Isotopically enriched n-alkan-2-ones with even chain predominance in a torbanite from the Sydney Basin, Australia

Abstract n-Alkan-2-ones (methyl ketones) ranging from C14 to C31, were recovered from the polar fraction of an extract of a torbanite from the Sydney Basin, Australia. There was a very slight even-over-odd predominance between C20 and C24 n-alkan-2-ones, together with remarkably high contents of the C24, C26 and C28 homologues. A range of C16 to C28 macrocyclic alkanes, as well as their corresponding methylated counterparts (C19–C24), was also detected in the saturate fraction obtained after molecular sieve treatments. Due to the relatively high contents of the n-alkan-2-ones and macrocyclic alkanes, we were able to carry out GC–irMS analysis to determine the δ13C values of specific compounds. The even-numbered n-alkan-2-ones from C24 to C28 were significantly 13C-enriched, compared to the other n-alkan-2-ones, which had similar isotopic values to the macrocyclic alkanes. The majority of the n-alkan-2-ones detected here probably had the same algal source as the macrocyclic alkanes, which is likely to be lipids and algaenan of the green microalga Botryococcus braunii. However, the distribution and 13C enrichment of C24, C26 and C28 n-alkan-2-ones indicates that an additional source must be present.