Other
Scientific paper
Jul 1992
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1992metic..27q.224g&link_type=abstract
Meteoritics, vol. 27, no. 3, volume 27, page 224
Other
4
Scientific paper
Previous studies using compound-specific isotope analysis of aromatic fractions from the Orgueil and Murchison carbonaceous chondrites suggested that there may be variations in delta^13C values between different PAH (1). Although the Orgueil meteorite was found to be too heavily contaminated with terrestrial PAH to be of any value, strong evidence that the PAH in Murchison were indigenous came from their relatively heavy isotope compositions of between -8 and -11o/oo. Aromatic fractions were isolated from two new samples of Murchison using solvent extraction followed by column chromatography on silica gel. The compounds were tentantively identified using GCMS and positive confirmation obtained for some by running authentic standards. The carbon isotope compositions of individual PAH were measured by isotope ratio monitoring-GCMS (irm-GCMS) on a Finnigan-MAT Delta S/GC. The most striking feature in the distribution of PAH in Murchison is the overwhelming predominace of phenanthrene, fluoranthene, and pyrene, with the latter two compounds being the most abundant. Anthracene is present at about 50 times less than phenanthrene with most other higher molecular weight PAH present in similar or smaller quantities. The isotopic compositions of parental PAH (no side chains) are listed in the table. The carbon isotope compositions range from -6 to -22o/oo. The range is similar to the isotopic compositions observed in stepped combustion of Murchison macromolecular material where the bulk delta^13C is -17o/oo. In general the higher molecular weight PAH appear to be isotopically lighter; however, at first sight there is no systematic trend. For instance both phenanthrene and fluorathene have similar delta^13C values, as do pyrene and benzo(j)fluoranthene. However, there are apparently systematic differences between some isomers; for example, anthracene is 6.5o/oo heavier that phenanthrene, fluoranthene is 7o/oo heavier than pyrene, and benzo(j)fluoranthene is 7o/oo heavier than benzo(e)pyrene. A decrease in delta^13C with increasing number of aromatic rings would be consistent with an origin for the higher homologues by condensation reactions with each closure of a benzene ring resulting in a step down in delta^13C, assuming a kinetic isotope effect during the condensation reaction. The differences in isotope composition of PAH in Murchison may represent two distinct populations synthesized at different locations, e.g., on the parent body vs. in the solar nebula. The systematic difference of 7o/oo between isomers may provide one clue. Addition of benzene rings to fluoranthene and pyrene, the most abundant PAH, from a common source would preserve an isotopic difference between isomers containing a S-membered ring and those without as is seen with benzo(e)pyrene and benzo(j)fluoranthene. Compound...............Molecular weight...delta^13C (o/oo)PDB Phenanthrene...........178................-7.5 Anthracene.............178................-13.1 Fluoranthene...........202................-S.9 Pyrene.................202................-13.1 Chrysene+Triphenylene..228................-14.5 Benzo(ghi)fluoranthene.226................-14.2 Benzo(e)pyrene.........252................-22.3 Benzo(j)fluoranthene...252................-15.4 References 1. Gilmour et al. (1991) Meteoritics, 26, 337-338
Gilmour Iain
Pillinger Colin
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