Gas chromatography/isotope-ratio mass spectrometry method for high-precision position-dependent 15N and 18O measurements of atmospheric nitrous oxide

Hydrochemical and isotopic characteristics of water sources for biological activity across a massive evaporite basin on the Tibetan Plateau: Implications for aquatic environments on early Mars

Covered by vast eolian landforms, gravel deposits, and playas, the worldwide typical evaporite deposit land, Qaidam Basin, in northwestern China is analogous to early Mars when the aridification process had lasted for millions of years since the end of a wetter climate. This study aims to investigate the chemical and isotopic characteristics of waters in an evaporite-rich environment, as well as the habitable conditions therein, that have undergone a transformation similar to early Mars. In May 2023, a total of 26 water samples were collected across the representative central axis of a longitudinal aridity gradient in the Qaidam Basin, including categories of meteoric water, freshwater, standing water accumulated after precipitation, salty lacustrine water, and hypersaline brines to inspect compounds made up of carbon, nitrogen, phosphorus, sulfur, halogen, and metallic elements. As evaporation intensified, the salt types transformed from HCO3-Ca·Na to Cl·SO4-Na or ClMg. The dominance of carbonate will gradually be replaced by sulfate and chloride, leaving much more dilute and less detectable contents. The presence of trace ClO4-, ClO3-, ClO2-, and BrO3- was confirmed in a few of the sampled Qaidam waters, indicating the preservation of oxyhalides in waters within an arid region and possibly the presence of relevant microbial enzymes. The isotopes of water, carbonaceous, and nitrogenous compounds provide valuable references for either abiogenic or biogenic signatures. With undetectable amount, phosphorus was found to be the limiting nutrient in evaporative aquatic environments but not necessarily antibiosignatures. Overall, these results suggest that the paleo-lacustrine environments on Mars are more likely to preserve biosignatures if they feature the dominance of carbonate minerals, bioavailable nitrate, phosphorus, and organic carbon, the presence of thermodynamically unstable oxyhalides, and isotope ratios that point to the involvement of biological activity.

Pyisotopomer: A Python package for obtaining intramolecular isotope ratio differences from mass spectrometric analysis of nitrous oxide isotopocules

RATIONALE

Obtaining nitrous oxide isotopocule measurements with isotope ratio mass spectrometry (IRMS) involves analyzing the ion current ratios of the nitrous oxide parent ion (N₂ O⁺ ) as well as those of the NO⁺ fragment ion. The data analysis requires correcting for "scrambling" in the ion source, whereby the NO⁺ fragment ion obtains the outer N atom from the N₂ O molecule. While descriptions exist for this correction, and interlaboratory intercalibration efforts have been made, there has yet to be published a package of code for implementing isotopomer calibrations.

METHODS

We developed a user-friendly Python package (pyisotopomer) to determine two coefficients (γ and κ) that describe scrambling in the IRMS ion source, and then used this calibration to obtain intramolecular isotope deltas in N₂ O samples.

RESULTS

With two appropriate reference materials, γ and κ can be determined robustly and accurately for a given IRMS system. An additional third reference material is needed to define the zero-point of the delta scale. We show that IRMS scrambling behavior can vary with time, necessitating regular calibrations. Finally, we present an intercalibration between two IRMS laboratories, using pyisotopomer to calculate γ and κ, and to obtain intramolecular N₂ O isotope deltas in lake water unknowns.

CONCLUSIONS

Given these considerations, we discuss how to use pyisotopomer to obtain high-quality N₂ O isotopocule data from IRMS systems, including the use of appropriate reference materials and frequency of calibration.

Sources of nitrous oxide emissions from hydroponic tomato cultivation: Evidence from stable isotope analyses

Introduction

Hydroponic vegetable cultivation is characterized by high intensity and frequent nitrogen fertilizer application, which is related to greenhouse gas emissions, especially in the form of nitrous oxide (N₂O). So far, there is little knowledge about the sources of N₂O emissions from hydroponic systems, with the few studies indicating that denitrification could play a major role.

Methods

Here, we use evidence from an experiment with tomato plants (Solanum lycopersicum) grown in a hydroponic greenhouse setup to further shed light into the process of N₂O production based on the N₂O isotopocule method and the ¹⁵N tracing approach. Gas samples from the headspace of rock wool substrate were collected prior to and after ¹⁵N labeling at two occasions using the closed chamber method and analyzed by gas chromatography and stable isotope ratio mass spectrometry.

Results

The isotopocule analyses revealed that either heterotrophic bacterial denitrification (bD) or nitrifier denitrification (nD) was the major source of N₂O emissions, when a typical nutrient solution with a low ammonium concentration (1-6 mg L^-1) was applied. Furthermore, the isotopic shift in ¹⁵N site preference and in δ¹⁸O values indicated that approximately 80-90% of the N₂O produced were already reduced to N₂ by denitrifiers inside the rock wool substrate. Despite higher concentrations of ammonium present during the ¹⁵N labeling (30-60 mg L^-1), results from the ¹⁵N tracing approach showed that N₂O mainly originated from bD. Both, ¹⁵N label supplied in the form of ammonium and ¹⁵N label supplied in the form of nitrate, increased the ¹⁵N enrichment of N₂O. This pointed to the contribution of other processes than bD. Nitrification activity was indicated by the conversion of small amounts of ¹⁵N-labeled ammonium into nitrate.

