Dissolved Nitrate δ15N and δ18O Analysis

Sample Material: Freshwater or seawater

Sample collection and storage: Approximately 30 ml of sample water is filtered through a pre-combusted (450°C) GFF filter and then placed in an acid-cleaned 50 ml Nalgene bottle leaving a small amount of headspace. The sample should be frozen as soon as possible after filtration.

Sample volume: Depends on the nitrate concentration. Note, we cannot analyze samples with <1 µmol/l. nitrate.

Precision: ±0.2 permil for δ15N and ±0.4 permil for δ18O

Method:
The isotopic composition of dissolved nitrate is analyzed using the denitrifier method of Sigman et al. (2001) and Casciotte et al. (2002). In brief, the sample is added via syringe to a He purged vial containing a solution of denitrifying bacteria (Pseudomonas aureofaciens, ATCC #13985).The bacteria are allowed to react with the sample at room temperature overnight during which time the NO3 is converted to N2O. The resulting N2O is analyzed by continuous-flow mass spectrometry using the PreCon-GasBench-Delta V system.

 

 

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Suggested Reading:

Casciotti, K.L., Sigman, D.M., Galanter Hastings, M., Bohlke, J.K., and Hilkert, A., 2002, Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Analytical Chemistry, 74, 4905-4912.

Casciotti, K.L. and McIlvin, M.R., 2007. Isotopic analyses of nitrate and nitrite from reference mixtures and application to Eastern Tropical Pacific waters. Marine Chemistry, 107, 184-201.

Casciotti, K.L,, Bohlke, J.K., McIlvin, M.R., Mroczkowski, S.J., and Hannon, J.E., 2007, Oxygen isotopes in nitrite: Analysis, Calibration, and Equilibration. Analytical Chemistry, 79, 2427-2436.

Coplen, T.B., Bohlke, J.K., and Casciotti, K.L., 2004. Using dual-bacterial denitrification to improve d15N determinations of nitrates containing mass-independent 17O. Rapid Communications in Mass Spectrometry, 18, 245-250.

Granger, J. et al., 2006. A method for the nitrite removal in nitrate N and O isotope analyses. Limnology and Oceanography: Methods, 4, 205-212.

Sigman, D.M., Casciotti, K.L., Andreani, M., Barford, C., Galanter, M., and Bohlke,J.K., 2001. A bacterial method for the nitrogen isotopic analysis of nitrate in seawater and freshwater. Analytical Chemistry, 73, 4145-4153.