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Dep. of Microbial Ecology, Inst. of Biological Sciences, Univ. of Aarhus, Ny Munkegade, Bd. 540, DK-8000 Aarhus C, Denmark
National Environment Research Inst., Dep. of Freshwater Ecology, Vejlsøvej 25, DK-8600 Silkeborg, Denmark
*Corresponding author (nilsr{at}pop.bio.aau.dk).
ABSTRACT
A limiting factor for work with biological N transformations has been unsatisfactory procedures for analysis of 15N isotope labeling in small amounts of NH+4. Using the procedure described here, it is now possible to perform accurate determinations in NH+4 pools as small as 2 nmol. By diffusion of NH3 through a gas phase, N in NH+4 is transferred from the sample and into a hypobromite solution, where an oxidation to N2 occurs. The N2 originating from the oxidation is mixed with N2 initially dissolved in the sample, which functions as a carrier during subsequent mass spectrometry. Random isotopic pairing of the N atoms originating from NH3 oxidation makes it possible to calculate the original 15N abundance in the NH+4 pool from the relative amounts of 14N15N and 15N15N in the reaction container. The combined microdiffusion-hypobromite oxidation method for 15NH+4 analysis proved to be very precise (standard error <0.15 atom %, n = 5) when applied on samples with an 15N enrichment >1 atom %, and interference was found only for volatile methyl amines.
Supported by the Danish National Science Research Council, The Danish Research Academy, and The Danish Center for Microbial Ecology. We thank Preben Sørensen for skillful technical assistance and Anne Haxen for linguistic corrections.
Received for publication April 20, 1994.
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