Rapid profiling of carboxylic acids in reservoir biodegraded crude oils using gas purge microsyringe extraction coupled to comprehensive two-dimensional gas chromatography-mass spectrometry

Carboxylic acids (CAs) are important molecular markers in petroleum geochemistry, which can indicate the origin, migration and biodegradation of oils. In this work, an easy and fast method was developed for the separation and analysis of CAs in crude oils by the combination of gas purge microsyringe extraction (GP-MSE) and comprehensive two-dimensional gas chromatography-mass spectrometry (GC × GC–MS). CAs in oils were firstly reacted with a silylation reagent to improve their volatility and thermal stability. The silylated CAs were then separated from oils together with light hydrocarbons with GP-MSE treatment at a relatively low temperature (200 °C). Subsequently, direct analysis of the extract solutions by GC × GC–MS was performed. Profited from the extremely high peak capacity of GC × GC, the signals of silylated CAs could be easily distinguished from hydrocarbon signals. Under optimal conditions, the extraction time and the solvent consumption of GP-MSE were 8 min and 150 μL, respectively. In the analysis of reservoir crude oils with different biodegradation degrees, abundant CA signals were successfully detected, and the contents of CAs were found to vary regularly with the biodegradation degree.