GC/MS quantitation of diamondoid compounds in crude oils and petroleum products

This study presents a quantitative gas chromotography/mass spectrometry (GC/MS) method for the analysis of adamantane, diamantane, and their alkylated homologues in 14 crude oils and 22 petroleum products including light and mid-range distillate fuels, residual fuels, and lubricating oils collected from various sources. The method detection limits for five target diamondoids were in the range of 0.06 to 0.14 mu g/g oil. The total concentration of adamantane and its 16 alkylated homologues commonly range from approximately 40 to 500 mu g/g in most crude oils and from 0.6 to 1,300 mu g/g in refined products, but reaching values of up to 2,000 mu g/g for the south Louisiana crude oil and the Jet A fuel. Diamantanes occur in all crude oils and lighter to middle distillates, and their total concentration was in a range of 5 to 200 mu g/g and with maximum values near 600 mu g/g in weathered diesel fuel, but they were not detected in very light distillates and most lubricating oils. Laboratory distillation of crude oils demonstrated that adamantane series were highly enriched in the diesel distillation range between 180 to 287 degrees C, while diamantanes were largely found in the distillation fraction from 280 to 320 degrees C. The concentrations of five groups of biomarker compounds in the saturated hydrocarbon fraction decrease in the order of sesquiterpanes > terpanes >= steranes > adamantanes > diamantanes for most crude oils, while their concentrations in various refined products differ widely. The absolute concentrations of diamondoid compounds and their molecular indices offer potential diagnostic means for oil source identification and oil correlation, particularly for lighter refined products such as jet and diesel fuels in which the high molecular weight biomarkers have been removed during the refining processes.