Discussion/Conclusion

Comparing the results from N₂O isotopocule analyses and the ¹⁵N tracing approach, likely a combination of bD, nD, and coupled nitrification and denitrification (cND) was responsible for the vast part of N₂O emissions observed in this study. Overall, our findings help to better understand the processes underlying N₂O and N₂ emissions from hydroponic tomato cultivation, and thereby facilitate the development of targeted N₂O mitigation measures.

Observational Evidence of Large Contribution from Primary Sources for Carbon Monoxide in the South Asian Outflow

South Asian air is among the most polluted in the world, causing premature death of millions and asserting a strong perturbation of the regional climate. A central component is carbon monoxide (CO), which is a key modulator of the oxidizing capacity of the atmosphere and a potent indirect greenhouse gas. While CO concentrations are declining elsewhere, South Asia exhibits an increasing trend for unresolved reasons. In this paper, we use dual-isotope (δ¹³C and δ¹⁸O) fingerprinting of CO intercepted in the South Asian outflow to constrain the relative contributions from primary and secondary CO sources. Results show that combustion-derived primary sources dominate the wintertime continental CO fingerprint (f_primary ∼ 79 ± 4%), significantly higher than the global estimate (f_primary ∼ 55 ± 5%). Satellite-based inventory estimates match isotope-constrained f_primary-CO, suggesting observational convergence in source characterization and a prospect for model-observation reconciliation. This "ground-truthing" emphasizes the pressing need to mitigate incomplete combustion activities for climate/air quality benefits in South Asia.

Quantifying soil N2O emissions from soil and anaerobically digested swine manure, nitrification and denitrification using 15N isotope labeling method

Increasing use of anaerobically digested swine manure in the farmland makes it necessary to understand its impact on N2O emissions, regarding the source of N2O and the corresponding mechanism of action. We used a 15N-labeled sulfate modifying the soil in order to identify the sources of N2O and the pathways of nitrification and denitrification. Three soil moisture contents (50% WHC, 75% WHC, and 100% WHC) along with three levels of anaerobically digested swine manure (0 g·kg-1, 10 g·kg-1, and 25 g·kg-1) were tested using randomized block design. Although the combined effect of contents of anaerobically digested swine manure and the soil moisture contents added to the system stimulated the utilization of soil N and promoted denitrification, the process of nitrification dominated. In anaerobically digested swine manure-treated soils, the rate of contribution of anaerobically digested swine manure to N2O accounted for 68.6-99.8%. In the 25 g·kg-1 treatment, the maximum of N2O produced by denitrification and nitrification were 14.1% and 93.1%.

Can N Fertilizer Addition Affect N2O Isotopocule Signatures for Soil N2O Source Partitioning?

Isotopocule signatures of N₂O (δ¹⁵N^bulk, δ¹⁸O and site preference) are useful for discerning soil N₂O source, but sometimes, N fertilizer is needed to ensure that there is enough N₂O flux for accurate isotopocule measurements. However, whether fertilizer affects these measurements is unknown. This study evaluated a gradient of NH₄NO₃ addition on N₂O productions and isotopocule values in two acidic subtropical soils. The results showed that N₂O production rates obviously amplified with increasing NH₄NO₃ (p < 0.01), although a lower N₂O production rate and an increasing extent appeared in forest soil. The δ¹⁵N^bulk of N₂O produced in forest soil was progressively enriched when more NH₄NO₃ was added, while becoming more depleted of agricultural soil. Moreover, the N₂O site preference (SP) values collectively elevated with increasing NH₄NO₃ in both soils, indicating that N₂O contributions changed. The increased N₂O production in agricultural soil was predominantly due to the added NH₄NO₃ via autotrophic nitrification and fungal denitrification (beyond 50%), which significantly increased with added NH₄NO₃, whereas soil organic nitrogen contributed most to N₂O production in forest soil, probably via heterotrophic nitrification. Lacking the characteristic SP of heterotrophic nitrification, its N₂O contribution change cannot be accurately identified yet. Overall, N fertilizer should be applied strictly according to the field application rate or N deposition amount when using isotopocule signatures to estimate soil N₂O processes.

Development and verification of a novel isotopic N2 O measurement technique for discrete static chamber samples using cavity ring-down spectroscopy

RATIONALE

N₂ O isotopomers are a useful tool to study soil N cycling processes. The reliability of such measurements requires a consistent set of international N₂ O isotope reference materials to improve inter-laboratory and inter-instrument comparability and avoid reporting inaccurate results. All these are the more important given the role of N₂ O in anthropogenic climate change and the pressing need to develop our understanding of soil N cycling and N₂ O emission to mitigate such emissions. Cavity ring-down spectroscopy (CRDS) could potentially overcome resource requirements and technical challenges, making N₂ O isotopomer measurements more feasible and less expensive than previous approaches (e.g., gas chromatography [GC] and isotope ratio mass spectrometry [IRMS]).

METHODS

A combined laser spectrometer and small sample isotope module (CRDS & SSIM) method enabled N₂ O concentration, δ¹⁵ N^bulk , δ¹⁵ N^α , δ¹⁵ N^β and site preference (SP) measurements of sample volumes <20 mL, such as static chamber samples. Sample dilution and isotopic mixing as well as N₂ O concentration dependence were corrected numerically. A two-point calibration procedure normalised δ values to the international isotope-ratio scales. The CRDS & SSIM repeatability was determined using a reference gas (Ref Gas). CRDS & SSIM concentration measurements were compared with those obtained by GC, and the isotope ratio measurements from two different mass spectrometers were compared.

