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

Subsurface biogeochemical cycling of nitrogen in the actively serpentinizing Samail Ophiolite, Oman

Nitrogen (N) is an essential element for life. N compounds such as ammonium ( NH 4 + ) may act as electron donors, while nitrate ( NO 3 - ) and nitrite ( NO 2 - ) may serve as electron acceptors to support energy metabolism. However, little is known regarding the availability and forms of N in subsurface ecosystems, particularly in serpentinite-hosted settings where hydrogen (H₂) generated through water-rock reactions promotes habitable conditions for microbial life. Here, we analyzed N and oxygen (O) isotope composition to investigate the source, abundance, and cycling of N species within the Samail Ophiolite of Oman. The dominant dissolved N species was dependent on the fluid type, with Mg²⁺- HCO 3 - type fluids comprised mostly of NO 3 - , and Ca²⁺-OH^- fluids comprised primarily of ammonia (NH₃). We infer that fixed N is introduced to the serpentinite aquifer as NO 3 - . High concentrations of NO 3 - (>100 μM) with a relict meteoric oxygen isotopic composition (δ¹⁸O ~ 22‰, Δ¹⁷O ~ 6‰) were observed in shallow aquifer fluids, indicative of NO 3 - sourced from atmospheric deposition (rainwater NO 3 - : δ¹⁸O of 53.7‰, Δ¹⁷O of 16.8‰) mixed with NO 3 - produced in situ through nitrification (estimated endmember δ¹⁸O and Δ¹⁷O of ~0‰). Conversely, highly reacted hyperalkaline fluids had high concentrations of NH₃ (>100 μM) with little NO 3 - detectable. We interpret that NH₃ in hyperalkaline fluids is a product of NO 3 - reduction. The proportionality of the O and N isotope fractionation (¹⁸ε / ¹⁵ε) measured in Samail Ophiolite NO 3 - was close to unity (¹⁸ε / ¹⁵ε ~ 1), which is consistent with dissimilatory NO 3 - reduction with a membrane-bound reductase (NarG); however, abiotic reduction processes may also be occurring. The presence of genes commonly involved in N reduction processes (narG, napA, nrfA) in the metagenomes of biomass sourced from aquifer fluids supports potential biological involvement in the consumption of NO 3 - . Production of NH 4 + as the end-product of NO 3 - reduction via dissimilatory nitrate reduction to ammonium (DNRA) could retain N in the subsurface and fuel nitrification in the oxygenated near surface. Elevated bioavailable N in all sampled fluids indicates that N is not likely limiting as a nutrient in serpentinites of the Samail Ophiolite.

Determination of the triple oxygen isotopic composition of tropospheric ozone in terminal positions using a multistep nitrite-coated filter-pack system

RATIONALE

The triple oxygen isotopic composition (Δ¹⁷ O) of tropospheric ozone (O₃ ) is a useful tracer for identifying the source and is essential for clarifying the atmospheric chemistry of oxidants. However, the single nitrite-coated filter method is inaccurate owing to the nitrate blank produced through the reaction of nitrite and oxygen compounds other than O₃ .

METHODS

A multistep nitrite-coated filter-pack system is newly adopted to transfer the O-atoms in terminal positions of O₃ to nitrite on each filter to determine the Δ¹⁷ O of O₃ in terminal positions (denoted as Δ¹⁷ O(O₃ )_term ). The NO₃ ^- produced by this reaction is chemically converted into N₂ O, and continuous-flow isotope ratio mass spectrometry (CF-IRMS) is used to determine the oxygen isotopic compositions.

RESULTS

The reciprocal of the NO₃ ^- quantities on the nitrite-coated filters in each sample showed a strong linear relationship with Δ¹⁷ O of NO₃ ^- . Using the linear relation, we corrected the changes in Δ¹⁷ O of NO₃ ^- on the filters. We verified the accuracy of the new method through the measurement of artificial O₃ with known Δ¹⁷ O(O₃ )_term value that had been determined from the changes in Δ¹⁷ O of O₂ . The Δ¹⁷ O(O₃ )_term of tropospheric O₃ was in agreement with previous studies.

