Online Two-Dimensional Ion Chromatography Coupled with Orbitrap MS for Measuring the Triple Isotopic Composition (δ15N, δ18O, Δ17O) of Nitrate

Stable isotope signatures of nitrate (NO3−) are useful tracers in environmental studies, but high-throughput applications are constrained by the extensive preparatory steps required to convert NO3− to N2O for GC-IRMS analysis. Additionally, most laboratories are limited to δ15N and δ18O analysis, as the determination of Δ17O requires specialized instrumentation. Electrospray ionization (ESI) Orbitrap-MS offers advantages over conventional IRMS by preserving native molecular structures of target analytes; however, existing Orbitrap-MS NO3−isotope techniques rely on the direct infusion of pure analyte solutions and have only been optimized for the analysis of NO3− in methanol. Here, we demonstrate the feasibility of coupling ion chromatography to ESI-Orbitrap for the analysis of NO3− isotopologues in aqueous solutions. MS parameters were optimized to enhance precision, and correction strategies were developed to account for method fractionation effects and instrument drift. We applied the optimized method to conduct replicate measurements of NO3− isotopic standards (IAEA-NO-3, USGS-34, and USGS-35) and developed a calibration framework to express Orbitrap-measured ratios on the measurement scales used in conventional stable isotope studies. Pooled standard deviations for the isotopic standards of 0.9‰, 2.0‰, and 2.0‰ were obtained for δ15N, δ18O, and Δ17O, respectively, with offsets from expected delta values between −0.1 and 0.2‰. This technique enables high-throughput analysis with minimal pretreatment compared to the hours to days required for offline NO3−purification. While these results demonstrate the potential of online IC-ESI-Orbitrap-MS as a practical approach for rapid nitrate isotope analysis, we also outline several avenues for future method improvement.