RESULTS

The repeatability (mean ± 1σ; n = 10) of the CRDS & SSIM measurements of the Ref Gas was 710.64 ppb (± 8.64), 2.82‰ (± 0.91), 5.41‰ (± 2.00), 0.23‰ (± 0.22) and 5.18‰ (± 2.18) for N₂ O concentration, δ¹⁵ N^bulk , δ¹⁵ N^α , δ¹⁵ N^β and SP, respectively. The CRDS & SSIM concentration measurements were strongly correlated with GC (r = 0.99), and they were more precise than those obtained using GC except when the N₂ O concentrations exceeded the specified operating range. Normalising CRDS & SSIM δ values to the international isotope-ratio scales using isotopic N₂ O standards (AK1 and Mix1) produced accurate results when the samples were bracketed within the range of the δ values of the standards. The CRDS & SSIM δ¹⁵ N^bulk and SP precision was approximately one order of magnitude less than the typical IRMS precision.

CONCLUSIONS

CRDS & SSIM is a promising approach that enables N₂ O concentrations and isotope ratios to be measured by CRDS for samples <20 mL. The CRDS & SSIM repeatability makes this approach suitable for N₂ O "isotopomer mapping" to distinguish dominant source pathways, such as nitrification and denitrification, and requires less extensive lab resources than the traditionally used GC/IRMS. Current study limitations highlighted potential improvements for future users of this approach to consider, such as automation and physical removal of interfering trace gases before sample analysis.

Nitrite isotope characteristics and associated soil N transformations

Nitrite (NO₂⁻) is a crucial compound in the N soil cycle. As an intermediate of nearly all N transformations, its isotopic signature may provide precious information on the active pathways and processes. NO₂⁻ analyses have already been applied in ¹⁵N tracing studies, increasing their interpretation perspectives. Natural abundance NO₂⁻ isotope studies in soils were so far not applied and this study aims at testing if such analyses are useful in tracing the soil N cycle. We conducted laboratory soil incubations with parallel natural abundance and ¹⁵N treatments, accompanied by isotopic analyses of soil N compounds (NO₃⁻, NO₂⁻, NH₄⁺). The double ¹⁵N tracing method was used as a reference method for estimations of N transformation processes based on natural abundance nitrite dynamics. We obtained a very good agreement between the results from nitrite isotope model proposed here and the ¹⁵N tracing approach. Natural abundance nitrite isotope studies are a promising tool to our understanding of soil N cycling.

Influence of δ18 O of water on measurements of δ18 O of nitrite and nitrate

RATIONALE

Oxygen isotope ratio measurements of NO₂ ^- and NO₃ ^- by the azide method and denitrifier method are sensitive to the δ¹⁸ O value of the sample water. However, the influence of δ¹⁸ O_H2O on those measurements has not been quantitatively evaluated and documented so far. Therefore, we investigated the influence of δ¹⁸ O_H2O of a sample on the δ¹⁸ O analysis of NO₂ ^- and NO₃ ^- .

METHODS

We prepared NO₂ ^- and NO₃ ^- standards (with known δ¹⁸ O_NO2- and δ¹⁸ O_NO3- values) dissolved in waters having different δ¹⁸ O_H2O values (δ¹⁸ O_H2O  = -12.6, 25.9, 56.7, and 110.1‰). Nitrite and nitrate were converted into N₂ O using the azide method and the denitrifier method, respectively. The isotope ratios of the generated N₂ O were measured with a Sercon purge-and-trap gas chromatography/isotope ratio mass spectrometry (PT-GC/IRMS) system. The measured δ¹⁸ O values of the produced N₂ O were plotted against known δ¹⁸ O_NO2- and δ¹⁸ O_NO3- values to evaluate the influence of exchange of an oxygen atom with H₂ O during the conversion of NO₂ ^- into N₂ O and NO₃ ^- into N₂ O, respectively.

RESULTS

The degree of oxygen isotope exchange was 10.8 ± 0.3% in the azide method and 5.5 ± 1.0% in the denitrifier method, indicating that the azide method is more susceptible to artifacts arising from differences in the δ¹⁸ O_H2O value of water than the denitrifier method. Thus, the intercept of the standard calibration curve must be corrected to account for differences in δ¹⁸ O_H2O . Abiotic NO₂ -H₂ O equilibrium isotope effect experiments yielded a rate constant of (1.13 ± 007) × 10^-2 (h^-1 ) and an equilibrium isotope effect of 11.9 ± 0.1‰ under the condition of pH = 7.5, 30°C, and 2.5% salinity.

CONCLUSIONS

Oxygen isotope ratio measurements of NO₂ ^- by the azide method are highly sensitive to δ¹⁸ O_H2O as a result of significant oxygen isotope exchange between NO₂ ^- and H₂ O. Therefore, to obtain the most accurate measurements water with the same δ¹⁸ O_H2O value as that of the sample must be used to make the NO₂ ^- and NO₃ ^- standards.

