Membrane inlet mass spectrometric analysis of N-isotope labelling for aquatic denitrification studies

Techniques are described for measuring the isotope distribution in dissolved nitrate and N₂ using membrane inlet mass spectrometry, which allows several gases to be measured in a water sample without the need for any separation steps. The isotope distribution in dissolved nitrate was measured using denitrifying Pseudomonas nautica to reduce the nitrate to N₂ which was then measured by mass spectrometry. Pseudomonas nautica NCIMB 1967 was easily grown in nitrate-limited continuous culture minimising intra- or extracellular nitrate or nitrite pools, and the bioassay was tolerant of a range of salinities. The precision of the bioassay when measuring samples with high ¹⁵NO₃⁻ contents (0.5 μmol) was 0.05 atom%; with 0.1 μmol ¹⁵NO₃⁻, the precision was around 0.2 atom%. Differences in labelling of N₂ in preserved samples obtained from ¹⁵NO₃⁻ incubations of water-covered sediment cores were measured on parallel samples with membrane inlet MS and GC-MS. The membrane inlet technique was accurate but the precision on ratio measurements was lower than by GC-MS.