Triple oxygen isotope analysis of nitrate using isotope exchange cavity ringdown laser spectroscopy

Abstract

Rationale: Triple oxygen isotopes (¹⁶ O/¹⁷ O/¹⁸ O) in nitrate are a valuable tool to ascertain the pathways of nitrate formation in the atmosphere and the fate of nitrate in ecosystems. Here we present a new method for determining Δ¹⁷ O values in nitrates, based on nitrate-water isotope equilibration (IE) and subsequent isotopic analysis of water using cavity ringdown laser spectroscopy (CRDS).

Methods: Nitrate oxygen (O-NO₃ ^- ) is equilibrated with water oxygen (O-H₂ O) at low pH and 80°C. Subsequently, the δ¹⁷ O and δ¹⁸ O values of equilibrated water are determined by CRDS, scaled to V-SMOW and V-SLAP and calibrated against nitrate standards (USGS-34, USGS-35 and IAEA-NO3). We provide isotopic measurements of synthetic and natural nitrates and a direct inter-lab comparison with the classic method of thermal-decomposition of nitrate followed by isotope ratio mass spectrometry of O₂ (TD-IRMS).

Results: For synthetic NaNO₃ , the precision (1SD) of the IE-CRDS method is 0.8‰ for δ¹⁷ O values, 1.7‰ for δ¹⁸ O values and 0.2‰ for Δ¹⁷ O values when using an O-NO₃ ^- /O-H₂ O ratio greater than 0.0114 ± 0.0001 (e.g. 12 μmol of NO₃ ^- in 50 μL of acid solution). For natural samples, after purification of nitrates by column chemistry and reprecipitation as AgNO₃ , the precision is better than 1.8‰ for δ¹⁷ O values, 3.2‰ for δ¹⁸ O values and 1‰ for Δ¹⁷ O values. IE-CRDS and TD-IRMS yield Δ¹⁷ O values within the analytical errors of the two methods.

Conclusions: The IE-CRDS method for determining Δ¹⁷ O values in nitrates utilizes a user-friendly and relatively cheaper benchtop analytical instrument, representing an alternative to IRMS-based methods for certain applications.