Investigation of the solvent extractable organic compounds in Tagish Lake. The results are used to test the interstellar parent body hypthesis for the origin of extraterrestrial organic matter

Gilmour, I.

Pearson, V. K.

Sephton, Mark

English

Sephton, M.A.

Open Research Online (Open)

Open Research OnlineThe Open University’s repository of research publicationsand other research outputsThe Tagish Lake chondrite and the interstellar parentbody hypothesisConference or Workshop ItemHow to cite:Gilmour, I.; Sephton, Mark and Pearson, V. K. (2001). The Tagish Lake chondrite and the interstellar parentbody hypothesis. 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/1969.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.ukTHE TAGISH LAKE CHONDRITE AND THE INTERSTELLAR PARENT BODY HYPOTHESIS.  I. Gil-mour, M.A. Sephton and V.K. Pearson, Planetary and Space Sciences Research Institute, The Open University,Milton Keynes MK7 6AA, United Kingdom (I.Gilmour@open.ac.uk).Introduction: The fall of the Tagish Lake meteor-ite and the subsequent suggestion that it may representa new type of chondrite that is more primitive than type1 CI chondrites [1] provides a unqiue opportunity toreexamine ideas on the origin of extraterrestrial organicmatter. To date, most studies of the organic matter inmeteorites have focussed on the Murchison meteoritewhich fell in 1969.We have undertaken a gas chromatography-massspectrometry investigation of solvent extractable or-ganic compounds in Tagish Lake. The results are usedto test the interstellar parent body hypthesis for theorigin of extraterrestrial organic matter proposed by[2].Experimental:  A crushed and powdered sample(ca. 450 mg) of Tagish Lake was solvent extractedusing dichloromethane (3 ml, Distol grade) by sonica-tion for 20 minutes. The extract was removed using apasteur pipette after centrifugation and concentrated toaround 50 µl under a stream of dry nitrogen. The ex-tract (1 µl) was injected onto a GCMS (Agilent Tech-nologies). The GC oven was operated under the fol-lowing conditions: initial isothermal hold for 2 min at50°C; temperature programmed at 5°C/min to 300°Cand final isothermal hold for 8 min. Compound identi-fications were performed by comparison with the NISTmass spectral database.Results:  Figure 1a shows the total ion currentchromatogram obtained for the extract. A wide rangeof organic compounds were observed including aro-matic and alkyl-aromatic compounds, phenols, nitrilesand aliphatic hydrocarbons.Figure 1b is a m/z 57 reconstructed ion chroma-togram (RIC) that selectively displays the distributionof aliphatic hydrocarbons. The majority of the aliphatichydrocarbons show great structural diversity in contrastof the typical homologous series of n-alkanes associ-ated with terrestrial contamination [3]. However, somen-alkanes were observed, the most notable of whichhad 15-16 carbon atoms.Figure 1c is a RIC (m/z 105,106,119,120,134) thatselectively displays the distribution of C2-C4 alkylsubstituted benzenes.Toluene was not observed in thisanalysis due to its low molecular weight which pre-cluded adequate separation from the solvent. Alkyl-substituted benzenes are the predominant species ob-served for compounds with between 6 and 10 carbonatoms and a diverse range of isomers are present.However, there is a steady decrease in relative abun-dance with increasing degree of alkylation.Figure 1d is a RIC (m/z 103, 117, 131) that selec-tively displays the distrubtion of  C0-C2 alkyl substi-tuted benzonitriles. As with the alkyl benzenes a di-verse range of isomers is observed and the relativeabundance decrease with increasing degree of alkyla-tion.Figure 1e is a RIC (m/z 142, 153, 155) that hasbeen used to selectively display the distribution ofsome higher molecular weight N-containing com-pounds. N-containing compounds are major compo-nents of the total ion current chromatogram suggestingthat they are a significant proportion of the free organicinventory of Tagish Lake.Discussion:  The interstellar parent body hypothe-sis proposed by [2] suggests that meteoritic organicmatter is produced by aqueous transformation of inter-stellar precursors initiated by the internal heating of avolatile-rich parent body. A significant proportion ofthe aromatic moeties in Murchison display evidence ofbeing the relics of interstellar grain processes [4]. Thefree organic inventory in Murchison has been inter-preted, in part, as the products of the aqueous alterationof this interstellar precursor material on the parentbody.There are several notable differences in the free or-ganic inventory of Tagish Lake compared to Murchi-son. Foremost is the relative prominence of nitrile andother cyano-aromatic species. Organic molecules withsuch hydrogen-poor functional groups may reflectrelatively low levels of aqueous processing on theTagish Lake parent body. One consequence of whichmay be a relative decrease in the abundance of hydro-gen-rich niotrogen containing species such as aminoacids.Further investigations using compound specificisotope analysis may better constrain exactly where onthe interstellar-parent body continuum Tagish Lakeorganic matter lies.References: [1] Brown et al., 2000, Science 290,320. [2]  Cronin and Chang, 1993, Chemistry of life’sorigins, 209. [3] Sephton et al., 2001, Precamb. Res.106, 45. [4] Sephton and Gilmour, 2000, Ap. J., 540,588.Lunar and Planetary Science XXXII (2001) 1969.pdfTAGISH LAKE ORGANIC MATTER:  I. Gilmour, M.A. Sephton, V.K. PearsonFigure 1. Mass chromatograms of a solvent extract of Tagish Lake. (a) Total ion current chromatogram, (b) Reconstructed ion chromatogram (RIC) showing aliphatic hydrocarbons, (c) RIC for alkyl-benzenes, (d) RIC for benzonitrile and alkyl-benzonitriles, (e) Partial RIC for higher molecular weight N-containing species.Peak identites. 1. Dimethyl benzene, 2. Benzaldehyde, 3. Phenol, 4. Benzonitrile, 5. Benzene dithiol, 6. Napthalene, 7. Phthalic anhydride, 8. Phenyl pyridine, 9. Dimethyl benzenes, 10. C3-alkyl benzenes, 11, C4-alkyl benzenes, 12-13. C15, C16 normal alkanes, 14. Benzonitrile, 15. C1-alkyl benzonitrile, 16. C2-alkyl benzontrile, 17.  Di-cyano substituted benzene, 18. Phenyl pyridine, 19. Iso-cyano naphthalene7Lunar and Planetary Science XXXII (2001) 1969.pdf

The Tagish Lake chondrite and the interstellar parent body hypothesis

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The Tagish Lake chondrite and the interstellar parent body hypothesis

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