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Measurement of a Doubly-Substituted Methane Isotopologue, CHD, by Tunable Infrared Laser Direct Absorption Spectroscopy
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Shuhei Ono, David T Wang, Danielle S. Gruen, Barbara Sherwood Lollar, Mark Zahniser, Barry J. McManus, and David D. Nelson Anal. Chem., Just Accepted Manuscript • DOI: 10.1021/ac5010579 • Publication Date (Web): 04 Jun 2014 Downloaded from http://pubs.acs.org on June 10, 2014
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Analytical Chemistry
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Measurement of a Doubly-Substituted Methane
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Isotopologue, 13CH3D, by Tunable Infrared Laser
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Direct Absorption Spectroscopy
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Shuhei Ono,*1, David T. Wang1, Danielle S. Gruen1, Barbara Sherwood Lollar2, Mark S.
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Zahniser3, Barry J. McManus3, David D. Nelson3 1
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Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of
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Technology, Cambridge, MA.
[email protected] 2
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Department of Earth Sciences, University of Toronto, Toronto, ON, Canada
Center for Atmospheric and Environmental Chemistry, Aerodyne Research, Inc., Billerica,
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Massachusetts, USA
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Keywords: Tunable Laser Spectroscopy, Quantum Cascade Laser, Methane, Isotopologue,
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Clumped Isotope
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* Corresponding author,
[email protected] 15
A manuscript prepared for submission to Analytical Chemistry March 18, 2014
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5520 words, 7 figures (equivalent to ~1200 words), 2 tables (250 to 400 words)
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Analytical Chemistry
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Abstract
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Methane is an important energy resource and significant long-lived greenhouse gas. Carbon
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and hydrogen isotope ratios have been used to better constrain the sources of methane, but
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interpretations based on these two parameters alone can often be inconclusive. The precise
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measurement of a doubly-substituted methane isotopologue, 13CH3D, is expected to add a critical
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new dimension to source signatures by providing the apparent temperature at which methane was
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formed or thermally equilibrated. We have developed a new method to precisely determine the
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relative abundance of
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(TILDAS). The TILDAS instrument houses two continuous wave quantum cascade lasers; one
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tuned at 8.6 µm to measure
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13
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‰ (2ߪ) was achieved for the measurement of
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mmol) pure methane samples. Smaller quantity samples (ca. 0.5 mL STP) can be measured at
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lower precision.
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thermally-equilibrating methane with a range of δD values.
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corresponds to uncertainties of ±7°C at 25°C and ±20°C at 200°C for estimates of apparent
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equilibrium temperatures.
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determine 13CH3D in natural methane samples to distinguish geological and biological sources of
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methane in the atmosphere, hydrosphere, and lithosphere.
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CH3D by using tunable infrared laser direct absorption spectroscopy
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CH3D,
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CH3D and
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CH4, and the other at 7.5 µm to measure
CH4. Using an astigmatic Herriott cell with an effective pathlength of 76 m, a precision of 0.2 13
CH3D abundance in ca. 10 mL STP (i.e., 0.42
The accuracy of the ∆13CH3D measurement is 0.7‰ (2ߪ), evaluated by The precision of ±0.2‰
The TILDAS instrument offers a simple and precise method to
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Analytical Chemistry
Introduction
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Methane is the second most important long-lived greenhouse gas, and an increasingly
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important energy resource.1 Abrupt changes in reservoirs and fluxes of methane in the Earth’s
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surface may have been responsible for triggering abrupt climate change in the past2. The
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atmospheric methane level increased through much of the twentieth century, but the growth rate
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slowed and the concentration appeared to have plateaued between 1999 and 2007, at which point
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growth resumed.3 The main drivers for this secular trend have been debated, with explanations
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ranging from changes in source (e.g., wetlands vs. fossil fuel emissions) and/or sink (oxidation
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by OH radical) strengths.3-5
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The majority of environmental methane is of biogenic (originating from microbial
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methanogenesis) or thermogenic (produced by thermal cracking of higher molecular weight
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hydrocarbons, kerogen and coal) origin, with contributions from biomass burning and, in certain
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geologic environments, putative abiotic sources (e.g., serpentinite and deep crustal fluids).6-8
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Carbon (13C/12C) and hydrogen (D/H) isotope ratios of methane and associated short-
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chain hydrocarbons have been used to elucidate their sources9,10 but such data alone can often be
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inconclusive because of significant overlap in isotopic signatures associated with microbial,
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thermogenic and abiogenic gases. This is because the
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carbon source (e.g., CO2, acetate) and its isotopic composition,11 the isotope effects associated
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with microbial methanogenesis or thermal cracking, and the effects of secondary processes, such
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as oxidation and mixing.12,13 The D/H ratio of methane is also a complex function of reaction
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pathways, the D/H ratio of environmental water, and the D/H ratio of precursor compounds.14,15
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Therefore, the development of a new constraint, such as an isotopic thermometer, could unlock
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critical information to constrain the source of methane.
