Establishment of a Purge and Trap Continuous-Flow Isotope Ratio Mass Spectrometer System for Analysis of Stable...

Pollution of surface and shallow groundwater by nitrate ( ${\mathrm{NO}}_3^{-}$ ) is a global concern, resulting in the deterioration of drinking water quality. Stable isotopes of ${\mathrm{NO}}_3^{-}$ (δ¹⁵N and δ¹⁸O) can be used to trace its sources and identify prevailing biogeochemical processes.

Conversion of aqueous ${\mathrm{NO}}_3^{-}$ to N₂O headspace gas by Ti(III) reduction is a new method for analysis of ${\mathrm{NO}}_3^{-}$ 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 ${\mathrm{NO}}_3^{-}$ .

The P&T-IRMS base analytical precision was ±0.3‰ and ±0.2‰ for δ¹⁵N and δ¹⁸O, respectively. The limit of quantification (LOQ) for the N₂O gas standard was 1 μL L^-1 (1.1 nmol N₂O-N) for both δ¹⁵N and δ¹⁸O. The ${\mathrm{NO}}_3^{-}-N$ range for accurate measurement was 0.2-0.3 mg L^-1 (3.3-6.6 nmol). Based on the logarithmic trend of the N₂O peak area vs. R¹⁵N^/14N and R¹⁸O/¹⁶O, δ¹⁵N and δ¹⁸O corrections are presented for values outside the range of accuracy. A comparison of δ¹⁵N and δ¹⁸O from P&T-IRMS and Ti(III) reduction measurements with EA-IRMS values showed high accuracy. The measurement precision and uncertainties for our ${\mathrm{KNO}}_3^{-}$ 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 demonstrated acceptable accuracy and precision, similar to well-established methods for analysis of ${\mathrm{NO}}_3^{-}$ 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.