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A Novel Pretreatment Method of Crude Oil for the Determination of Diamondoids

Abstract

Diamondoids in petroleum are a class of saturated cage-like hydrocarbons with the number of diamond subunit ranging from 1 to 11. It has been improved that quantitative and related parameters of diamondoids can play an important role in determining the kerogen types, thermal maturity and sedimentary environments, tracing oil origins and thermal cracking levels in subsurface of sedimentary basins. Currently, the widely used methods for the pretreatment of crude oil is column chromatography, which is efficient for the extraction of semi-volatile biomarkers in crude oil, but usually failed in volatile compounds. Adamantane, diamantane and their alkyl substitutions are volatile molecules, and some steps such as rotary evaporating and volatilizing solvent would result in the loss of diamondoids during the pretreatment. Furthermore, the pretreatment is time-consuming and needs lots of toxic and expensive organic solvents. Some researchers inject samples directly in order to avoid the problems mentioned above, which is not good for the capillary column and ion source. Based on a homemade device, a novel sample pretreatment method tentatively termed as gas purge microsyringe extraction (GP-MSE) has been developed for the extraction of diamondoids in crude oils. The whole process of extraction takes only 10 minutes, and only 200μL of organic solvent is needed. Several parameters affecting extraction efficiency and the optimal conditions for this method are as follows: extraction temperature is 300°C, extraction time is 5 min, carrier gas flow rate is 2mL/min and trapping temperature is −2°C. The Limit of Detection of adamantines and diamantanes are 25.5ng/L and 45.42ng/L, respectively. The relative standard deviation of the method for adamantane and diamantane are 1.57% and 2.63%, respectively. Finally, The condensates, normal and heavy oils from the pearl mouth basin were extracted using the developed method, and then analyzed by GC×GC-TOFMS. Seven common used geochemistry parameters were calculated and RSD were all below 5%. Moreover, the results by using GP-MSE to pretreat are good matched with the results by injecting directly. The experimental results indicates that we can get the reliable results of diomandoids analysis by utilizing the new sample preparation method. The proposed method can provide an alternative to traditional sample preparation methods for the determination of diamondoids in crude oils and be also extended to other compounds in crude oils.