Dual isotope ratio normalization of nitrous oxide by bacterial denitrification of USGS reference materials

We measured the δ values of N₂O using gas chromatography isotope ratio mass spectrometry with a preconcentrator (precon-GC-IRMS). The instrumental precision of the mass spectrometer was restricted to below the shot noise limit, which agreed with the theoretical and experimental results of 0.02‰ (δ¹⁵N) and 0.04‰ (δ¹⁸O), respectively. The precision of the measured δ values was significantly improved by the temperature regulation protocol of the LN₂ preconcentrator, which was monitored by various temperature sensors placed along the U-trap. The reproducibility of the He-diluted N₂O gas measurements resulted in 0.063‰ (δ¹⁵N) and 0.075‰ (δ¹⁸O) due to additional sources of uncertainty in the vials used for autosampling and in the general preconcentration process. Multipoint normalization of the dual δ values of the measured N₂O samples was conducted using United States Geological Survey reference materials denitrified by Pseudomonas aureofaciens. Kaiser's ion correction method, based on International Atomic Energy Agency parameters, exhibited low bias for the atomic isotope ratio reduction of the nitrate reference material, for which the oxygen anomaly was considerably high. Dedicated corrections for net isotope fractionation and water exchange were important in improving uncertainties in the procedure for normalizing the oxygen isotope ratio. Blank measurements for correcting biases in isotope ratios caused by pre-dissolved nitrate and nitrite ions in the water solvent led to further improvements, i.e. beyond unevenly controlled net isotope fractionation, throughout the bacterial denitrification process. The uncertainty evaluation revealed that three-point normalization can significantly improve the normalization accuracy compared with two-point normalization. In addition, an alternative strategy was suggested for assigning δ¹⁸O using a CO₂ lab tank, allowing its use as a reference material for N₂O gas tanks.

Measurement of 18 O18 O and 17 O18 O in atmospheric O2 using the 253 Ultra mass spectrometer and applications to stratospheric and tropospheric air samples

RATIONALE

The doubly substituted isotopologues (e.g., ¹⁸ O¹⁸ O, ¹⁷ O¹⁸ O) in atmospheric O₂ are potential tracers for ozone photochemistry and atmospheric temperatures. Their low abundances and isobaric interference are the major analytical challenges. The 253 Ultra high-resolution stable isotope ratio mass spectrometer is suitable for resolving isobaric interferences.

METHODS

O₂ from air is purified using gas chromatography on a packed column filled with molecular sieve 5 Å and cooled to -78°C. The δ¹⁷ O, δ¹⁸ O, Δ¹⁷ O, Δ₃₅ and Δ₃₆ values are measured on the extracted O₂ with the 253 Ultra at medium mass resolution (M/ΔM ~10000) using Faraday detectors for the singly substituted isotopologues and ion counters for the doubly substituted isotopologues.

RESULTS

Interferences from isobars, mainly ³⁵ Cl for ¹⁷ O¹⁸ O and H³⁵ Cl and ³⁶ Ar for ¹⁸ O¹⁸ O, are sufficiently resolved to enable high-precision determination of Δ₃₅ and Δ₃₆ . The Δ₃₅ and Δ₃₆ values of O₂ after photochemical isotope equilibration at -63°C and heating to 850°C agree with the theoretical prediction. The stratospheric Δ₃₅ and Δ₃₆ values are close to isotopic equilibrium at the ambient temperatures. However, the values for tropospheric O₂ differ from those expected at equilibrium.

CONCLUSIONS

The 253 Ultra allows interference-free clumped isotope measurements of O₂ at medium mass resolution. The Δ₃₅ and Δ₃₆ signatures in atmospheric O₂ are mainly governed by O₃ photochemistry, temperature and atmospheric transport. Tropospheric O₂ is isotopically well mixed and retains a significant stratospheric signature.

Quantum cascade laser based absorption spectroscopy for direct monitoring of atmospheric N2O isotopes

A compact high-resolution spectroscopic sensor using a thermoelectrically (TE) cooled continuous-wave (CW) room temperature (RT) quantum cascade laser (QCL) operating at 4.6 μm, is employed for simultaneous detection of three main isotopic species (¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O and ¹⁴N¹⁴N¹⁶O). To enable a high-precision analysis of N₂O isotopic species at ambient mixing ratios, a liquid nitrogen-free preconcentration unit is built to trap and load atmospheric N₂O. The absorption spectra of ¹⁴N¹⁵N¹⁶O, ¹⁵N¹⁴N¹⁶O, and ¹⁴N¹⁴N¹⁶O between 2188.6 cm^-1 and 2189 cm^-1 are measured, and the respective ratios of the rare to the abundant isotopologues abundances are demonstrated. Moreover, spectroscopic parameters of pressure-broadening coefficient for selected absorption lines have been determined, and a good agreement is obtained by comparing with HITRAN database.

Sources and atmospheric processing of size segregated aerosol particles revealed by stable carbon isotope ratios and chemical speciation

Size-segregated aerosol particles were collected during winter sampling campaigns at a coastal (55°37' N, 21°03'E) and an urban (54°64' N, 25°18' E) site. Organic compounds were thermally desorbed from the samples at different temperature steps ranging from 100 °C to 350 °C. The organic matter (OM) desorbed at each temperature step is analysed for stable carbon isotopes using an isotope ratio mass spectrometer (IRMS) and for individual organic compounds using a Proton Transfer Reaction Time-of-Flight Mass Spectrometer (PTR-MS). The OM desorbed at temperatures <200 °C was classified as less refractory carbon and the OM desorbed at temperatures between 200 °C and 350 °C was classified as more refractory carbon. At the coastal site, we identified two distinct time periods. The first period was more frequently influenced by marine air masses than the second time period, which was characterized by Easterly wind directions and continental air masses. During the first period OM contained a large fraction of hydrocarbons and had a carbon isotopic signature typical of liquid fossil fuels in the region. Organic mass spectra provide strong evidence that shipping emissions are a significant source of OM at this coastal site. The isotopic and chemical composition of OM during the second period at the coastal site was similar to the composition at the urban site. There was a clear distinction in source contribution between the less refractory OM and the more refractory OM at these sites. According to the source apportionment method used in this study, we were able to identify fossil fuel burning as predominant source of the less refractory OM in the smallest particles (D₅₀ < 0.18 μm), and biomass burning as predominant source of the more refractory OM in the larger size range (0.32 < D₅₀ < 1 μm).