CONCLUSIONS

We accurately determined the δ¹⁸ O and Δ¹⁷ O values of tropospheric O₃ by blank correction using our new method. Measurements of Δ¹⁷ O(O₃ )_term of the ambient troposphere showed 1.1 ± 0.7‰ diurnal variations between daytime (higher) and nighttime (lower) due likely to the formation of the temperature inversion layer at night.

Precision Spectroscopy of Nitrous Oxide Isotopocules with a Cross-Dispersed Spectrometer and a Mid-Infrared Frequency Comb

As a potent greenhouse gas and an ozone-depleting agent, nitrous oxide (N2O) plays a critical role in the global climate. Effective mitigation relies on understanding global sources and sinks, which can be supported through isotopic analysis. We present a cross-dispersed spectrometer, coupled with a mid-infrared frequency comb, capable of simultaneously monitoring all singly substituted, stable isotopic variants of N2O. Rigorous evaluation of the instrument lineshape function and data treatment using a Doppler-broadened, low-pressure gas sample are discussed. Laboratory characterization of the spectrometer demonstrates sub-GHz spectral resolution and an average precision of 6.7 × 10-6 for fractional isotopic abundance retrievals in 1 s.

Triple oxygen isotope analysis of nitrate using isotope exchange cavity ringdown laser spectroscopy

RATIONALE

Triple oxygen isotopes (¹⁶ O/¹⁷ O/¹⁸ O) in nitrate are a valuable tool to ascertain the pathways of nitrate formation in the atmosphere and the fate of nitrate in ecosystems. Here we present a new method for determining Δ¹⁷ O values in nitrates, based on nitrate-water isotope equilibration (IE) and subsequent isotopic analysis of water using cavity ringdown laser spectroscopy (CRDS).

METHODS

Nitrate oxygen (O-NO₃ ^- ) is equilibrated with water oxygen (O-H₂ O) at low pH and 80°C. Subsequently, the δ¹⁷ O and δ¹⁸ O values of equilibrated water are determined by CRDS, scaled to V-SMOW and V-SLAP and calibrated against nitrate standards (USGS-34, USGS-35 and IAEA-NO3). We provide isotopic measurements of synthetic and natural nitrates and a direct inter-lab comparison with the classic method of thermal-decomposition of nitrate followed by isotope ratio mass spectrometry of O₂ (TD-IRMS).

RESULTS

For synthetic NaNO₃ , the precision (1SD) of the IE-CRDS method is 0.8‰ for δ¹⁷ O values, 1.7‰ for δ¹⁸ O values and 0.2‰ for Δ¹⁷ O values when using an O-NO₃ ^- /O-H₂ O ratio greater than 0.0114 ± 0.0001 (e.g. 12 μmol of NO₃ ^- in 50 μL of acid solution). For natural samples, after purification of nitrates by column chemistry and reprecipitation as AgNO₃ , the precision is better than 1.8‰ for δ¹⁷ O values, 3.2‰ for δ¹⁸ O values and 1‰ for Δ¹⁷ O values. IE-CRDS and TD-IRMS yield Δ¹⁷ O values within the analytical errors of the two methods.

CONCLUSIONS

The IE-CRDS method for determining Δ¹⁷ O values in nitrates utilizes a user-friendly and relatively cheaper benchtop analytical instrument, representing an alternative to IRMS-based methods for certain applications.

Evidence for fungal and chemodenitrification based N2O flux from nitrogen impacted coastal sediments

Although increasing atmospheric nitrous oxide (N₂O) has been linked to nitrogen loading, predicting emissions remains difficult, in part due to challenges in disentangling diverse N₂O production pathways. As coastal ecosystems are especially impacted by elevated nitrogen, we investigated controls on N₂O production mechanisms in intertidal sediments using novel isotopic approaches and microsensors in flow-through incubations. Here we show that during incubations with elevated nitrate, increased N₂O fluxes are not mediated by direct bacterial activity, but instead are largely catalysed by fungal denitrification and/or abiotic reactions (e.g., chemodenitrification). Results of these incubations shed new light on nitrogen cycling complexity and possible factors underlying variability of N₂O fluxes, driven in part by fungal respiration and/or iron redox cycling. As both processes exhibit N₂O yields typically far greater than direct bacterial production, these results emphasize their possibly substantial, yet widely overlooked, role in N₂O fluxes, especially in redox-dynamic sediments of coastal ecosystems.

