A gas chromatography/pyrolysis/isotope ratio mass spectrometry system for high-precision deltaD measurements of atmospheric methane extracted from ice cores

Fractionation of Methane Isotopologues during Preparation for Analysis from Ambient Air

Preconcentration of methane (CH4) from air is a critical sampling step in the measurement of singly and doubly substituted isotopologue ratios. We demonstrate the potential for isotope fractionation during preconcentration onto and elution from the common trapping material HayeSep-D and investigate its significance in laser spectroscopy measurements. By altering the trapping temperature for adsorption, the flow direction of CH4 through the trap and the time at which CH4 is eluted during a desorption temperature ramp, we explain the mechanisms behind fractionation affecting δ13C(CH4) and δ2H(CH4). The results highlight that carbon isotope fractionation is driven by advection and diffusion, while hydrogen isotope fractionation is driven by the interaction of CH4 with the adsorbing material (tending to smaller isotopic effects at higher temperatures). We have compared the difference between the measured isotope ratio of sample gases (compressed whole air and a synthetic mixture of CH4 at ambient amount fraction in an N2 matrix) and their known isotopic composition. An open-system Rayleigh model is used to quantify the magnitude of isotopic fractionation affecting measured δ13C(CH4) and δ2H(CH4), which can be used to calculate the possible magnitude of isotopic fractionation given the recovery percentage. These results provide a quantitative understanding of isotopic fractionation during the sample preparation of CH4 from ambient air. The results also provide valuable insights applicable to other cryogenic preconcentration systems, such as those for measurements that probe the distribution of rarer isotopologues.

Glacial/interglacial wetland, biomass burning, and geologic methane emissions constrained by dual stable isotopic CH4 ice core records

Atmospheric methane (CH4) records reconstructed from polar ice cores represent an integrated view on processes predominantly taking place in the terrestrial biogeosphere. Here, we present dual stable isotopic methane records [δ13CH4 and δD(CH4)] from four Antarctic ice cores, which provide improved constraints on past changes in natural methane sources. Our isotope data show that tropical wetlands and seasonally inundated floodplains are most likely the controlling sources of atmospheric methane variations for the current and two older interglacials and their preceding glacial maxima. The changes in these sources are steered by variations in temperature, precipitation, and the water table as modulated by insolation, (local) sea level, and monsoon intensity. Based on our δD(CH4) constraint, it seems that geologic emissions of methane may play a steady but only minor role in atmospheric CH4 changes and that the glacial budget is not dominated by these sources. Superimposed on the glacial/interglacial variations is a marked difference in both isotope records, with systematically higher values during the last 25,000 y compared with older time periods. This shift cannot be explained by climatic changes. Rather, our isotopic methane budget points to a marked increase in fire activity, possibly caused by biome changes and accumulation of fuel related to the late Pleistocene megafauna extinction, which took place in the course of the last glacial.

Hydrogen Isotopic Composition of Arctic and Atmospheric CH4 Determined by a Portable Near-Infrared Cavity Ring-Down Spectrometer with a Cryogenic Pre-Concentrator

In this study, near-infrared continuous wave cavity ring-down spectroscopy was applied to the measurement of the δ²H of methane (CH₄). The cavity ring-down spectrometer (CRDS) system consisted of multiple DFB laser diodes to optimize selection of spectral line pairs. By rapidly switching measurements between spectral line peaks and the baseline regions, the long-term instrumental drift was minimized, substantially increasing measurement precision. The CRDS system coupled with a cryogenic pre-concentrator measured the δ²H of terrestrial atmospheric CH₄ from 3 standard liters of air with a precision of ±1.7‰. The rapidity with which both C and H isotopic measurements of CH₄ can be made with the CRDS will enable hourly monitoring of diurnal variations in terrestrial atmospheric CH₄ signatures that can be used to increase the resolution of global climate models for the CH₄ cycle. Although the current instrument is not capable of measuring the δ²H of 10 ppbv of martian CH₄, current technology does exist that could make this feasible for future spaceflight missions. As biological and abiotic CH₄ sources have overlapping carbon isotope signatures, dual-element (C and H) analysis is key to reliable differentiation of these sources. Such an instrument package would therefore offer improved ability to determine whether or not the CH₄ recently detected in the martian atmosphere is biogenic in origin. Key Words: Arctic-Hydrogen isotopes-Atmospheric CH₄-CRDS-Laser. Astrobiology 16, 787-797.

