Compound-Specific δ13C of n-Alkanes: Clean-Up Methods Appraisal and Application to Recent Sediments of a Highly Contaminated Bay

Different clean-up methods to determine isotopic composition of n-alkanes were tested. Sources of organic matter in Guanabara Bay were re-examined through the VC of individual n-alkanes in surface sediment samples. The n-alkanes were efficiently isolated without significant losses of the compounds of interest. The delta C-13 of n-alkanes (n-C-17 to n-C-35) ranged between -34.0 and -26.4 parts per thousand. More depleted delta C-13 values associated to higher carbon preference index (CPI > 4) suggest prevalence of biogenic n-alkanes from terrestrial origin in the inner stations influenced by inputs from mangrove areas. Small isotopic differences between odd and even-numbered chains and C-13-enrichment in long-chain homologues indicate a petrogenic hydrocarbons contribution. Higher concentrations of short-chain compounds associated to a more C-13-enriched total organic carbon (TOC) were found near the most intense sewage discharges. Results show that delta C-13 of n-alkanes can be applied to a highly degraded system as the Guanabara Bay and help improve the characterization of the organic matter.