DETERMINING N-15 IN AMMONIUM BY PRODUCING NITROUS-OXIDE

Standard procedures for determining N-15 in the NH4+ fraction of 2 M K Cl soil extracts convert NH4+ to N-2 prior to isotope-ratio mass spect rometry. We developed a new gas-phase method for determining N-15 in N H4+ by producing N2O. Producing N2O means that atmospheric contaminati on by Nr at natural abundance is not a problem. The method is more sen sitive than other methods at lower enrichments, and as accurate and pr ecise as other methods. Nitrous oxide is produced as a by-product duri ng the oxidation of NH4+ by alkaline OBr-. The reaction is catalyzed b y Cu2+ and is pH-dependent. The best practical conditions for producin g N2O were when the NH3-absorbing solution contained 0.005 M H2SO4 wit h 0.5 mM CuSO4, and when the molarity of the NaOH in the NaOBr was 10 M. Background N (0.20 mu mol test(-1)) significantiy diluted the theor etical enrichments for amounts of N < 10 mu mol test(-1). Corrected en richment values were accurate for amounts of NH4+-N greater than or eq ual to 1 mu mol test(-1). For amounts of NH4+-N greater than or equal to 1 mu mol test(-1), precision was determined with a CV of 5.6% at 0. 10 atom % excess and with a CV of 0.3% at 10.00 atom % excess, The met hod is suitable for any matrix and has the potential for better perfor mance if the background contamination could be lowered.