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

Abstract

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 80oC. 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.