Pitfalls in the chromatographic determination of the alkenone U^k₃₇ index for paleotemperature estimation

Co-elution and irreversible adsorption are major error sources in the gas chromatographic determination of paleotemperatures from the analysis of C₃₇ di-and triunsaturated ketones. The former are related to the interference of methyl hexatriaconta-(7E,14E,21E)-trienoate with heptatriaconta-(15E,22E)-dien-2-one. These compounds can be resolved using capillary columns with a thin coating (ca. 0.1 μm) of 100% dimethylsiloxane. Irreversible adsorption concerns both the di-and triunsaturated species and has significant effects (deviations of temperature measurements higher than 0.5°C) when the absolute total amounts of ketones introduced into the system are smaller than 10 ng. The higher adsorption of the C_37:3 species gives rise to deviations of the U^k₃₇ measurements towards higher temperatures that become more important the lower is the proportion of this species in the mixture. This adsorption effect also increases the dispersion of the results as a higher standard deviation is observed the lower is the total amount of ketones and the proportion of the triunsaturated species.