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

BACKGROUND

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.