Method: Nitrate Source Tracking

Pretreatment

In order to analyze the sample we must chemically convert aqueous NO3 into headspace N2O. Nitrite (as another species along the reduction pathway) is first removed from the solution via Sulfamic Acid. As water sample chemistry can affect results samples are matched to standards of matching salinity and NO3 concentrations. Concentration is determined by spectrophotometry. The chemical reduction is carried out using Ti(III)Cl conditioned with Zinc powder preconditioned to remove Ti(IV). Samples are stoppered and crimped in gas-tight acid-washed vials purged with an inert gas. Following the incubation of the sample with the reducing reagent the sample is ready for analysis.

Isotope Measurement

Following its chemical reduction the sample is analyzed by continuous flow (CF) Isotope Ratio Mass Spectrometry (IRMS).The N2O is isolated via a modified GasBench(II) and cryotraps. Once the analyte has been cryo-trapped and separated from interfering gasses via a GC the isotopic composition is measured via Mass Spectrometry. (Casciotti et al., 2002; Foreman et al., 2016; and Altabet and Wassenaar, 2017). Isotope ratio data are reported as delta (δ) values in units of parts per thousand (per mill) (‰) (Coplen, 2011).

Nitrate Source Tracking Results

Isotope ratio data are reported as delta (δ) values in units of parts per thousand (per mill) (‰) (Coplen, 2011). Nitrogen isotope ratios are reported relative to N2 in air (Mariotti, 1983) and oxygen isotope ratios are reported relative to VSMOW reference water and normalized on a scale such that δ18OSLAP = -55.5‰ (Coplen, 1994; IAEA, 2017). The results are also presented graphically on a plot which includes representative areas of the isotopic composition (δ18O and δ15N) of various nitrate sources (Kendall et al, 2007; Hastings et al., 2013).

References

  • Altabet, Mark A., et al. “A Ti (III) reduction method for one‐step conversion of seawater and freshwater nitrate into N2O for stable isotopic analysis of 15N/14N, 18O/16O and 17O/16O.” Rapid Communications in Mass Spectrometry 33.15 (2019): 1227-1239.
  • Altabet, M.A. and L. I. Wassenaar. (2017). New Chemical methods for the precise determination of nitrate isotopic composition Presented at Chemical Oceanography at the 86th annual Gordon Research Conference; 2017 July, 23-28; Colby-Sawyer College, New London, NH.
  • Casciotti, K. L., Sigman, D. M., Hastings, M. G., Böhlke, J. K., & Hilkert, A. (2002). Measurement of the oxygen isotopic composition of nitrate in seawater and freshwater using the denitrifier method. Anal. Chem, 74(19), 4905-4912.
  • Coplen, T. B., (1994). Reporting of Stable Hydrogen, Carbon, and Oxygen Isotopic Abundances, Pure and Applied Chemistry, v. 66, p. 273-276.
  • Coplen, T. B., (2011). Guidelines and recommended terms for expression of stable-isotope-ratio and gas-ratio measurement results. Rapid Communications in Mass Spectrometry, v. 25, 2538–2560.
  • Foreman, R. K., Segura-Noguera, M., & Karl, D. M. (2016). Validation of Ti (III) as a reducing agent in the chemiluminescent determination of nitrate and nitrite in seawater. Marine Chemistry, 186, 83- 89.
  • Hastings, et al., (2013). Stable Isotopes as Tracers of Anthropogenic Nitrogen Sources, Deposition, and Impacts. Elements, 9(5), 339-344.
  • International Atomic Energy Agency (IAEA), (2017). Reference Sheet for International Measurement Standards.
  • Kendall, C., Elliott, E.M., and Wankel, S.D. (2007). Tracing anthropogenic inputs of nitrogen to ecosystems. Stable Isotopes in Ecology and Environmental Science, 2nd ed., Ch. 12.
  • Mariotti, A., (1983). Atmospheric nitrogen is a reliable standard for natural 15N abundance measurements: Nature, v. 303, p. 685-687.

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