Macromolecular material is, by far, the major organic component of meteorites. Flash pyrolysis GCMS has been used to investigate this organic component in Tagish Lake. It is more condensed, less susbtituted than Murchson

Gilmour, I.

Pearson, V. K.

Sephton, M. A.

English

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Open Research OnlineThe Open University’s repository of research publicationsand other research outputsAnalysis of Tagish Lake macromolecular organicmaterialConference or Workshop ItemHow to cite:Gilmour, I.; Pearson, V. K. and Sephton, M. A. (2001). Analysis of Tagish Lake macromolecular organicmaterial. 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/1993.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.ukANALYSIS OF TAGISH LAKE MACROMOLECULAR ORGANIC MATERIAL.  I.Gilmour, V.K. Pearsonand M.A. Sephton, Planetary and Space Sciences Research Institute, The Open University, Milton Keynes MK76AA, United Kingdom (I.Gilmour@open.ac.uk).Introduction:  The fall and recovery of the TagishLake meteorite [1] has provided a unique opportunityto study the organic composition of what is possibly anew type of carbonaceous chondrite. The bulk elementabundances of Tagish Lake have led to the suggestion[1] that it is more primitive than type 1 CI chondrites.Preliminary analysis indicated that it contains around3.6 –5.4 wt % C of which some 3.7 wt % is carbonate[1], much of the remainder being organic carbon. Themajority of this organic matter occurs as a complexmacromolecular material. We have undertaken a flashpyrolysis-gas chromatography-mass spectrometry studyof Tagish Lake to examine the chemical composition ofthis macromolecular material and have compared theresults with those obtained from the type 2 CM chon-drite Muchison.Experimental:  Around 30 mg of whole-rock meteoritethat had been kept cold since its fall (at ca. -20°C) wasloaded into a pre-cleaned quartz glass tube.  The sample wasflash pyrolysed at 610°C using a CDS 1000 pyroprobe (CDSAnalytical, Oxford, PA), within 15 s after insertion into theinterface, for 15 s in a flow of helium. The heating rate ofpyrolysis was 20°C ms-1. The interface was held at 260°Cand the GC injector maintained at 250°C. The GC oven wasoperated under the following conditions: initial isothermalhold for 2 min at 50°C; temperature programmed at 5°C/minto 300°Cand final isothermal hold for 8 min. The pyrolysatewas analysed on an Agilent Technologies 5973 GCMS oper-ated in fullscan electron ionization mode (50-500 Da, 1.8scan s-1, 70 eV electron energy).Following the intial pyrolysis at 610°C, the sample waspyrolysed a second time at 1000°C, all other conditions werethe same as for the first run. A similar-sized sample ofwhole-rock Murchison meteorite was also pyrolysed underidentical conditions for comparison.Results:  The total ion chromatogram for the610°C pyrolysis is shown in Figure 1a. In general, thepyrolysate is dominated by aromatic compounds, how-ever, two compounds predominate: the C16 polyaro-matic hydrocarbons fluoranthene and pyrene. Otherpolyaromatic hydrocarbons are also present, though inmuch lower quantities. Figures 1b – 1e are recon-structed ion chromatograms that show the distributionof 1-ring to 4-ring aromatic hydrocarbons.The pyrolysate of Tagish Lake is dominated byrelatively high-molecular weight aromatic compounds;small quantities of the C18 PAHs chrysene and benz-anthracene were observed in the pyrolysate. Relativelysmall quatities of one and two-ring species were ob-served, although chromatographic resolution of one-ring aromatics was difficult due to the release of sub-stantial quantites of sulfur which col-eluted with ben-zene and toluene.Discussion: Although representing by far the majororganic component in meteorites, macromolecular ma-terial has not been extensively studied. To date, studieshave been laregly restricted to the Murchison meteoriteso that the fall of Tagish Lake represents a unique op-portunity for comparison between the macromolecularmaterial in these two chondrites.Several distinct differences have been observedbetween Tagish Lake and Murchison. Aliphatic hydro-carbon moieties are present in significant amountswithin the Murchison macromolecular material. Py-rolysis studies have indicated that these entities existwithin or around the aromatic network [2] while NMRstudies have revealed abundant aliphatic units in theform of short, branched groups which link aromaticcenters [3]. Tagish Lake macromolecular material,however, is apparently more aromatic in character – thedominant pyrolysis products are two parental PAHs,alkyl-substituted PAHs being scarce by comparison.Pyrolysis experiments of Murchison have also releasedseveral heteroatom species such as O-containing phe-nols, S-containing thiopphenes and benzonthiophenesand N-containing species such as benzonitrile. Smallamounts of phenol were detected in Tagish Lake py-rolysates, however, the dominant S-containing moeitieswere small amounts of benzo-thiophenes and scarceamounts of thiophenes.However, the major difference between the twometeorites is in the molecular weight distribution of thepyrolysis products. Murchison contains substantialquantities of alkyl-benzenes and other monoaromaticspecies such as indanes that are either missing or ex-tremely scarce in the pyrolysates of Tagish Lake. Thissuggests that the macromolecular material in TagishLake has a more condensed  aromatic structure withhigher-molecular weight aromatic moeities predomi-nating.Conclusions: Pyrolysates of the Tagish Lake mete-orite are dominated by two high molecular weightPAHs suggsting that the macromolecular material inTagish Lake consists of a highly condensed aromaticnetwork.References: [1] Brown et al., 2000, Science 290,320. [2] Septhon et al., 2000, GCA 64, 321. [3] Croninet al., 1987, GCA, 51, 299.Lunar and Planetary Science XXXII (2001) 1993.pdfOrganic Analysis of Tagish Lake Macromolecular material:  I. Gilmour, V.K. Pearson and M.A. SephtonFigure 1. Mass chromatograms of pyrolysates released from a 610°C flash pyrolysis of Tagish Lake. (a) Total ion current chromatogram, (b) Mass chromatogram (m/z 128, 141, 142, 155, 156) showing C0-C3 alkyl naphthalenes, (c) Mass chromatogram (m/z 134, 147, 148, 161, 162) showing C0-C2 alkylbenzo[b]thiophenes, (d) Mass chromatogram (m/z 178, 192, 206) showing C0-C2 alkyl phenanthrenes, (d) Mass chromatogram (m/z 202, 216, 230) showing C0-C2 alkyl pyrenes/fluoranthenes.Lunar and Planetary Science XXXII (2001) 1993.pdf

Analysis of Tagish Lake macromolecular organic material

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Analysis of Tagish Lake macromolecular organic material

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