Dual isotopic composition (H and C) of methane extracted from a small sample of Murchison meteorite reveals a deuterium enrichment for this molecule, indicating the presence of interstellar hydrogen

Butterworth, A. L.

Gilmour, I.

Sephton, M. A.

English

Open Research Online (The Open University)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsDual isotopic composition of methane in MurchisonmeteoriteConference or Workshop ItemHow to cite:Butterworth, A. L.; Sephton, M. A. and Gilmour, I. (2001). Dual isotopic composition of methane in Murchisonmeteorite. In: 32nd Lunar and Planetary Science Conference, 12-16 Mar 2001, Houston, Texas, USA.For guidance on citations see FAQs.c© [not recorded]Version: [not recorded]Link(s) to article on publisher’s website:http://www.lpi.usra.edu/meetings/lpsc2001/pdf/1910.pdfCopyright and Moral Rights for the articles on this site are retained by the individual authors and/or other copyrightowners. For more information on Open Research Online’s data policy on reuse of materials please consult the policiespage.oro.open.ac.ukDUAL ISOTOPIC COMPOSITION OF METHANE IN MURCHISON METEORITE.  A. L. Butterworth, M.A. Sephton and I. Gilmour,  Planetary and Space Sciences Research Institute, The Open University, Walton Hall,Milton Keynes, MK7 6AA, UK (email: a.l.butterworth@open.ac.uk)Introduction:  Examining the record containedwithin the organic matter in meteorites can reveal thenature of the inputs and processes that contributed tothe early solar system environment. Specifically, iso-topic measurements for individual organic compoundscan reveal the reaction mechanisms and possible sourceenvironments from which the organic constituents wereformed. Hydrogen isotopic measurements have becomevaluable indicators of the contributions from interstel-lar environments [1] and carbon isotopic measurementshave been used successfully to indicate bond formationand destruction in a variety of meteoritic organic mole-cules [2,3]. Here, for the first time, we present dualisotopic data (H and C) for methane, the fundamentalbuilding block implicated in the stepwise formation ofmeteoritic hydrocarbons [2].D/H measurement of methane.  The D/H ratio ofmethane has been accomplished previously by a directmethod involving the catalytic combustion of methaneto CO2 and H2O and subsequent reduction of the waterto hydrogen gas for measurement by mass spectromet-ric analysis.  In this way, several D/H measurements ofatmospheric methane have been made using 70 µmolmethane, e.g. [4].  This large sample requirement hasprevented the direct method being used for the D/Hmeasurements of extraterrestrial methane.An alternative approach to obtaining D/H ratios ofmethane is by an indirect method [5].  By measuringthe combined isotopic composition of methane (mass17/16 ratio) and the 13C/12C ratio (by combustion toCO2), the D/H ratio may be calculated by the followingrelationship:HDCCMM 412131617+=where 17M and 16M represent the intensities ofmethane species (13CH4 + 12CH3D) and 12CH4 respec-tively.  A NIST standard methane, NGS#3, has beendefined as having δ17M 0.0 ‰ (17M/16M = 0.0109351)[5] so that the combined isotopic composition of amethane sample may be represented similarly to theUrey delta notation as follows:( )( ) 100011617161717 ×−=referencesampleMMMMMδThe dual isotopic composition of methane may beresolved from the combined isotopic signature usingthe equation:( ) ( ) 6.1490043.185528.17 1317 −⋅−⋅= CMD δδδThe advantage of using methane itself as the ana-lyte for mass spectrometric analysis is that chemicalconversions involving water are avoided, the associ-ated memory effects are eliminated and the requiredsample size is reduced.A further reduction in sample size is achieved byanalysing the methane in a static mass spectrometer.All of the sample is admitted to the mass spectrometerat once and compared to an identically sized aliquot ofreference methane, which is analysed immediately afterthe sample has been pumped away.  The mass spec-trometer developed for methane isotope analysis(MIRANDA) has been used to investigate the proper-ties of small samples (in 10 ml air) of atmosphericmethane [6].  In addition, the instrument has been usedto measure the combined isotopic composition ofmethane evolved during the stepped pyrolysis of somelunar soils [7].Method:  In an evacuated 3 ml Pyrex vessel, thevolatile component was extracted from a powderedsample of Murchison interior (76 mg) in 0.2 ml waterusing freeze/thaw cycling assisted by sonication at 25°C.  The vessel was then attached to the vacuum inletsystem of the static mass spectrometer (MIRANDA),allowing several hours for the joining volume to beevacuated.The water was then frozen in liquid nitrogen andthe headspace expanded into the high vacuum inlet forsample clean-up.  