N and O isotope (δ15 Nα , δ15 Nβ , δ18 O, δ17 O) analyses of dissolved NO3- and NO2- by the Cd-azide reduction method and N2 O laser spectrometry

Leonard I Wassenaar; Cedric Douence; Mark A Altabet; Pradeep K Aggarwal

doi:10.1002/rcm.8029

N and O isotope (δ¹⁵ N^α , δ¹⁵ N^β , δ¹⁸ O, δ¹⁷ O) analyses of dissolved NO₃^- and NO₂^- by the Cd-azide reduction method and N₂ O laser spectrometry

Rapid Commun Mass Spectrom. 2018 Feb 15;32(3):184-194. doi: 10.1002/rcm.8029.

Authors

Leonard I Wassenaar¹, Cedric Douence¹, Mark A Altabet², Pradeep K Aggarwal¹

Affiliations

¹ Isotope Hydrology Section, International Atomic Energy Agency, Vienna, Austria.
² Department of Estuarine and Ocean Sciences, University of Massachusetts Dartmouth, New Bedford, MA, USA.

PMID: 29131928
DOI: 10.1002/rcm.8029

Abstract

Rationale: The nitrogen and oxygen (δ¹⁵ N, δ¹⁸ O, δ¹⁷ O) isotopic compositions of NO₃^- and NO₂^- are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd-azide method was used to convert NO₃^- or NO₂^- to N₂ O for N and triple-O isotopic analyses by N₂ O laser spectrometry. A protocol for laser-based headspace isotope analyses was compared with isotope ratio mass spectrometry. Lasers provide the ability to directly measure ¹⁷ O anomalies which can help discern atmospheric N sources.

Methods: δ¹⁵ N, δ¹⁸ O and δ¹⁷ O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N₂ O using the Cd-azide method and headspace N₂ O laser spectrometry. A ¹⁵ N tracer test assessed the position-specific routing of N to the α or β positions in the N₂ O molecule. A data processing algorithm was used to correct for isotopic dependencies on N₂ O concentration, cavity pressure and water content.

Results: NO₃^- /NO₂^- nitrogen is routed to the ¹⁵ N^α position of N₂ O in the azide reaction; hence the δ¹⁵ N^α value should be used for N₂ O laser spectrometry results. With corrections for cavity pressure, N₂ O concentration and water content, the δ¹⁵ N^α_AIR , δ¹⁸ O_VSMOW and δ¹⁷ O_VSMOW values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO₃ ), -1.7 ± 0.1, -26.8 ± 0.8, -14.4 ± 1.1 (USGS34) and +2.6 ± 0.1, +57.6 ± 1.2, +51.2 ± 2.0 (USGS35), in agreement with their values and with the isotope ratio mass spectrometry results. The ¹⁷ O excess for USGS35 was +21.2 ± 9‰, in good agreement with previous results.

Conclusions: The Cd-azide method yielded excellent results for routine determination of δ¹⁵ N, δ¹⁸ O and δ¹⁷ O values (and the ¹⁷ O excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N₂ O laser spectrometers. The ¹⁵ N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high ¹⁵ N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of ¹⁷ O.