Analysis of Nitrate Stable Isotopes by Cavity Ring-Down Spectroscopy

Nitrate contamination of surface and groundwater is a serious environmental and public health issue.  Identifying the source of this pollutant is an important step in addressing the problem. Nitrogen and oxygen isotopes (δ¹⁵N and δ¹⁸O values) are a powerful tool for tracing the source(s) of nitrate and understanding processes that impact its cycling in the environment.  Traditionally nitrate isotopes are measured via isotope ratio mass spectrometry (IRMS) but in recent years laser spectroscopy has become a practical option.

We evaluated Picarro’s new PI5131-i isotopic and gas concentration analyser for determining bulk δ¹⁵N and δ¹⁸O values of N₂O converted from dissolved nitrate using the Titanium III chloride method. The PI5131-i analyser is based on a robust mid-infrared, laser-based cavity ring-down spectrometry (CRDS) technology. This system when combined with Picarro’s Sage gas autosampler allowed us to analyse the isotopic composition of dissolved nitrate to a level matching IRMS precision and at concentrations as low as 0.05 mg/L NO3-N. 

In 40 mL reaction vials, Ti (III) chloride was added to 10 mL sample at a 1:20 ratio (v/v, reagent to sample). After 24 hours of reaction time enough N₂O was produced for laser spectroscopy analysis. Prior to analysis, the headspace N₂O was transferred into 12 mL exetainers to fit in the Sage autosampler. We compared a direct transfer protocol where 2 mL N₂O from the reaction vial is injected into exetainers and a 2-steps protocol where the N₂O is injected into purged exetainers (evacuated and pressurized with synthetic air).

Both transfer methods performed well in a blind nitrate intercomparison exercise (NICO).  The direct transfer workflow required fewer preparation steps but required a blank correction, whereas the two-step protocol was more labour-intensive due to the purge and fill process.