SI-traceable total analysis of nitrate and nitrite by isotope dilution optical spectroscopy and its application to Berlin surface waters

Abstract

Accurate nitrate and nitrite data support water-quality regulation, yet routine methods rely on external calibration and rarely achieve SI traceability. We report a calibration-free determination of nitrate and nitrite by combining isotope dilution with high-resolution continuum-source graphite furnace molecular absorption spectrometry (ID-HR-CS-GF-MAS). A ¹⁵N-enriched nitrate spike (its concentration verified by reverse isotope dilution against the standard reference material NIST 3185) provides the SI link, and it is gravimetrically added to samples; nitrate and residual nitrite are converted in situ to nitric oxide (NO), whose 215 nm band is recorded at a pixel resolution of λ/Δλ ≈ 140 000. The 0.2127 nm shift between ¹⁴NO and ¹⁵NO electronic spectra is resolved, and a three-latent-variable partial least squares regression model yields the ¹⁵N/¹⁴N ratio with 0.3% precision. Instrumental LoD values of 4.8 ng (¹⁴N) and 3.2 ng (¹⁵N) translate to a method LoD of 4.8 ng of nitrogen (equivalent to 1.05 mg L⁻¹ NO₃⁻ for a 20 μL aliquot). The furnace program allows for successive drying/pyrolysis loops, so additional 20 μL aliquots can be layered onto the graphite platform. Alternatively, a 10 mL anion-exchange solid-phase extraction step concentrates nitrate and nitrite fivefold, allowing for the analysis of even lower sample concentrations. Results for four certified reference materials (2.9 to 1000 mg L⁻¹ NO₃⁻) agreed with certified values, giving relative expanded uncertainties of 2 to 4%. Analysis of twenty Berlin surface-water samples revealed concentrations ranging from 0.10 to 7.3 mg L⁻¹ NO₃⁻, indicating that the Panke River and Teltow Canal are the primary sources of nitrogen. ID-HR-CS-GF-MAS thus delivers ID-MS-level accuracy in a few minutes per run with bench-top optics, and, with optional on-platform or SPE pre-concentration, extends SI-traceable nitrate/nitrite monitoring into the low-ng regime.