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.