An automated, laser‐based measurement system for nitrous oxide isotope and isotopomer ratios at nanomolar levels

Abstract

Rationale Nitrous oxide (N2O) is an atmospheric trace gas regulating Earth's climate, and is a key intermediate of many nitrogen cycling processes in aquatic ecosystems. Laser‐based technology for N2O concentration and isotopic/isotopomeric analyses has potential advantages, which include high analytical specificity, low sample size requirement and reduced cost. Methods An autosampler with a purge‐and‐trap module is coupled to a cavity ring‐down spectrometer to achieve automated and high‐throughput measurements of N2O concentrations, N2O isotope ratios (δ15Nbulk and δ18O values) and position‐specific isotopomer ratios (δ15Nα and δ15Nβ values). The system provides accuracy and precision similar to those for measurements made by traditional isotope ratio mass spectrometry (IRMS) techniques. Results The sample sizes required were 0.01–1.1 nmol‐N2O. Measurements of four N2O isotopic/isotopomeric references were cross‐calibrated with those obtained by IRMS. With a sample size of 0.50 nmol‐N2O, the measurement precision (1σ) for δ15Nα, δ15Nβ, δ15Nbulk and δ18O values was 0.61, 0.33, 0.41 and 0.43‰, respectively. Correction schemes were developed for sample size‐dependent isotopic/isotopomeric deviations. The instrumental system demonstrated consistent measurements of dissolved N2O concentrations, isotope/isotopomer ratios and production rates in seawater. Conclusions The coupling of an autosampler with a purge‐and‐trap module to a cavity ring‐down spectrometer not only significantly reduces sample size requirements, but also offers comprehensive investigation of N2O production pathways by the measurement of natural abundance and tracer level isotopes and isotopomers. Furthermore, the system can perform isotopic analyses of dissolved and solid nitrogen‐containing samples using N2O as the analytical proxy.

N<sub>2</sub>O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

“…Although the magnitude of this effect ultimately varied across the analyzers and was dependent on N 2 O mixing ratios, the effect of a change in O 2 composition of 20.5 % was typically on the order of 10 ‰ to 30 ‰ for δ values. Similar O 2 dependencies have been reported by Erler et al (2015) for CRDS N 2 O isotope laser spectrometers, as well as for CRDS H 2 O isotope analyzers (Johnson and Rella, 2017). The underlying reason for this effect is differences in N 2 versus O 2 (and Ar) broadening parameters of the probed N 2 O isotopocule lines.…”

Temporal and Vertical Oxygen Gradients Modulate Nitrous Oxide Production in a Seasonally Anoxic Fjord: Saanich Inlet, British Columbia

“…The PT‐CRDS is an automated, laser‐based system that provides comparable accuracy and precision to gas chromatography and isotope ratio mass spectrometer techniques for N 2 O concentration and isotope/isotopomer ratio measurements, respectively. A full description of this technique is outlined in Ji and Grundle (2019). Isotopic analysis determines the relative abundances of nitrogen isotopes (δ 15 N bulk ) and oxygen isotopes (δ 18 O) of the entire N 2 O pool, whereas isotopomeric analysis discerns the intramolecular nitrogen isotope substitution (isotopomers) on the linear asymmetric N 2 O molecule (N = N = O).…”

“…Heating to 480 K released the gaseous N 2 O, after which concentrations and isotope/isotopomer ratios were measured by quantitative light dissipation at specific wavelengths ( 14 N = 15 N = 16 O, 15 N = 14 N = 16 O, and 14 N = 14 N = 18 O at 2195.76195, 2198.79576, and 2195.95102 cm −1 , respectively). Raw isotopic measurements were corrected for sample size‐dependent isotopic deviations caused by varying N 2 O concentrations (Ji & Grundle, 2019) and calibrated with isotopic reference materials (see Table S2). Because the optimal quantity of N 2 O for highly accurate isotopic/isotopomeric analyses by PT‐CRDS is 0.5–0.8 nmol‐N 2 O, samples from 130 and 160 m in April and 160 m samples in June did not contain enough N 2 O, and, as such, those results are not reported.…”

“…For example, 15 N tracer experiments determine potential and instantaneous N 2 O production rates and pathways (Ji et al, 2018), whereas N 2 O isotopomeric measurements reveal the dominant N 2 O production pathway integrated over the history of a water mass (Fujii et al, 2013). The isotope measurements reported here were conducted on a recently developed laser‐based analytical system that is able to deliver accuracy and precision comparable to traditional isotope ratio mass spectrometer techniques (Ji & Grundle, 2019). This work combines 15 N tracer incubation, N 2 O concentration, and natural abundance isotopic/isotopomeric measurements to (1) elucidate the N 2 O dynamics in a model coastal marine system with temporal and vertical oxygen gradients, (2) examine oxygen regulation of N 2 O production pathways during bottle incubations and under in situ conditions, and (3) identify the major N 2 O production pathways potentially contributing to atmospheric emissions.…”

N<sub>2</sub>O isotopocule measurements using laser spectroscopy: analyzer characterization and intercomparison

“…To the best of our knowledge, the work presented in Lee et al (2017) and Ji and Grundle (2019) are the only published uses of G5131-i models. A prior model (the G5101-i), which employs a different 5 spectral region and does not offer the capability for  18 O was used by Peng et al (2014), Erler et al (2015), Li et al (2015), Lebegue et al (2016) and Winther et al (2018).…”