Preliminary assessment of stable nitrogen and oxygen isotopic composition of USGS51 and USGS52 nitrous oxide reference gases and perspectives on calibration needs

RATIONALE

Despite a long history and growing interest in isotopic analyses of N₂ O, there is a lack of isotopically characterized N₂ O isotopic reference materials (standards) to enable normalization and reporting of isotope-delta values. Here we report the isotopic characterization of two pure N₂ O gas reference materials, USGS51 and USGS52, which are now available for laboratory calibration (https://isotopes.usgs.gov/lab/referencematerials.html).

METHODS

A total of 400 sealed borosilicate glass tubes of each N₂ O reference gas were prepared from a single gas filling of a high vacuum line. We demonstrated isotopic homogeneity via dual-inlet isotope-ratio mass spectrometry. Isotopic analyses of these reference materials were obtained from eight laboratories to evaluate interlaboratory variation and provide preliminary isotopic characterization of their δ¹⁵ N, δ¹⁸ O, δ¹⁵ N^α , δ¹⁵ N^β and site preference (S_P ) values.

RESULTS

The isotopic homogeneity of both USGS51 and USGS52 was demonstrated by one-sigma standard deviations associated with the determinations of their δ¹⁵ N, δ¹⁸ O, δ¹⁵ N^α , δ¹⁵ N^β and S_P values of 0.12 mUr or better. The one-sigma standard deviations of S_P measurements of USGS51 and USGS52 reported by eight laboratories participating in the interlaboratory comparison were 1.27 and 1.78 mUr, respectively.

CONCLUSIONS

The agreement of isotope-delta values obtained in the interlaboratory comparison was not sufficient to provide reliable accurate isotope measurement values for USGS51 and USGS52. We propose that provisional values for the isotopic composition of USGS51 and USGS52 determined at the Tokyo Institute of Technology can be adopted for normalizing and reporting sample data until further refinements are achieved through additional calibration efforts.

Estimating N2 O processes during grassland renewal and grassland conversion to maize cropping using N2 O isotopocules

RATIONALE

Enhanced nitrous oxide (N₂ O) emissions can occur following grassland break-up for renewal or conversion to maize cropping, but knowledge about N₂ O production pathways and N₂ O reduction to N₂ is very limited. A promising tool to address this is the combination of mass spectrometric analysis of N₂ O isotopocules and an enhanced approach for data interpretation.

METHODS

The isotopocule mapping approach was applied to field data using a δ¹⁵ N^sp_N2O and δ¹⁸ O_N2O map to simultaneously determine N₂ O production pathways contribution and N₂ O reduction for the first time. Based on the isotopic composition of N₂ O produced and literature values for specific N₂ O pathways, it was possible to distinguish: (i) heterotrophic bacterial denitrification and/or nitrifier denitrification and (ii) nitrification and/or fungal denitrification and the contribution of N₂ O reduction.

RESULTS

The isotopic composition of soil-emitted N₂ O largely resembled the known end-member values for bacterial denitrification. The isotopocule mapping approach indicated different effects of N₂ O reduction on the isotopic composition of soil-emitted N₂ O for the two soils under study. Differing N₂ O production pathways in different seasons were not observed, but management events and soil conditions had a significant impact on pathway contribution and N₂ O reduction. N₂ O reduction data were compared with a parallel ¹⁵ N-labelling experiment.

CONCLUSIONS

The field application of the isotopocule mapping approach opens up new prospects for studying N₂ O production and consumption of N₂ O in soil simultaneously based on mass spectrometric analysis of natural abundance N₂ O. However, further studies are needed in order to properly validate the isotopocule mapping approach.

Quantifying nitrous oxide production pathways in wastewater treatment systems using isotope technology - A critical review

Nitrous oxide (N₂O) is an important greenhouse gas and an ozone-depleting substance which can be emitted from wastewater treatment systems (WWTS) causing significant environmental impacts. Understanding the N₂O production pathways and their contribution to total emissions is the key to effective mitigation. Isotope technology is a promising method that has been applied to WWTS for quantifying the N₂O production pathways. Within the scope of WWTS, this article reviews the current status of different isotope approaches, including both natural abundance and labelled isotope approaches, to N₂O production pathways quantification. It identifies the limitations and potential problems with these approaches, as well as improvement opportunities. We conclude that, while the capabilities of isotope technology have been largely recognized, the quantification of N₂O production pathways with isotope technology in WWTS require further improvement, particularly in relation to its accuracy and reliability.

Isotopocule analysis of biologically produced nitrous oxide in various environments

Natural abundance ratios of isotopocules, molecules that have the same chemical constitution and configuration, but that only differ in isotope substitution, retain a record of a compound's origin and reactions. A method to measure isotopocule ratios of nitrous oxide (N₂ O) has been established by using mass analysis of molecular ions and fragment ions. The method has been applied widely to environmental samples from the atmosphere, ocean, fresh water, soils, and laboratory-simulation experiments. Results show that isotopocule ratios, particularly the ¹⁵ N-site preference (difference between isotopocule ratios ¹⁴ N¹⁵ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O and ¹⁵ N¹⁴ N¹⁶ O/¹⁴ N¹⁴ N¹⁶ O), have a wide range that depends on their production and consumption processes. Observational and laboratory studies of N₂ O related to biological processes are reviewed and discussed to elucidate complex material cycles of this trace gas, which causes global warming and stratospheric ozone depletion. © 2015 Wiley Periodicals, Inc. Mass Spec Rev 36:135-160, 2017.