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C/12C ratio of methane depends on the
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Analytical Chemistry
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In this study, we report a methane isotope thermometry method based on the
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measurements of the doubly-substituted (“clumped”) methane isotopologue, 13CH3D. Following
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earlier studies of doubly-substituted carbon dioxide (13C16O18O),19 the precise measurements of
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four or more isotopologues of methane (12CH4,
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estimation of the temperature at which a sample of methane was formed or thermally
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equilibrated.20,21
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CH4,
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CH3D, and
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CH3D) may allow
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Carbon (13C/12C) and hydrogen (D/H) isotope ratios of methane have been routinely measured
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by gas-source isotope-ratio mass spectrometry (IRMS). Conventional IRMS techniques for
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measuring carbon- and hydrogen-isotope ratios involve combustion or pyrolysis of methane,
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respectively, and measurement of the isotopologue ratios of the product CO2 or H2.9,22 Direct
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measurement of
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13
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3 to 8 ppm) and interferences from adduct (13CH5+) and
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CH3+, CH2+) also complicate mass-16 (12CH4+) and mass-17 (12CH3D+ and
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Resolving these isobars requires the use of double-focusing high resolution gas-source
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IRMS.21,23 Stolper et al. (2014)21 report the first precise measurements of methane clumped
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isotopologue abundance (combined
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(M/∆M) from 16,000 to 25,000; contributions of adduct ions (13CH5+ on 13CH3D+, and 12CH5+ on
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CH3D+ ion is required, however, for mass-spectrometric determination of
CH3D. This is technically challenging because of the low fractional abundance of 13CH3D (ca.
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CH2D2+ ions. Fragment ions (e.g., 13
CH4+) signals.
CH3D+12CH2D2) by using medium mass-resolving power
CH3D+) were corrected as a function of mass-16 ion current. In this study, we report the application of tunable infrared laser direct absorption spectroscopy 13
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(TILDAS) for the fully-resolved measurement of a clumped isotopologue of methane,
CH3D.
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TILDAS is a virtually non-destructive technique, and offers a promising approach for precise
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and direct measurements of methane isotopologues.
Bergamaschi et al. (1994)24 reported
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Analytical Chemistry
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measurements of
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(ߥ3 band). Precisions of 0.44 and 5.1 ‰ were achieved for δ13C and δD measurements,
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respectively (δ values are defined as the deviation of the isotope ratios
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with respect to reference materials). Recently, a tunable laser spectroscopy instrument
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with an interband cascade laser tuned at 3.27μm was used for the detection of methane on
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the Mars Curiosity rover mission.25 Tsuji et al., (2012)26 reported the first optical measurement
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of
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analytical precision of 20‰ (1σ), however, was too large to resolve the thermodynamically-
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predicted natural range of variation of 13CH3D abundances (ca. 6‰).
