COMBINED MICRODIFFUSION-HYPOBROMITE OXIDATION METHOD FOR DETERMINING N-15 ISOTOPE IN AMMONIUM

A limiting factor for work with biological N transformations has been unsatisfactory procedures for analysis of N-15 isotope labeling in small amounts of NH4+. Using the procedure described here, it is now possible to perform accurate determinations in NH4+ pools as small as 2 nmol, 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 original 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 proved to be very precise (standard error <0.15 atom %, n = 5) when applied on samples with an N-15 enrichment >1 atom %, and interference was found only for volatile methyl amines.