Updates to instrumentation and protocols for isotopic analysis of nitrate by the denitrifier method

RATIONALE

The denitrifier method allows for highly sensitive measurement of the (15) N/(14) N (δ(15) N value) and (18) O/(16) O (δ(18) O value) of nitrate dissolved in natural waters and for highly sensitive δ(15) N measurement of other N forms (e.g., organic N) that can be converted into nitrate. Here, updates to instrumentation and protocols are described, and improvements in data quality are demonstrated.

METHODS

A 'heart cut' of the N2 O was implemented in the extraction system to (1) minimize introduction of contaminants into the mass spectrometer, reducing isotopic drift and (2) decrease the fraction of sample lost at the open split to improve sensitivity. Referencing protocols were updated, including a correction scheme for a weak dependence of nitrate δ(18) O values on nitrate concentration. Analyses of samples from the US GEOTRACES North Atlantic Program and of reference solutions from the same analysis batches were used to characterize performance.

RESULTS

The drift is typically <0.1‰ for both δ(15) N and δ(18) O values. Within-batch and inter-batch replication yields 1 standard deviation (SD) of ≤0.06‰ for δ(15) N values and ≤0.14‰ for δ(18) O values down to 5 μM nitrate and ≤0.08‰ and ≤0.23‰ at 2 and 1 μM. The blank is typically 0.06 nmol N, 0.3% of the N in a 20 nmol N sample. Differences between reference materials in seawater are indistinguishable from reported differences for δ(15) N values, with a contraction for δ(18) O values of ≤5%.

CONCLUSIONS

The new instrumentation and protocols yield nitrate isotopic data with external precision of ≤0.1‰ for large sample sets such as those derived from oceanographic sections. Further study should investigate the causes of (1) the weak dependence of nitrate δ(18) O values on nitrate concentration and (2) the inter-batch variation in the δ(18) O contraction (due mostly to oxygen atom exchange with water). Nevertheless, comprehensive correction schemes are in place for the measurement of both the δ(15) N and δ(18) O values of nitrate. Copyright © 2016 John Wiley & Sons, Ltd.

Isotope fractionation factors controlling isotopocule signatures of soil-emitted N₂O produced by denitrification processes of various rates

RATIONALE

This study aimed (i) to determine the isotopic fractionation factors associated with N2O production and reduction during soil denitrification and (ii) to help specify the factors controlling the magnitude of the isotope effects. For the first time the isotope effects of denitrification were determined in an experiment under oxic atmosphere and using a novel approach where N2O production and reduction occurred simultaneously.

METHODS

Soil incubations were performed under a He/O2 atmosphere and the denitrification product ratio [N2O/(N2 + N2O)] was determined by direct measurement of N2 and N2O fluxes. N2O isotopocules were analyzed by mass spectrometry to determine δ(18)O, δ(15)N and (15)N site preference within the linear N2O molecule (SP). An isotopic model was applied for the simultaneous determination of net isotope effects (η) of both N2O production and reduction, taking into account emissions from two distinct soil pools.

RESULTS

A clear relationship was observed between (15)N and (18)O isotope effects during N2O production and denitrification rates. For N2O reduction, diverse isotope effects were observed for the two distinct soil pools characterized by different product ratios. For moderate product ratios (from 0.1 to 1.0) the range of isotope effects given by previous studies was confirmed and refined, whereas for very low product ratios (below 0.1) the net isotope effects were much smaller.

CONCLUSIONS

The fractionation factors associated with denitrification, determined under oxic incubation, are similar to the factors previously determined under anoxic conditions, hence potentially applicable for field studies. However, it was shown that the η(18)O/η(15)N ratios, previously accepted as typical for N2O reduction processes (i.e., higher than 2), are not valid for all conditions.

Proper interpretation of dissolved nitrous oxide isotopes, production pathways, and emissions requires a modelling approach

Stable isotopes ([Formula: see text]15N and [Formula: see text]18O) of the greenhouse gas N2O provide information about the sources and processes leading to N2O production and emission from aquatic ecosystems to the atmosphere. In turn, this describes the fate of nitrogen in the aquatic environment since N2O is an obligate intermediate of denitrification and can be a by-product of nitrification. However, due to exchange with the atmosphere, the [Formula: see text] values at typical concentrations in aquatic ecosystems differ significantly from both the source of N2O and the N2O emitted to the atmosphere. A dynamic model, SIDNO, was developed to explore the relationship between the isotopic ratios of N2O, N2O source, and the emitted N2O. If the N2O production rate or isotopic ratios vary, then the N2O concentration and isotopic ratios may vary or be constant, not necessarily concomitantly, depending on the synchronicity of production rate and source isotopic ratios. Thus prima facie interpretation of patterns in dissolved N2O concentrations and isotopic ratios is difficult. The dynamic model may be used to correctly interpret diel field data and allows for the estimation of the gas exchange coefficient, N2O production rate, and the production-weighted [Formula: see text] values of the N2O source in aquatic ecosystems. Combining field data with these modelling efforts allows this critical piece of nitrogen cycling and N2O flux to the atmosphere to be assessed.

