Combined Microdiffusion-Hypobromite Oxidation Method for Determining Nitrogen-15 Isotope in Ammonium
- Nils Risgaard-Petersen ,
- Niels Peter Revsbech and
- Søren Rysgaard
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.
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