A Proof-of-Principle Study for δ¹⁵N Measurements of Aqueous Dissolved Nitrate With a Modified LC-IRMS Interface

The analysis of nitrogen isotopes in aqueous dissolved nitrate is an effective method for identifying pollution sources and offers the potential to study the nitrogen cycle. However, the measurement of nitrogen isotope ratios of nitrate still requires extensive sample preparation or derivatization.

In this study, a modified commercially available liquid chromatography-isotope ratio mass spectrometer (LC-IRMS) interface is presented that enables automated measurement of δ¹⁵N signatures from nitrate by online reduction of nitrate in two consecutive steps. First, vanadium(III) chloride is used as a reducing agent to convert NO₃ ^- to N_xO_y under acidic conditions. The mix of nitrogen oxides is then transferred into a stream of helium and reduced to nitrogen (N₂) analysis gas via a hot copper reactor. Prior to the online conversion of aqueous nitrate into elemental nitrogen, the sample was chromatographically separated from potential matrix effects on a PGC column.

Precision was achieved at a level below 1.4‰ by injecting 10 μL of 50 mg L^-1 N, using five different nitrate standards and reference materials. These materials spanned a range of more than 180‰ in δ¹⁵N. To demonstrate the applicability of the method, we measured water samples from an enrichment experiment, where isotopically enriched ammonium chloride was administered into a small river over the course of 2 weeks. In contrary to our expectation, the δ¹⁵N values of river nitrate showed values between +0.4 ± 0.4‰ and +4.1 ± 0.3‰, varying over a small range of 3.7‰.

Our study showed that the measurement of nitrate nitrogen isotope ratios with a modified LC-IRMS system is possible but that further modifications and improvements would be necessary for a robust and user-friendly instrument.