MgO to solution;  trap the NH3 on a small acidified filter disk; dry
disk and combust it an elemental analyzer connected to a mass spec (N15 from
NH4+); add Devarda alloy to remaining solution (reduce NO3- to NH4+); trap
NH3 as before; dry and measure disk (N15 from NO3-).

some references:

P.D.Brooks,J.M.Stark,B.B.McInteer,T.Preston; Soil Sci. Am. J. 53:1707-1711
(1989)
Diffusion Method To prepare Soil Extracts For Automated Nitrogen-15 Analysis

N.S.Saghir, F.P.Mungwari, R.L.Mulvaney, F.Azam; Comm. Soil Sci. Plant Anal.,
24, 2747-2763 (1993)
Determination of Nitrogen by Microdiffusion in Mason Jars

P.Sorensen, E.S.Jensen; Analytica Chimica Acta, 252:201-203 (1991)
Sequential diffusion of ammonia and nitrate from soil extracts to a PTFE
trap for N-15 determination

Hope it helps

Leo Mayr
FAO/IAEA Agriculture and Biotechnology Laboratory
Soil Science Unit
A-2444 Seibersdorf, Austria
Tel. (+43 1) 2600-28305
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