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

Rationale: Triple oxygen isotopes (¹⁶O/¹⁷O/¹⁸O) 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 Δ¹⁷O 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-H₂O) at low pH and 80^oC. Subsequently, the δ¹⁷O and δ¹⁸O 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 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 δ¹⁷O values, 1.7‰ for δ¹⁸O values and 0.2‰ for Δ¹⁷O values when using a O-NO₃^-/O-H₂O 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 δ¹⁷O values, 3.2‰ for δ¹⁸O values and 1‰ for Δ¹⁷O values. IE-CRDS and TD-IRMS yield Δ¹⁷O values within the analytical errors of the two methods. Conclusions The IE-CRDS method for determining Δ¹⁷O values in nitrates utilizes a user-friendly and relatively cheaper benchtop analytical instrument, representing an alternative to IRMS-based methods for certain applications.