Analysis of N2 & N2O by GasBench-Precon-IRMS
The SIF provides isotope analysis (15N, 18O) of N2 and/or N2O in gas mixtures that may also contain O2 and CO2. This method is also used to analyze N2 and/or N2O purged from water samples with helium.
Analysis of 15N and 18O in Gas Mixtures
Stable isotope ratios of nitrogen (δ15N) and/or oxygen (δ18O) are measured using a ThermoScientific GasBench + Precon 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 5-100 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 (15 m x 0.53 mm 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 two 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operated in series such that the N2O is held in the first trap until the non-condensing portion of the sample gas has been replaced by helium carrier, then passed to the second, smaller trap.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 Poraplot Q GC column (25 m 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 15N delta values are calculated by adjusting the provisional values for changes in linearity and instrumental drift such that correct 15N delta values for laboratory reference materials are obtained. Two laboratory reference materials are analyzed every 10 samples. The laboratory reference materials 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 ‰).
N2O isotopomers (δ15Nα and δ15Nβ)
Analysis of N2O isotopomers (δ15Nα and δ15Nβ) are performed using the same multi-collector isotope-ratio mass spectrometer as above. Simultaneous measurement of N2O (m/z 44, 45, 46) and its NO fragment (m/z 30 and 31) allow determination of the central nitrogen atom (δ15Nα). Isotopic composition of the terminal nitrogen atom (δ15Nβ) can be calculated as the difference between the central position and the overall (δ15N) bulk measurement.
Limit of Quantitation and Long-term standard deviation for N2 & N2O Analysis by GasBench-Precon-IRMS
N2O : Limit of Quantitation: approx. 150 picomoles
Long-term standard deviation: 15N, 0.1 ‰; 18O, 0.3 ‰
N2: Limit of Quantitation: approx. 150 nanomoles
Long-term standard deviation: 0.1 ‰
Maximum measurable gas concentrations are dependent upon both concentration and isotopic enrichment. Please contact us if you intend to submit isotopically enriched samples at gas concentrations more than ten times that of ambient concentration.