A SENSITIVE ASSAY FOR DETERMINATION OF N-14/N-15 ISOTOPE DISTRIBUTION IN NO3-

Bacterial denitrification by use of an enrichment culture was used as a bioassay for conversion of (NO3-)-N-14 and (NO3-)-N-15 to N2 gas os N-28(2), N-29(2) and N-30(2), which was subsequently analyzed at high accuracy by mass spectrometry. The N-15 labeling of the NO3- could then be calculated from the ratio between N-29(2) and N-30(2) in the analyzed gas. The bacterial reduction was carried out on 2-ml samples contained in 3.4 ml butyl rubber stoppered blood collection tubes. Removal of the N2 dissolved in the original sample was not necessary for the assay. The method proved to be very precise (0.2 atom% in standard deviation on the measured values) even when applied to nanomole amounts of (NO3-)-N-15. After complete reduction of the NO3-, the isotopic composition of N2 in the blood collection tubes was stable for many weeks, and the tubes were therefore used to store extracted gas samples for later analysis. Isotope distributions in evolved N2 gas and overlying water NO3- during light and dark incubations of a sediment core are shown to illustrate the quality of data which can be obtained by the analytical procedures described in this paper.