Soluble Organic Compounds in the Tagish Lake Meteorite. R.W. Hilts1 and C.D.K

Lunar and Planetary Science XXXIX (2008) 1737.pdf

Soluble Organic Compounds in the Tagish Lake Meteorite. R.W. Hilts1 and C.D.K. Herd2, 1Chemistry Depart- ment, Grant MacEwan College, Edmonton, Alberta, Canada T5J 4S2 ([email protected]), 2Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada T6G 2E3 ([email protected]).

Introduction: The Tagish Lake meteorite is an the sample over the range 4000-650 cm-1. Omnic soft- ungrouped carbonaceous chondrite that represents ware (7.1) was used to collect and process the spectra. some of the most primitive material available for The baseline was manually corrected in all spectra. study. It contains a high amount of organic carbon The GC-MS data were acquired with an Agilent tech- (~2.6 wt%); however, only 2% of this is soluble in nologies 5975-C gas chromatograph-mass spectrome- polar solvents [1]. As with other organic-rich meteor- ter equipped with an HP-5MS column packed with ites, terrestrial contamination is a concern. Many po- (5% phenyl)methyl polysiloxane. The lower limit for tential sources of terrestrial contamination exist that the mass spectra was 30 m/z. Water-soluble organics can interfere with analysis of indigenous molecules in were drawn out by heating aqueous slurries (~0.4 mL meteorites. As such, sample handling and curation are ultrapure H2O/0.3 g dust) of the dust at reflux paramount. (~100oC) for 24 h. The LC-MS data were gathered on This study takes advantage of the pristine nature an Agilent Technologies HP1100Ms equipped with a of the Tagish Lake meteorites collected a few days Luna C18 HPLC column. after their fall [2] and which are curated at the Univer- Results and Discussion: Optimal extraction of sity of Alberta to 1) use available resources to identify non-polar, weakly polar and moderately polar solvent- organic molecules in Tagish Lake and develop a soluble organic species was achieved with neat di- method that can be readily applied in the context of chloromethane. A substantially smaller assortment of other (esp. mineralogical and petrological) studies, and compounds was extracted with 50:50 (v/v) toluene- 2) identify terrestrial contamination and trace its methanol solution. A comparison is given in Table 1. source as part of efforts to establish curation and handling protocols. Table 1: Selected Compounds Extracted from Tagish Materials: Pristine samples of the Tagish Lake Lake meteorite are stored in a research grade freezer at -28 Compound MEOH-Toluene DCM °C at the University of Alberta. A 1.685 g subsample of sample 11v forms the source of the material used in 1,3,5-trimethybenzene YES YES this study. Sample 11v consists of 5.65 g of dust and styrene YES NO disaggregated material that was collected from the napthalene YES YES surface of Tagish Lake in January 2000. It has been d-limonene YES YES stored in a Ziploc bag at temperatures below 0 °C cyclic octaatomic sulfur YES YES since its collection. Field Emission (FE)SEM analysis oleamide(terrestrial) YES YES of the sample showed that it is dominated by a matrix undecane YES YES of serpentine/saponite with sparse chondrules (Figure dimethyl tetrasulfide YES NO 1), similar to what has been reported previously [3]. 2-methylnapthalene NO YES Methods: The subsample was transferred to an 1-methylnapthalene YES NO Ar-containing Schlenk vessel which was then stored in n-hexadecanoic acid NO YES eicosane NO YES a -30°C freezer in the laboratory. Solvents used for heptadecane NO YES extraction include neat dichloromethane, 50:50 (v/v) tetradecane NO YES toluene-methanol, and ultrapure water. A typical extraction involved adding ~0.4 mL of solvent to ~0.3 g o The GC-MS trace for the DCM (dichloromethane) of material in a Schlenk vessel under Ar, kept at -78 C extract is shown in Figure 2. By correlating the GC- in a dry ice-acetone bath. The resulting black slurry MS empirical results for this extract to a comprehen- was allowed to warm to room temperature and lightly sive mass spectra database we have been able to un- tapped, causing the insoluble material to settle out. The ambiguously identify thirty-three organic compounds, slightly cloudy brown supernatant (~0.2 mL) was ex- ten of which are likely terrestrial; an additional thirty- tracted and split into two 0.1 mL portions which were four less confidently identified extraterrestrial com- used for GC-MS and FTIR. pounds were also observed (the organic subclass to The FTIR spectra were recorded as casts on a which each belongs is known with good confidence, Thermo-Nicolet Nic Plan FTIR microscope attached to but the absolute identity in each case remains ambigu- a Magna 760 FTIR spectrometer. For each analysis, ous). Long-chain saturated hydrocarbons (up to n- 256 scans were collected for both the background and Lunar and Planetary Science XXXIX (2008) 1737.pdf

C44H90), and branched alkanes (e.g., 2,2- dimethyldecane) dominate the suite of extraterrestrial compounds. Alkyl benzenes (e.g. 1,3,5- trimethylbenzene) and low molar mass PAHs (e.g. naphthalene) are also relatively plentiful. Findings similar to ours were reported by [4, 5]. It is noteworthy that the largest peak in the GC-MS is not an organic species, but rather extraterrestrial cyclic octaatomic sulfur, or S8 (Figure 2). In addition, styrene was found in the methanol-toluene extract. Given the reactive nature of this compound (its lifetime in the Earth’s Serpentine matrix atmosphere is short and it tends to polymerize), we interpret it as indigenous. Fe,Mg olivine chondrule The FTIR spectrum of the dichoromethane extract (Figure 3) contains bands corresponding to alkane C-H bonds between 2800 and 3000 cm-1 and aromatic C-H -1 Figure 1. FE-SEM image of material used in this absorptions at ~3090 cm , which are congruent with study. Scale bar is 100 microns. the GC-MS data, and similar to previous results [6]. The LC-MS spectra of the water extract display an envelope of poorly-resolved peaks ascribed to low- molar mass (< 300m/z) monocarboxylic acids. The S8 unequivocal identification of these acids will be estab- lished either through the use of reference standards or ester derivitization. The most prevalent terrestrial contaminant found is oleamide (Figure 2), a plasticizer used in the manu- facture of Ziploc (and other) bags, and is derived from the bag in which the sample was stored. Conclusions: We have identified a suite of in- oleamide digenous soluble organic compounds, and several terrestrial contaminants. Indigenous compounds include those that readily sublime at room temperature (e.g., naphthalene), reactive substances (e.g., styrene), and high amount of octaatomic sulfur. Besides providing a potential explanation behind anecdotal reports of “me- tallic” smells emanating from samples of Tagish Lake warmed to room temperature, our results demonstrate the importance of cold curation of organic-rich meteor-

ites for the preservation of organic compounds. Figure 2. GC-MS trace for the DCM extract. Acknowledgements: This study was funded through the NRCan contribution to the purchase of the pristine Tagish Lake meteorites. Dr. Joe Takats is thanked for the use of his laboratory. References: [1] Grady M. M. et al. (2002) Meteor- itics & Planet. Sci., 37, 713-735. [2] Hildebrand A. R. et al. (2006) Meteoritics & Planet. Sci., 41, 407-431. [3] Zolensky M. E. et al. (2002) Meteoritics & Planet. Sci., 37, 737-761. [4] Gilmour I. (2003) in Treatise on Geochemistry, D.H. Heinrich and K.T. Karl, Eds, Per- gamon: Oxford. p. 269-290. [5] Pizzarello S. et al. (2001) Science, 293, 2236-2239. [6] Matrajt G. et al. (2004) Astronomy & Astrophysics, 416, 983-990. Figure 3. FTIR spectrum (DCM cast).