MEMBRANE INLET MASS-SPECTROMETRIC ANALYSIS OF N-ISOTOPE LABELING FOR AQUATIC DENITRIFICATION STUDIES

Techniques are described for measuring the isotope distribution in dis solved nitrate and N-2 using membrane inlet mass spectrometry, which a llows several gases to be measured in a water sample without the need for any separation steps. The isotope distribution in dissolved nitrat e was measured using denitrifying Pseudomonas nautica to reduce the ni trate to N-2 which was then measured by mass spectrometry. Pseudomonas nautica NCIMB 1967 was easily grown in nitrate-limited continuous cul ture minimising intra- or extracellular nitrate or nitrite pools, and the bioassay was tolerant of a range of salinities. The precision of t he bioassay when measuring samples with high (NO3-)-N-15 contents (0.5 mu mol) was 0.05 atom% with 0.1 mu mol (NO3-)-N-15, the precision was around 0.2 atom%. Differences in labelling of N-2 in preserved sample s obtained from (NO3-)-N-15 incubations of water-covered sediment core s were measured on parallel samples with membrane inlet MS and GC-MS. The membrane inlet technique was accurate but the precision on ratio m easurements was lower than by GC-MS.