Abstract:

A novel sample pretreatment method, gas purge micro-syringe extraction (GP-MSE), coupled to comprehensive two-dimensional gas chromatography/time-of-flight mass spectrometry (GC×GC/TOFMS) has been developed for the characterization of volatile and semi-volatile compounds in crude oils. In the sample pretreatment process, the analytes were carried to the microsyringe barrel by inert gas, and at the same time, trapped by an organic solvent. The whole process of extraction takes less than 10 min, and only 20 μ L of organic solvent was needed. Using two custom standard solutions containing alkanes and polycyclic aromatic hydrocarbons (PAHs), the influences of the extraction conditions were investigated. The optimized conditions were as follows: 5 mg crude oil, 20 μ L hexane (extraction solvent), extraction for 3 min at 300 ℃, condensation temperature set at -2 ℃, gas flow rate set at 2 mL/min. Under the optimized conditions, a real crude oil sample was extracted and then analyzed in detail. It showed that the proposed method was very effective in simultaneously analyzing the normal and branched alkanes, cycloalkanes, aromatic hydrocarbons, and biomarkers of crude oil such as steranes and terpanes. The recoveries obtained ranged from 82.0% to 107.3% and the detection limits ranged from 34 to 93 μg/L. The correlation coefficients (R²) were more than 0.99. The relative standard deviations (RSDs, n=5) for all the analytes were below 10%. The results indicate that the proposed method is suitable for the characterization of volatile and semi-volatile compounds in crude oils with easy operation, high sensitivity and efficiency.

Key words: comprehensive two-dimensional gas chromatography (GC×GC), crude oils, gas purge micro-syringe extraction (GP-MSE), mass spectrometry (MS)