N and O isotope (15N, 15N, 18O, 17O) analyses of dissolved NO3- and NO2- by the Cd-azide reduction method and N2O laser spectrometry

RationaleThe nitrogen and oxygen (N-15, O-18, O-17) 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 N2O for N and triple-O isotopic analyses by N2O 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-17 anomalies which can help discern atmospheric N sources. Methods(15)N, O-18 and O-17 values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N2O using the Cd-azide method and headspace N2O laser spectrometry. A N-15 tracer test assessed the position-specific routing of N to the or positions in the N2O molecule. A data processing algorithm was used to correct for isotopic dependencies on N2O concentration, cavity pressure and water content. ResultsNO(3)(-)/NO2- nitrogen is routed to the N-15 position of N2O in the azide reaction; hence the N-15 value should be used for N2O laser spectrometry results. With corrections for cavity pressure, N2O concentration and water content, the N-15(AIR), O-18(VSMOW) and O-17(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 O-17 excess for USGS35 was +21.2 +/- 9 parts per thousand, in good agreement with previous results. ConclusionsThe Cd-azide method yielded excellent results for routine determination of N-15, O-18 and O-17 values (and the O-17 excess) of nitrate or nitrite by laser spectrometry. Disadvantages are the toxicity of Cd-azide chemicals and the lack of automated sampling devices for N2O laser spectrometers. The N-15-enriched tracer test revealed potential for position-specific experimentation of aqueous nutrient dynamics at high N-15 enrichments by laser spectrometry, but exposed the need for memory corrections and improved spectral deconvolution of O-17.