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Abstract

Hydrocarbon contamination of organic-lean rocks is thought to occur principally during lubricantassisted core drilling and improper sample storage, while in-laboratory contamination can be monitored and controlled via the use of procedural blanks. Contamination by fine particles settling from the air inside laboratories has not been considered a serious threat. We have used passive silica gel samplers to show that laboratory atmospheres can contain airborne hydrocarbons that preferentially accumulate in fume hoods. The molecular inventory consisted of a range of mature hydrocarbons, including diagnostic hopanes, steranes and, oddly, aryl isoprenoids. Accumulation rates were linear and reached a level that could pose a threat to lean samples after exposure for as little as 24 h. Temporal and spatial variability indicated multiple contaminant sources. During common laboratory operation no single source could be identified and hydrocarbons probably represented a blend of rock dust produced in previous projects and oil used in roughing pumps. During floor waxing, performed as building maintenance on a regular basis in some laboratories, we observed that common background levels were raised by a factor of ca. 10 and shifted towards a different molecular fingerprint. This aspect of sample contamination has failed to be acknowledged hitherto but might explain some of the false positive hydrocarbon signatures reported from otherwise overmature rocks.