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.

Automated system measuring triple oxygen and nitrogen isotope ratios in nitrate using the bacterial method and N2 O decomposition by microwave discharge

RATIONALE

Triple oxygen and nitrogen isotope ratios in nitrate are powerful tools for assessing atmospheric nitrate formation pathways and their contribution to ecosystems. N₂ O decomposition using microwave-induced plasma (MIP) has been used only for measurements of oxygen isotopes to date, but it is also possible to measure nitrogen isotopes during the same analytical run.

METHODS

The main improvements to a previous system are (i) an automated distribution system of nitrate to the bacterial medium, (ii) N₂ O separation by gas chromatography before N₂ O decomposition using the MIP, (iii) use of a corundum tube for microwave discharge, and (iv) development of an automated system for isotopic measurements. Three nitrate standards with sample sizes of 60, 80, 100, and 120 nmol were measured to investigate the sample size dependence of the isotope measurements.

RESULTS

The δ¹⁷ O, δ¹⁸ O, and Δ¹⁷ O values increased with increasing sample size, although the δ¹⁵ N value showed no significant size dependency. Different calibration slopes and intercepts were obtained with different sample amounts. The slopes and intercepts for the regression lines in different sample amounts were dependent on sample size, indicating that the extent of oxygen exchange is also dependent on sample size. The sample-size-dependent slopes and intercepts were fitted using natural log (ln) regression curves, and the slopes and intercepts can be estimated to apply to any sample size corrections. When using 100 nmol samples, the standard deviations of residuals from the regression lines for this system were 0.5‰, 0.3‰, and 0.1‰, respectively, for the δ¹⁸ O, Δ¹⁷ O, and δ¹⁵ N values, results that are not inferior to those from other systems using gold tube or gold wire.

CONCLUSIONS

An automated system was developed to measure triple oxygen and nitrogen isotopes in nitrate using N₂ O decomposition by MIP. This system enables us to measure both triple oxygen and nitrogen isotopes in nitrate with comparable precision and sample throughput (23 min per sample on average), and minimal manual treatment. Copyright © 2016 John Wiley & Sons, Ltd.

On-line triple oxygen isotope analysis of nitrous oxide using decomposition by microwave discharge

RATIONALE

The oxygen isotope anomaly, Δ(17)O, of N2O and nitrate is useful to elucidate nitrogen oxide dynamics. The previously developed method for Δ(17)O measurement presents difficulty in maintaining optimal conditions of the gold tube for thermal decomposition of N2O to O2 and the Δ(17)O value is also sample size dependent.

METHODS

Trace amounts (5-40 nmol) of N2O were decomposed quantitatively to O2 in a quartz tube by microwave discharge. The O2 was purified using gas chromatography. Triple oxygen isotopes were measured using isotope ratio mass spectrometry. Each step was connected online and was applied to the analysis of nitrate in precipitation samples collected in Yokohama, Japan.

RESULTS

Precision (1σ) of Δ(17)O analysis was better than 0.26‰ when more than 20 nmol of N2O with a small Δ(17)O value (approx. 1‰) was measured. It was better than 0.76‰ when more than 60 nmol of nitrate was converted into N2O using the denitrifier method and then measured on the developed system. The obtained Δ(17)O values in precipitation samples (14.5-26.4‰) agreed with findings from previous studies.

CONCLUSIONS

A novel on-line analytical method was developed to measure the triple oxygen isotopes of N2O using microwave discharge to decompose N2O. This easy-to-use method is free from conditioning of reaction devices, and is applicable to molecules other than N2O such as NO and NO2.

Stable hydrogen isotopic analysis of nanomolar molecular hydrogen by automatic multi-step gas chromatographic separation