Fully automated, high-throughput instrumentation for measuring the δ13C value of methane and application of the instrumentation to rice paddy samples

RATIONALE

The stable carbon isotope ratio ((13)C/(12)C or δ(13)C value) of methane (CH4) produced in methanogenic environments contains information about primary source material, CH4 production pathways, degree of oxidation, and transport. However, the availability of δ(13)C-CH4 data is severely limited because isotope analysis methods are low throughput, owing primarily to the need for manual processing steps. High-throughput, fully automated measurement is necessary to facilitate the use of the δ(13)C signature in understanding CH4 biogeochemistry.

METHODS

We modified a conventional continuous-flow (CF) gas chromatography/combustion/isotope ratio mass spectrometry (IRMS) instrument system by incorporating (i) automated sample injection, (ii) a newly developed temperature-control unit for preconcentration and cryofocus traps, and (iii) an automatic system for liquid-nitrogen refilling. The system, which could run unattended for 1 day, was used to obtain δ(13)C-CH4 data for CH4 samples collected from an irrigated rice paddy with an automated closed-chamber system.

RESULTS

Using the fully automated CF-IRMS system, we measured δ(13)C-CH4 data for 77 samples during a 21.5-h continuous run (17 min per sample) with high precision (1σ = 0.11‰, reproducibility) and moderate consumption of liquid nitrogen (11 L). Application of the system to CH4 samples obtained from the rice paddy revealed distinct seasonal and diurnal variations in δ(13)C values with the highest temporal resolution ever reported.

CONCLUSIONS

A fully automated, high-throughput system for the measurement of δ(13)C-CH4 values was developed and used to analyze air samples obtained from a rice paddy. Our results demonstrate the high potential of this system for obtaining δ(13)C data useful for process-level understanding of CH4 biogeochemistry with respect to spatiotemporal variation of CH4 sources and how that variation is affected by environmental and management factors.

'Extreme mass spectrometry': the role of mass spectrometry in the study of the Antarctic environment

A focus on the studies of the Antarctic environment that have been performed by mass spectrometry is presented herein; our aim is to give evidence of the essential role of this instrumental technique in the framework of the scientific research in Antarctica, with a comprehensive review on the main literature of the last two decades. Due to the wideness of the topic, the present review is limited to the determination of organic pollutants, natural molecules and biomarkers in Antarctica, thus excluding elemental analysis and studies on inorganic species. The work has been divided into five sections, on the basis of the considered environmental compartment: air; ice and snow; seawater, pack ice and lakes; soil and sediments; and organisms and biomarkers.

δ13C and δ2H measurement of methane from ecological and geological sources by gas chromatography/combustion/pyrolysis isotope-ratio mass spectrometry

RATIONALE

The carbon and hydrogen isotopes of methane are useful in differentiating biological (e.g. wetlands, ruminants, biomass burning) and geological methane sources (e.g. fossil fuels, gas hydrates), as well as quantifying pathways of methanotrophism. Continuous-flow isotopic measurements of methane present a set of analytical challenges, including sample size restrictions and separation of CH4 from atmosphere, hydrocarbons, and CO2 .

METHODS

Small-scale modifications were made to a commercial trace-gas preconcentration and sampling unit (Thermo Scientific PreCon-GasBench) for improved isotopic analysis of methane (δ(13)C/δ(2)H) across a range of gas concentrations.

RESULTS

The long-term reproducibility of δ(13)C-CH4 values is less than ±0.2‰ (1σ). The limit-of-quantitation of δ(13)C-CH4 values is less than 0.8 nmol, conveniently measurable within standard gas sampling vials. A reproducibility of better than ±4‰ (1σ) is regularly achieved for δ(2) H values from sample sizes greater than 2 nmol. The range of measurement, for both δ(13)C and δ(2)H values, is easily extended from ambient concentration (~1.7 ppm-v) for preconcentrated samples to percent methane concentrations under subsampling.

CONCLUSIONS

The automated measurement of δ(13)C-CH4 and δ(2)H-CH4 values, from ambient to percentage concentrations, is possible with minimal modifications to a commercial preconcentration/gas chromatography inlet. Sample matrix interferences (CO2 , Cn Hy , air) are eliminated and simultaneous isotopic measurements of methane and CO2 and/or C1 -C4 light hydrocarbons are possible, while still retaining functionality for isotopic measurements of other gas species (e.g. CO2, N2, O2).