The SIF provides isotope analysis (¹⁵N, ¹⁸O) of NO₃ in water by bacteria denitrification assay.

Analysis of ¹⁵N and ¹⁸O in Gas Mixtures
Trace gas isotope ratios are measured using a ThermoFinnigan GasBench + PreCon trace gas concentration system interfaced to a ThermoScientific Delta V Plus isotope-ratio mass spectrometer (Bremen, Germany). Gas samples are purged from vials through a double-needle sampler into a helium carrier stream (20 mL/min). Then, N₂ and N₂O are isolated and concentrated in preparation for isotopic analysis. First, N₂ gas is sampled by a rotary 8-port valve fitted with a 20-100 µL sampling loop and timed to capture the peak N₂ concentration in the carrier gas stream. This gas sample is passed to the IRMS through a molecular sieve 5A GC column (15m x 0.53mm ID,  25°C, 3 mL/min). A reference N₂ peak is used to calculate provisional isotope ratios of the sample N₂ peak.

As N₂ is analyzed, the rest of the gas sample passes through a CO₂ scrubber (Ascarite) and N₂O is trapped and concentrated in 2 liquid nitrogen cryo-traps operated in series such that the N₂O is held in the first trap until the non-condensing portion of the sample gas has been replaced by helium carrier, then passed to the second, smaller trap. Finally the second trap is warmed to ambient and the N₂O is carried by helium to the IRMS via a Poroplot Q GC column (25m x 0.53 mm,  25°C, 1.8 mL/min). This column separates N₂O from residual CO₂. A reference N₂O peak is used to calculate provisional isotope ratios of the sample N₂O peak.

Final δ¹⁵N values are calculated by adjusting the provisional values such that correct δ¹⁵N values for working standards are obtained. Two working standards are analyzed every 10 samples. The working standards are mixtures of N₂ and N₂O (e.g., 3% N₂ + 1 ppm N₂O with the balance He or 1 ppm N₂O with balance N₂). The N₂ is calibrated against an Oztech N₂ standard (Oztech Trading Co., δ15N vs air = -0.61). The calibration of the N₂O is problematic since there are no suitable international standards. Thus, we calibrated ¹⁵N and ¹⁸O by thermally decomposing N₂O in a heated gold tube (800°C) to convert N₂O to N₂ + O₂. The resulting N₂ was calibrated against the Oztech N₂ standard, and the O₂ was calibrated against an Oztech O₂ standard (δ¹⁸O vs VSMOW = 27.48).