Determination of 15 N Abundance in Nanogram Pools of NO 3 - and NO 2 - by Denitrification Bioassay and Mass Spectrometry

Abstract

Suspensions of two strains of Pseudomonas aeruginosa (ON12 and ON12-1) were used to reduce NO 3 - and NO 2 - , respectively, to N 2 O. The evolved N 2 O was quantified by gas chromatography with electron capture detection, and the 15 N abundance was determined by mass spectrometry with a special inlet system and triple-collector detection. Sample gas containing unknown N 2 O pools as small as 0.5 ng of N was analyzed by use of a spike technique, in which a reference gas of N 2 O of natural 15 N abundance was added to obtain enough total N for the mass spectrometer. In NO 3 - or NO 2 - pools, the 15 N abundance could be determined in samples as small as approximately 3.5 ng of N. No cross-contamination took place between the NO 3 - and NO 2 - pools. The excellent separation of NO 3 - and NO 2 - pools, small sample size required, and low contamination risk during N 2 O analysis offer great advantages in isotope studies of inorganic N transformations by, e.g., nitrifying or denitrifying bacteria in the environment.