Establishment of a purge and trap continuous flow isotope ratio mass spectrometer system for analysis of stable nitrate isotopes (δ15N and δ18O) in water samples by Ti(III) reduction

Pollution of surface and shallow groundwater by nitrate (NO₃^-) is a global concern resulting in deterioration of drinking water quality. Stable isotopes of NO₃^- (δ¹⁵N and δ¹⁸O) can be used to trace its sources which have distinct N and O isotopic signatures. The isotopic values can also be used to identify areas of natural remediation through biogeochemical processes such as denitrification. Conversion of aqueous NO₃^- to N₂O headspace gas by Ti(III) reduction is a new method for analysis of NO₃^- stable isotopes. Previous literature introduces the analytical procedure but provides limited guidelines for instrument set-up and operation. Here, we present an automated purge and trap isotope ratio mass spectrometer (P&T-IRMS) combined with the Ti(III) reduction method for analysis of δ¹⁵N and δ¹⁸O in NO₃^-. The P&T-IRMS base analytical precision was ±0.3‰ and ±0.2‰ for δ¹⁵N and δ¹⁸O, respectively. Isotopic values were quantified down to an N₂O gas concentration of 1 µl L^-1 for δ¹⁵N and 2 µl L^-1 for δ¹⁸O. The target NO₃^--N concentration needed for accurate measurements was 0.2 mg L^-1. Comparison of δ¹⁵N and δ¹⁸O measured using the P&T-IRMS by Ti(III) reduction with EA-IRMS values showed high accuracy. The measurement precision (SD) and uncertainties (u) for our KNO₃^- internal standard were ±0.2 (±0.6) and ±0.2 (±0.9) for δ¹⁵N and δ¹⁸O, respectively. The P&T-IRMS and Ti(III) reduction method set-up showed low quantification limits and acceptable accuracy and precision in line with other well-established methods for analysis of NO₃^- stable isotopes. The provided guidelines will assist laboratories which utilize IRMS headspace gas instrumentation with the process of IRMS set-up and operation and establishment of an independent analytical procedure for the Ti(III) reduction method.