N and O isotope (δ N , δ N , δ O, δ O) analyses of dissolved NO and NO by the Cd-azide reduction method and N O laser spectrometry.

Rationale: The nitrogen and oxygen (δ N, δ O, δ O) isotopic compositions of NO and NO are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd-azide method was used to convert NO or NO to N O for N and triple-O isotopic analyses by N 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 O anomalies which can help discern atmospheric N sources.

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

Results: NO /NO nitrogen is routed to the N position of N O in the azide reaction; hence the δ N value should be used for N O laser spectrometry results. With corrections for cavity pressure, N O concentration and water content, the δ N , δ O and δ O values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO ), -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 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 δ N, δ O and δ O values (and the 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 N O laser spectrometers. The N-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high N enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of O.