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N and O isotope ( δ 15 N α , δ 15 N β , δ 18 O, δ 17 O) analyses of dissolved NO 3 − and NO 2 − by the Cd‐azide reduction method and N 2 O laser spectrometry
Author(s) -
Wassenaar Leonard I.,
Douence Cedric,
Altabet Mark A.,
Aggarwal Pradeep K.
Publication year - 2018
Publication title -
rapid communications in mass spectrometry
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.528
H-Index - 136
eISSN - 1097-0231
pISSN - 0951-4198
DOI - 10.1002/rcm.8029
Subject(s) - chemistry , isotope , radiochemistry , oxygen 18 , physics , nuclear physics , nuclear chemistry , isotopes of oxygen
Rationale The nitrogen and oxygen ( δ 15 N, δ 18 O, δ 17 O) isotopic compositions of NO 3 − and NO 2 − are important tracers of nutrient dynamics in soil, rain, groundwater and oceans. The Cd‐azide method was used to convert NO 3 − or NO 2 − to N 2 O for N and triple‐O isotopic analyses by N 2 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 δ 15 N, δ 18 O and δ 17 O values were measured on N/O stable isotopic reference materials (IAEA, USGS) by conversion to N 2 O using the Cd‐azide method and headspace N 2 O laser spectrometry. A 15 N tracer test assessed the position‐specific routing of N to the α or β positions in the N 2 O molecule. A data processing algorithm was used to correct for isotopic dependencies on N 2 O concentration, cavity pressure and water content. Results NO 3 − /NO 2 − nitrogen is routed to the 15 N α position of N 2 O in the azide reaction; hence the δ 15 N α value should be used for N 2 O laser spectrometry results. With corrections for cavity pressure, N 2 O concentration and water content, the δ 15 N α AIR , δ 18 O VSMOW and δ 17 O VSMOW values (‰) of international reference materials were +4.8 ± 0.1, +25.9 ± 0.3, +12.7 ± 0.2 (IAEA NO 3 ), −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 δ 15 N, δ 18 O and δ 17 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 N 2 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.