Establishment of a Purge and Trap Continuous‐Flow Isotope Ratio Mass Spectrometer System for Analysis of Stable Nitrate Isotopes (δ 15 N and δ 18 O) in Water Samples by Ti(III) Reduction

ABSTRACT Rationale Pollution of surface and shallow groundwater by nitrate () is a global concern, resulting in the deterioration of drinking water quality. Stable isotopes of (δ 15 N and δ 18 O) can be used to trace its sources and identify prevailing biogeochemical processes. Methods Conversion of aqueous to N 2 O headspace gas by Ti(III) reduction is a new method for analysis of 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 δ 15 N and δ 18 O in . Results The P&T‐IRMS base analytical precision was ±0.3‰ and ±0.2‰ for δ 15 N and δ 18 O, respectively. The limit of quantification (LOQ) for the N 2 O gas standard was 1 μL L −1 (1.1 nmol N 2 O‐N) for both δ 15 N and δ 18 O. The range for accurate measurement was 0.2–0.3 mg L −1 (3.3–6.6 nmol). Based on the logarithmic trend of the N 2 O peak area vs. R 15 N /14 N and R 18 O/ 16 O, δ 15 N and δ 18 O corrections are presented for values outside the range of accuracy. A comparison of δ 15 N and δ 18 O from P&T‐IRMS and Ti(III) reduction measurements with EA‐IRMS values showed high accuracy. The measurement precision and uncertainties for our internal standard were ±0.2 (±0.6) and ±0.2 (±0.9) for δ 15 N and δ 18 O, respectively. Conclusion The P&T‐IRMS and Ti(III) reduction method demonstrated acceptable accuracy and precision, similar to well‐established methods for analysis of stable isotopes. This publication will assist laboratories which utilize IRMS headspace gas instrumentation with the process of IRMS set‐up and operation, establishment of an independent analytical procedure for the Ti(III) reduction method, and data evaluation and correction.