Complete Measurement of Stable Isotopes in N2O (δ15N, δ15Nα, δ15Nβ, δ18O, δ17O) Using Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS)

Abstract:

Nitrate contamination in water is a worldwide environmental problem and source apportionment is critical to managing nitrate pollution. Fractionation caused by physical, chemical and biological processes alters the isotope ratios of nitrates (¹⁵N/¹⁴N, ¹⁸O/¹⁶O and ¹⁷O/¹⁶O) and biochemical nitrification and denitrification impart different intramolecular site preference (¹⁵N¹⁴NO vs. ¹⁴N¹⁵NO). Additionally, atmospheric nitrate is anomalously enriched in ¹⁷O compared to other nitrate sources. The anomaly (Δ¹⁷O) is conserved during fractionation processes, providing a tracer of atmospheric nitrate. All of these effects can be used to apportion nitrate in soil.

Current technology for measuring nitrate isotopes is complicated and costly – it involves conversion of nitrate to nitrous oxide (N₂O), purification, preconcentration and measurement by isotope ratio mass spectrometer (IRMS). Site specific measurements require a custom IRMS. There is a pressing need to make this measurement simpler and more accessible.

Los Gatos Research has developed a next generation mid-infrared Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS) analyzer to quantify all stable isotope ratios of N₂O (δ¹⁵N, δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O, δ¹⁷O). We present the latest performance data demonstrating the precision and accuracy of the OA-ICOS based measurement. At an N₂O concentration of 322 ppb, the analyzer quantifies [N₂O], δ¹⁵N, δ¹⁵N^a, δ¹⁵N^b, and δ¹⁸O with a precision of ±0.05 ppb, ±0.4 ‰, ±0.45 ‰, and ±0.6 ‰, and ±0.8 ‰ respectively (1σ, 100s; 1σ, 1000s for δ¹⁸O). Measurements of gas standards demonstrate accuracy better than ±1 ‰ for isotope ratios over a wide dynamic range (200 – 100,000 ppb). The measurement of δ¹⁷O requires a higher concentration (1 - 50 ppm), easily obtainable through conversion of nitrates in water. For 10 ppm of N₂O, the instrument achieves a δ¹⁷O precision of ±0.05 ‰ (1σ, 1000s). This performance is sufficient to quantify atmospheric nitrate in soil and groundwater and may be used to differentiate other sources of nitrate for which the range of Δ¹⁷O values is much smaller.

By measuring δ¹⁵N, δ¹⁵N^α, δ¹⁵N^β, δ¹⁸O and δ¹⁷O, our instrument will help researchers unravel the complicated nitrate mixing problem to determine the sources and sinks of nitrate pollution.