Nitrate (NO3-) is quietly contaminating water worldwide. Fertilizers, animal manure, and wastewater wash into rivers, lakes, and groundwater, in turn triggering algal blooms, ocean dead zones, and risks to drinking water quality.1,2
To fix the problem, users first need to understand where nitrate actually comes from.
Stable Isotopes as a Fingerprinting Tool
Measuring the isotopic signatures of nitrogen and oxygen locked inside nitrate molecules (δ15N, δ18O, δ17O), we can trace nitrate sources and, in turn, reveal:
- Where they came from: synthetic fertilizer, organic waste, or atmospheric deposition
- Whether nature is cleaning them up: bacterial denitrification leaves a telltale isotopic trail
- How land use is changing: insights that concentration measurements alone simply cannot provide
The Challenge with Traditional Methods
Conventional nitrate isotope analysis relies on microbial or cadmium (Cd) reduction coupled with GC-IRMS, a process that involves toxic chemicals and labor-intensive, multi-step conversions. More critically, it cannot directly measure δ17O, the very signature needed to distinguish atmospheric nitrate from nutrient-derived sources.
These limitations become especially costly in atmospheric chemistry and water quality monitoring, where isotope signatures must be captured rapidly and repeatedly; traditional lab workflows simply cannot support this.
Source: ABB Measurement and Analytics Analytical Products
| isotopologue |
m/z |
rel. abundance |
| 15N14N16O |
45 |
3.64 × 10-3 |
| 14N15N16O |
45 |
3.64× 10-3 |
| 14N14N17O |
45 |
3.69 × 10-4 |
Three isotopologues with m/z = 45 share the same mass. This leads to isobaric interference, which prevents direct measurements with IRMS.
ABB’s Laser-Based GLA451-N2OI3 Overcomes Each of These Barriers
Based on Off-Axis Integrated Cavity Output Spectroscopy (OA-ICOS), it simultaneously and directly measures δ15N (bulk, α and β site-specific), δ18O, and δ17O with no prior chemical conversion and minimal sample preparation.
Paired with a headspace autoinjector, the system runs fully unattended across entire sample batches at a speed of 12 minutes per sample, enabling the high-temporal-resolution studies that were previously out of reach.
GLA451-N2OI3 with Headspace Autoinjector. Image Credit: ABB Measurement and Analytics Analytical Products
Measured N2O spectrum. Image Credit: ABB Measurement and Analytics Analytical Products
Methods
Instrument Details
The analyzer is based on Off-Axis Integrated Cavity Output Spectroscopy,2 which combines high precision with robustness essential for field applications. Laser light at the desired frequency is injected into the cavity off-axis to suppress cavity-mode noise. The beam reflects off two highly reflective mirrors (R = 99.998%), forming a pattern before exiting the cavity. This technique enhances the effective path length to several kilometers while being robust to mechanical vibrations.
Off-axis integrated cavity output spectroscopy. Image Credit: ABB Measurement and Analytics Analytical Products
Performance with Headspace Autoinjector
The Headspace autoinjector seamlessly connects with the analyzer, facilitating autonomous sample runs. Up to 36 nitrate samples can be programmed to be introduced into the analyzer using the headspace autoinjector, with each injection taking 12 minutes.
Image Credit: ABB Measurement and Analytics Analytical Products
δ15N repeatability of 80 sequential injections using the ACC-headspace auto injector. Mean = -13.3 ‰ and Standard deviation = 0.68 ‰. Image Credit: ABB Measurement and Analytics Analytical Products
Performance
Precision
The precision of the GLA451-N2013 was characterized by measuring a time series of 2 ppm N2O and calculating the Allan-Werle Plot.
Allan-Werle plot showing the precision of N2O isotopologues. Image Credit: ABB Measurement and Analytics Analytical Products
Linearity and Concentration Dependence
OA-ICOS has an inherently linear response since the amount of light is directly proportional to the amount of gas in the sample. A 10 ppm gas cylinder is diluted with zero air to characterize the linearity and concentration dependence of the analyzer.
Image Credit: ABB Measurement and Analytics Analytical Products
Conclusion
- High Precision: Allan deviation shows 1σ = 0.3‰ for δ15N and δ18O, and 3‰ for δ17O at 300 s integration
- Excellent linearity and high dynamic range: Across the full 0–10 ppm N2O range, with a calibration slope of b = 1.0006, confirming negligible concentration dependence
- High Repeatability: 0.6‰ (1σ) repeatability over sequential injections validates suitability for long, unattended automated sample runs bracketed with references
- Direct δ17O Measurement: Unlike GC-IRMS, OA-ICOS measures δ15N, δ18O, and δ17O simultaneously, enabling discrimination between atmospheric and nutrient-derived nitrate sources.
- High Selectivity: The laser-based OA-ICOS offers high selectivity, thereby overcoming the isobaric interference of conventional GC-IRMS.
- Fast Measurements: A faster, safer alternative to conventional GC-IRMS workflows
References
- Wassenaar, L.I., et al. (2023). Automated rapid triple-isotope (δ15N, δ18O, δ17O) analyses of nitrate by Ti(III) reduction and N2O laser spectrometry. Isotopes in Environmental and Health Studies, 59(3), pp.297–308. DOI: 10.1080/10256016.2023.2222222. https://www.tandfonline.com/doi/full/10.1080/10256016.2023.2222222.
- Kreitler, C. W. (1974). Determining the source of nitrate in groundwater by nitrogen isotope studies. Texas ScholarWorks. DOI: 10.15781/T2ZS2KX2J. https://repositories.lib.utexas.edu/items/9394ae2b-f2da-4644-bb69-6de30ab47547.
This information has been sourced, reviewed and adapted from materials provided by ABB Measurement and Analytics Analytical Products.
For more information on this source, please visit ABB Measurement and Analytics Analytical Products.