A chemical method for the conversion of nitrate to nitrous oxide for isotopic analysis

Abstract

The nitrate (NO3-) isotope method described here is a modified version of published chemical denitrification methods (e.g. McIlvin and Altabet, 2005; Ryabenko et al. 2009) that determine the δ15N and δ18O values of nitrate by sequentially converting nitrate to nitrous oxide gas (N2O) and measuring its isotopic composition. To summarize, samples containing 2 μg of NO3 -- nitrogen (N) are freeze dried and then redissolved in a 3 mL sodium chloride – imidazole solution in a 3.7 mL Exetainer® vial. Following redissolution, activated cadmium is added to each vial to reduce nitrate to nitrite over a 24-hour period. Samples are then syringe filtered into helium-filled 20 mL serum vials and buffer solution (acetic acid and sodium azide) added to convert nitrite to nitrous oxide. After allowing the reaction to proceed to completion, sodium hydroxide solution is added to quench the reaction. Depending on the instrument used for the isotopic analysis, nitrous oxide from the headspace of the vials can be directly injected to the instrument or sub-sampled into a suitable autosampler vial. The isotopic composition of the original nitrate is determined by creating a correction equation using nitrate standards included in each run. The correction accounts for method blank effects, isotopic fractionation during the various reactions, oxygen exchange between water and NOx, and the fact that one N of the N2O comes from the original nitrate and one from azide. Replicate samples and standards within each run are typically within ±0.5‰ or less for both δ15N and δ18O.