Fully automated, high-precision instrumentation for the isotopic analysis of tropospheric N2O using continuous flow isotope ratio mass spectrometry

RATIONALE

Measurements of the isotopic composition of nitrous oxide in the troposphere have the potential to bring new information about the uncertain N2O budget, which mole fraction data alone have not been able to resolve. Characterizing the expected subtle variations in tropospheric N2O isotopic composition demands high-precision and high-frequency measurements. To enable useful observations of N2O isotopic composition in tropospheric air to reduce N2O source and sink uncertainty, it was necessary to develop a high-precision measurement system with fully automated capabilities for autonomous deployment at remote research stations.

METHODS

A fully automated pre-concentration system for high-precision measurements of N2O isotopic composition (δ(15)N(β) , δ(15)N(α), δ(18)O) in tropospheric air has been developed which combines a custom liquid-cryogen-free cryo-trapping system and gas chromatograph interfaced to a continuous flow isotope ratio mass spectrometry (IRMS) system. A quadrupole mass spectrometer was coupled in parallel to the IRMS system during development to evaluate peak interference. Multi-port inlet and fully-automated capabilities allow streamlined analyses between in situ air inlet, air standards, flask air sample, or other gas source in exactly replicated analysis sequences.

RESULTS

The system has the highest precision to date for (15)N site-specific composition results (δ(15) N(α) ±0.11‰, δ(15)N(β) ±0.14‰ (1σ)), attributed mostly to uniformity of analytical cycles and particular attention to fluorocarbon interference noted for (15)N site-specific measurements by IRMS. Air measurements demonstrated the fully automated capacity and performance.

CONCLUSIONS

The system makes substantial headway in measurement precision, possibly defining the limits of IRMS measurement capabilities in low concentration N2O air samples, with fully automated capabilities to enable high-frequency in situ measurements.

Novel laser spectroscopic technique for continuous analysis of N2O isotopomers--application and intercomparison with isotope ratio mass spectrometry

RATIONALE

Nitrous oxide (N(2)O), a highly climate-relevant trace gas, is mainly derived from microbial denitrification and nitrification processes in soils. Apportioning N(2)O to these source processes is a challenging task, but better understanding of the processes is required to improve mitigation strategies. The N(2)O site-specific (15)N signatures from denitrification and nitrification have been shown to be clearly different, making this signature a potential tool for N(2)O source identification. We have applied for the first time quantum cascade laser absorption spectroscopy (QCLAS) for the continuous analysis of the intramolecular (15)N distribution of soil-derived N(2)O and compared this with state-of-the-art isotope ratio mass spectrometry (IRMS).

METHODS

Soil was amended with nitrate and sucrose and incubated in a laboratory setup. The N(2)O release was quantified by FTIR spectroscopy, while the N(2)O intramolecular (15)N distribution was continuously analyzed by online QCLAS at 1 Hz resolution. The QCLAS results on time-integrating flask samples were compared with those from the IRMS analysis.

RESULTS

The analytical precision (2σ) of QCLAS was around 0.3‰ for the δ(15)N(bulk) and the (15)N site preference (SP) for 1-min average values. Comparing the two techniques on flask samples, excellent agreement (R(2)= 0.99; offset of 1.2‰) was observed for the δ(15)N(bulk) values while for the SP values the correlation was less good (R(2 )= 0.76; offset of 0.9‰), presumably due to the lower precision of the IRMS SP measurements.

CONCLUSIONS

These findings validate QCLAS as a viable alternative technique with even higher precision than state-of-the-art IRMS. Thus, laser spectroscopy has the potential to contribute significantly to a better understanding of N turnover in soils, which is crucial for advancing strategies to mitigate emissions of this efficient greenhouse gas.

Isotopologue enrichment factors of N(2)O reduction in soils

Isotopic signatures can be used to study sink and source processes of N(2)O, but the success of this approach is limited by insufficient knowledge on the isotope fractionation factors of the various reaction pathways. We investigated isotope enrichment factors of the N(2)O-to-N(2) step of denitrification (epsilon) in two arable soils, a silt-loam Haplic Luvisol and a sandy Gleyic Podzol. In addition to the epsilon of (18)O (epsilon(18O)) and of average (15)N (epsilon(bulk)), the epsilon of the (15)N site preference within the linear N(2)O molecule (epsilon(SP)) was also determined. Soils were anaerobically incubated in gas-tight bottles with N(2)O added to the headspace to induce N(2)O reduction. Pre-treatment included the removal of NO(3) (-) to prevent N(2)O production. Gas samples were collected regularly to determine the dynamics of N(2)O reduction, the time course of the isotopic signatures of residual N(2)O, and the associated isotope enrichment factors. To vary reduction rates and associated fractionation factors, several treatments were established including two levels of initial N(2)O concentration and anaerobic pre-incubation with or without addition of N(2)O. N(2)O reduction rates were affected by the soil type and initial N(2)O concentration. The epsilon(18O) and epsilon(bulk) ranged between -13 and -20 per thousand, and between -5 and -9 per thousand, respectively. Both quantities were more negative in the Gleyic Podzol. The epsilon of the central N position (epsilon(alpha)) was always larger than that of the peripheral N-position (epsilon(beta)), giving epsilon(SP) of -4 to -8 per thousand. The ranges and variation patterns of epsilon were comparable with those from previous static incubation studies with soils. Moreover, we found a relatively constant ratio between epsilon(18O) and epsilon(bulk) which is close to the default ratio of 2.5 that had been previously suggested. The fact that different soils exhibited comparable epsilon under certain conditions suggests that these values could serve to identify N(2)O reduction from the isotopic fingerprints of N(2)O emitted from any soil.