We have developed a new automated analytical system that employs a continuous flow isotope ratio mass spectrometer to determine the stable hydrogen isotopic composition (δD) of nanomolar quantities of molecular hydrogen (H(2)) in an air sample. This method improves previous methods to attain simpler and lower-cost analyses, especially by avoiding the use of expensive or special devices, such as a Toepler pump, a cryogenic refrigerator, and a special evacuation system to keep the temperature of a coolant under reduced pressure. Instead, the system allows H(2) purification from the air matrix via automatic multi-step gas chromatographic separation using the coolants of both liquid nitrogen (77 K) and liquid nitrogen + ethanol (158 K) under 1 atm pressure. The analytical precision of the δD determination using the developed method was better than 4‰ for >5 nmol injections (250 mL STP for 500 ppbv air sample) and better than 15‰ for 1 nmol injections, regardless of the δD value, within 1 h for one sample analysis. Using the developed system, the δD values of H(2) can be quantified for atmospheric samples as well as samples of representative sources and sinks including those containing small quantities of H(2) , such as H(2) in soil pores or aqueous environments, for which there is currently little δD data available. As an example of such trace H(2) analyses, we report here the isotope fractionations during H(2) uptake by soils in a static chamber. The δD values of H(2) in these H(2)-depleted environments can be useful in constraining the budgets of atmospheric H(2) by applying an isotope mass balance model.

An injection method for measuring the carbon isotope content of soil carbon dioxide and soil respiration with a tunable diode laser absorption spectrometer

We present a novel technique in which the carbon isotope ratio (delta(13)C) of soil CO(2) is measured from small gas samples (<5 mL) injected into a stream of CO(2)-free air flowing into a tunable diode laser absorption spectrometer (TDL). This new method extends the dynamic range of the TDL to measure CO(2) mole fractions ranging from ambient to pure CO(2), reduces the volume of sample required to a few mL, and does not require field deployment of the instrument. The measurement precision of samples stored for up to 60 days was 0.23 per thousand. The new TDL method was applied with a simple gas well sampling technique to obtain and measure gas samples from shallow soil depth increments for CO(2) mole fraction and delta(13)C analysis, and subsequent determination of the delta(13)C of soil-respired CO(2). The method was tested using an artificial soil system containing a controlled CO(2) source and compared with an independent method using the TDL and an open soil chamber. The profile and chamber estimates of delta(13)C of an artificially produced CO(2) flux were consistent and converged to the delta(13)C of the CO(2) source at steady state, indicating the accuracy of both methods under controlled conditions. The new TDL method, in which a small pulse of sample is measured on a carrier gas stream, is analogous for the TDL technique to the development of continuous-flow configurations for isotope ratio mass spectrometry. While the applications presented here are focused on soil CO(2), this new TDL method could be applied in a number of situations requiring measurement of delta(13)C of CO(2) in small gas samples with ambient to high CO(2) mole fractions.

Simultaneous determination of delta(15)N and delta(18)O of N2O and delta(13)C of CH4 in nanomolar quantities from a single water sample

We have developed a rapid, sensitive, and automated analytical system to simultaneously determine the concentrations and stable isotopic compositions (delta(15)N, delta(18)O, and delta(13)C) of nanomolar quantities of nitrous oxide (N(2)O) and methane (CH(4)) in water, by combining continuous-flow isotope-ratio mass spectrometry and a helium-sparging system to extract and purify the dissolved gases. Our system, which is composed of cold traps and a capillary gas chromatograph that use ultra-pure helium as the carrier gas, achieves complete extraction of N(2)O and CH(4) in a water sample and separation among N(2)O, CH(4), and the other component gases. The flow path following exit from the gas chromatograph was periodically changed to pass the gases through the combustion furnace to convert CH(4) and the other hydrocarbons into CO(2), or to bypass the combustion furnace for the direct introduction of eluted N(2)O into the mass spectrometer, for determining the stable isotopic compositions through monitoring the ions of m/z 44, 45, and 46 of CO(2) (+) and N(2)O(+). The analytical system can be operated automatically with sequential software programmed on a personal computer. Analytical precisions better than 0.2 per thousand and 0.3 per thousand and better than 1.4 per thousand and 2.6 per thousand were obtained for the delta(15)N and delta(18)O of N(2)O, respectively, when more than 6.7 nmol and 0.2 nmol of N(2)O, respectively, were injected. Simultaneously, analytical precisions better than 0.07 per thousand and 2.1 per thousand were obtained for the delta(13)C of CH(4) when more than 5.5 nmol and 0.02 nmol of CH(4), respectively, were injected. In this manner, we can simultaneously determine stable isotopic compositions of a 120 mL water sample with concentrations as low as 1.7 nmol/kg for N(2)O and 0.2 nmol/kg for CH(4).