There were two main purificationstages; the first was to remove any water, since watercracks in a mass spectrometer source to produce iso-baric interference at mass 17.  The second stage was tominimise adverse pressure effects by removing almostall other gases present.  Nanomole quantities of nitro-gen were very difficult to avoid, but are too small tomake gas chromatographic separation necessary, sincethe static system requires all carrier gas to be removedLunar and Planetary Science XXXII (2001) 1910.pdfD/H of Methane in Murchison:  A. L. Butterworth et al.before the sample may be admitted to the mass spec-trometer.  Fortunately active gases may be getteredusing SAES St172 at 50 °C without affecting the iso-topic composition of the methane.  Only traces of noblegases remained in the analyte, which do not affect them/z 17/16 ratio.A “blank” of the entire procedure was carried outwhich resulted in 7 pmol methane detected in the massspectrometer.Results:  A total of 0.266 nmol methane was re-covered from the sample, which equates to 3.5 nmol/g,a lower yield than 8.9 nmol/g achieved by Yuen et al.[2].As shown in Fig.1, assuming δ13C +9.2 ±1.0 ‰ (V-PDB) from [2], a δD value of +906 ± 50 ‰ (V-SMOW) was calculated from the measured δ17M of+148 ± 2 ‰ (NGS#3).-200020040060080010001200-80 -60 -40 -20 0 20 40Murchison methaneatmospheric methaneNGS#3 methanemonocarboxylic acids [1]δD V-SMOWδ13C V-PDB12010080604020140this workYuen etal. [2]δ17M80Figure 1: The plot of δD against δ13C shows possibleisotope combinations that result in particular δ17M val-ues plotted in steps of 20 ‰.  Atmospheric methaneand the reference NGS#3 (defined as having δ17M =0.0 ‰) are shown for comparison.Discussion:  The δD value obtained for Murchisonmethane is significantly higher than bulk measurementsof dried, solvent extracted aliphatic fractions from thismeteorite but is comparable with bulk measurementsfor more polar dried organic fractions [2]. The appar-ent discordance between methane and aliphatic frac-tions from Murchison is unsurprising since bulk meas-urements are averages of a wide range of isotopicallyand structurally different molecules, including contri-butions from terrestrial contaminants. Furthermore, thebulk fractions will have had their most volatile compo-nents, such as methane, removed by drying prior toanalysis.The high δD value of Murchison methane suggeststhe presence of some interstellar hydrogen in the mole-cule. D-enrichments are thought to result from gasphase ion-molecule and other reactions in the inter-stellar medium [8]. As methane is a building block ofat least one class of organic molecule [2] and is also acommon cracking product of high molecular weightorganic matter, its δ13C value may reflect the degree ofcarbon-carbon bond destruction that has occurredwithin the organic inventory of a meteorite. Addition-ally, as methane that has originated from a crackingprocess can be expected to contain less D than inher-ited material due to the high abundance of solar systemnormal hydrogen on meteorite parent bodies, the δDvalues may also be an indicator of the level of secon-dary processing.It should be noted that the mass spectrometry tech-nique has a tendency for a possible stretching of theδ17M scale, a problem heightened by the use of a D-depleted methane standard for calibration for this work.Further work will address these calibration issueswhich may produce a slightly higher than true δDvalue.  By disaggregating chips of meteorite, ratherthan a ground sample, we hope to increase methaneyields.In essence, this work is a study of the possibilitiesfor measurements of the dual isotopic composition ofvery small samples of methane.  Since Yuen et al. [2]analysed 209 nmol CH4 to obtain δ13C, advances incontinuous flow mass spectrometry have reduced thesample size needed (to ~0.4 nmol) [9].  The optimumsample size for 17M/16M measurement is 0.3 nmol (cf.methane D/H sample 70µmol [4]).  For the first time itis possible to measure the dual isotopic composition ofmethane from less than 1 nmol total sample (120 mgMurchison).References:  [1] Krishnamurthy R. V. et al. (1992)GCA 56, 4045-4058.  [2] Yuen G. et al. (1984) Nature307, 252-254.  [3] Sephton, M.A., et al. (1998) GCA62(10), 1821-1828.  [4] Levin I. et al. (1993) Chemos-phere 26(1-4) 161-177.  [5] Morse A. D. et al. (1996)Rapid Commun. Mass Spectrom. 10, 1743-1746.  [6]Jackson et al. (1999) Rapid Commun. Mass Spectrom.13,1329-1333.  [7] Lécluse C. et al. (1995) Meteoritics30, 534  [8] Sandford, S.A. (1996) Meteorit. Planet.Sci. 31: 449-476.  [9] Merritt et al. (1995) J. Geophys.Res. 100, 1317.Lunar and Planetary Science XXXII (2001) 1910.pdf

Dual isotopic composition of methane in Murchison meteorite

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Dual isotopic composition of methane in Murchison meteorite

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