Determination of N2O isotopomers with quantum cascade laser based absorption spectroscopy

We present an analytical technique based on direct absorption laser spectroscopy for high precision and simultaneous determination of the mixing ratios of the most abundant nitrous oxide isotopic species: (14)N(15)N(16)O, (15)N(14)N(16)O and (14)N(2) (16)O. A precision of 0.5 ??? was achieved for the site specific isotope ratios of N(2)O at 90 ppm using an averaging time of 300 s.

A review of stable isotope techniques for N2O source partitioning in soils: recent progress, remaining challenges and future considerations

Nitrous oxide is produced in soil during several processes, which may occur simultaneously within different micro-sites of the same soil. Stable isotope techniques have a crucial role to play in the attribution of N(2)O emissions to different microbial processes, through estimation (natural abundance, site preference) or quantification (enrichment) of processes based on the (15)N and (18)O signatures of N(2)O determined by isotope ratio mass spectrometry. These approaches have the potential to become even more powerful when linked with recent developments in secondary isotope mass spectrometry, with microbial ecology, and with modelling approaches, enabling sources of N(2)O to be considered at a wide range of scales and related to the underlying microbiology. Such source partitioning of N(2)O is inherently challenging, but is vital to close the N(2)O budget and to better understand controls on the different processes, with a view to developing appropriate management practices for mitigation of N(2)O. In this respect, it is essential that as many of the contributing processes as possible are considered in any study aimed at source attribution, as mitigation strategies for one process may not be appropriate for another. To aid such an approach, here the current state of the art is critically examined, remaining challenges are highlighted, and recommendations are made for future direction.

Determination of the 15N/14N, 17O/16O, and 18O/16O ratios of nitrous oxide by using continuous-flow isotope-ratio mass spectrometry

We developed a rapid, sensitive, and automated analytical system to determine the delta15N, delta18O, and Delta17O values of nitrous oxide (N2O) simultaneously in nanomolar quantities for a single batch of samples by continuous-flow isotope-ratio mass spectrometry (CF-IRMS) without any cumbersome and time-consuming pretreatments. The analytical system consisted of a vacuum line to extract and purify N2O, a gas chromatograph for further purification of N2O, an optional thermal furnace to decompose N2O to O2, and a CF-IRMS system. We also used pneumatic valves and pneumatic actuators in the system so that we could operate it automatically with timing software on a personal computer. The analytical precision was better than 0.12 per thousand for delta15N with >4 nmol N2O injections, 0.25 per thousand for delta18O with >4 nmol N2O injections, and 0.20 per thousand for Delta17O with >20 nmol N2O injections for a single measurement. We were also easily able to improve the precision (standard errors) to better than 0.05 per thousand for delta15N, 0.10 per thousand for delta18O, and 0.10 per thousand for Delta17O through multiple analyses with more than four repetitions with 190 nmol samples using the automated analytical system. Using the system, the delta15N, delta18O, and Delta17O values of N2O can be quantified not only for atmospheric samples, but also for other gas or liquid samples with low N2O content, such as soil gas or natural water. Here, we showed the first ever Delta17O measurements of soil N2O.

Carbon Position-Specific Isotope Analysis of Alanine and Phenylalanine Analogues Exhibiting Nonideal Pyrolytic Fragmentation

Recent advances in gas chromatography combustion-isotope ratio mass spectrometry (GCC-IRMS) has made compound-specific isotope analysis routine, but reports on position-specific isotopic analysis are still scarce. On-line GC-pyrolysis (Py) coupled to GCC-IRMS is reported here for isolation and isotopic characterization of alaninol and phenethylamine, analogues of alanine and phenylalanine, respectively. Ideally, pyrolytic fragments will originate from unique sites within the parent molecule, and isotope ratios for each position within the parent can either be measured directly or calculated from fragment isotope ratios without substantially degrading the analytical precision. Alaninol pyrolysis yielded several fragments, of which CO and CH4 were used for isotope ratio calculations. Isotope labeling experiments showed that CO derived entirely from the C(1) position, while all three positions of alaninol contributed to CH4 (29.0 +/- 0.3% from C(1), 3.6 +/- 0.2% from C(2), and 66.9 +/- 1.1% from C(3)). We demonstrate iterative use of mass balance to calculate isotope ratios from all positions despite the nonideal positional fidelity of CH4. Pyrolysis of phenethylamine generated benzene and toluene fragments. Benzene derived entirely from C(ring), and toluene was proportionately formed from C(3) and C(ring). Relative intramolecular isotope ratios (Deltadelta13C) were calculated directly from delta13C of fragments or indirectly by mass balance. Though the C(3) isotope ratio was calculated from the benzene and toluene fragments, propagation of errors showed that the final precision of the determination was degraded due to the small contribution that C(3) makes to toluene. Samples of each amino acid from four different vendors showed natural variability between sources, especially at the C(1) position of alaninol (range of Deltadelta13C approximately 50 per thousand). The average precision was SD(Deltadelta13C) < 0.20 per thousand for directly measured positions of alaninol and phenethylamine. The precision of indirectly measured positions was poorer (SD(Deltadelta13C) = 0.94 per thousand for alaninol, 6.54 per thousand for phenethylamine) due to propagation of errors. These data demonstrate that GC-Py-GCC-IRMS data can be used to extract high-precision isotope ratios from amino acids despite nonideal positional fidelity in fragments and that natural intramolecular variability in delta13C can be used to distinguish different sources of amino acids.