Determination of 15N Abundance in Nanogram Pools of NO3- and NO2- by Denitrification Bioassay and Mass Spectrometry

Suspensions of two strains of Pseudomonas aeruginosa (ON12 and ON12-1) were used to reduce NO₃^- and NO₂^-, respectively, to N₂O. The evolved N₂O was quantified by gas chromatography with electron capture detection, and the ¹⁵N abundance was determined by mass spectrometry with a special inlet system and triple-collector detection. Sample gas containing unknown N₂O pools as small as 0.5 ng of N was analyzed by use of a spike technique, in which a reference gas of N₂O of natural ¹⁵N abundance was added to obtain enough total N for the mass spectrometer. In NO₃^- or NO₂^- pools, the ¹⁵N abundance could be determined in samples as small as approximately 3.5 ng of N. No cross-contamination took place between the NO₃^- and NO₂^- pools. The excellent separation of NO₃^- and NO₂^- pools, small sample size required, and low contamination risk during N₂O analysis offer great advantages in isotope studies of inorganic N transformations by, e.g., nitrifying or denitrifying bacteria in the environment.