N. Risgaardpetersen et al., COMBINED MICRODIFFUSION-HYPOBROMITE OXIDATION METHOD FOR DETERMINING N-15 ISOTOPE IN AMMONIUM, Soil Science Society of America journal, 59(4), 1995, pp. 1077-1080
A limiting factor for work with biological N transformations has been
unsatisfactory procedures for analysis of N-15 isotope labeling in sma
ll amounts of NH4+. Using the procedure described here, it is now poss
ible to perform accurate determinations in NH4+ pools as small as 2 nm
ol, By diffusion of NH3 through a gas phase, N in NH4+ is transferred
from the sample and into a hypobromite solution, where an oxidation to
N-2 occurs, The N-2 originating from the oxidation is mixed with N-2
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 orig
inal N-15 abundance in the NH4+ pool from the relative amounts of (NN)
-N-14-N-15 and (NN)-N-15-N-15 in the reaction container, The combined
microdiffusion-hypobromite oxidation method for (NH4+)-N-15 analysis p
roved to be very precise (standard error <0.15 atom %, n = 5) when app
lied on samples with an N-15 enrichment >1 atom %, and interference wa
s found only for volatile methyl amines.