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.