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CH4 and
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CH3D using a lead salt diode laser tuned in the ~3.3 µm regions
13C/12C
and D/H
CH3D using a difference-frequency-generation laser in the 3.4µm region. Their reported
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The TILDAS instrument used in this study takes advantage of quantum cascade lasers
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(QCLs) that allow for continuous wave (cw) operation near room temperature, with high power
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(tens of mW) and narrow linewidths (1,000K), and referred to
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as a stochastic distribution.
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predicts that the equilibrium for (1) lies slightly toward the right (K≃1.006 at room temperature,
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see Supporting Information, SI).
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At lower temperatures, however, statistical mechanics theory
Defining ∆13CH3D as the natural logarithm for equation (2) gives: ଵଷ
Δ CHଷ D = ln( = )ܭln
ൣ 13CH3 D൧ ൣ 12CH3 D൧
− ln
ൣ 13CH4 ൧ ൣ 12CH4 ൧
(3)
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The definition of ∆13CH3D in equation (3) yields values that are practically identical to those
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calculated using the definition used by Stolper et al. (2014)21 (i.e., the deviation from stochastic
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distribution. see SI).
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In this study, we use conventional delta notation to express ratios of isotopologue abundance in a sample with respect to those of a reference. For example, ߜ 13CH3 D =
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(ሾ 13CH3 Dሿ/ሾ 12CH4 ሿ)sample −1 (ሾ 13CH3 Dሿ/ሾ 12CH4 ሿ)reference
(4)
For practical purposes (under expected ranges of multiply-substituted isotopologue abundance for natural samples), δ13CH4 is identical to δ13C and δ12CH3D is identical to δD. -6ACS Paragon Plus Environment
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Analytical Chemistry
Experimental Section
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Synthesis of Methane Isotopologues
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Methane isotopologues,
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CH3D, CH3D (natural abundance
C/12C), and CH4 (D-depleted)
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were synthesized for spectral characterization and to produce a series of working standard gases.
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The clumped methane isotopologue,
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iodomethane and D2O. D-depleted methane was synthesized from aluminum carbide (Al4C3,
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natural C isotope abundance) and D-depleted water (D content 2-3 ppm) (See SI for detailed
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description of synthesis procedures).
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CH3D, was synthesized by the Grignard reaction from
C-iodomethane and D2O. Similarly, CH3D (with natural-abundance 13C) was synthesized from
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Selection of Absorption Lines Among several possible methane infrared band systems, a spectral region around 8.6 µm was Tsuji et al. (2012)26 reported measurements of
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selected.
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However, they used dry ice to cool the absorption cell in order to reduce hot bands and high-J
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fundamental bands from major isotopologues. The absorption band system at 8.6 µm is highly
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advantageous because of less interference from hot bands in this region, and the availability of
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line parameters in the HITRAN database.29 A single deuterium substitution in CH4 reduces its
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symmetry from Td to C3v. As a result, the triply degenerate C-H bending mode of CH4 centered
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at 1300 cm-1 splits into C-D bending of 13CH3D and 12CH3D. The resultant C-D bending mode is
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about 150 cm-1 lower in frequency than the C-H bending vibration.
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frequency shift by deuterium substitution allows us to measure 13CH3D and 12CH3D lines that are
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free from strong lines originating from the much more abundant 12CH4 (Figure 1). -7ACS Paragon Plus Environment
CH3D in the 3.4 µm region.
This relatively large
Analytical Chemistry
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The spectral window between 1168.885 and 1168.995 cm-1 was chosen to access three
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13
CH3D,
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CH3D and
12
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isotopologue lines of
CH4 (Figure 1 and 2). These three lines are
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virtually-free from spectral interference, and have relatively low lower-state energies (59, 470,
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and 861 cm-1 for 13CH3D, 12CH3D, and 12CH4, respectively), such that temperature-dependence is
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expected to be minor (
