SOLID PHASE EXTRACTION COMBINED WITH COMPREHENSIVE TWO-DIMENSIONAL GAS CHROMATOGRAPHY FOR ANALYSIS OF CARBOXYLIC ACIDS IN BIODEGRADABLE OILS

The carboxylic acid compound in crude oil is one of the main products of biodegradation, including fatty acids, naphthenic acids and aromatic acids, and has the general formula of C_nH_mO_x.Compared with normal crude oil, biodegradable oil contains more carboxylic acid compounds. Due to the complexity of the crude oil sample matrix, it is necessary to separate the carboxylic acid compounds in the crude oil before analysis. To solve this problem, we used solid phase extraction (SPE) to separate the carboxylic acid compounds in crude oil, and compared with the liquid-liquid extraction method, we found that the operation is simpler and faster, the selectivity is better, and the test results are more stable and reliable. On the other hand, considering the wide variety of carboxylic acid compounds in crude oil and the existence of a large number of isomers, we analyzed them by Comprehensive Two-Dimensional Gas Chromatography(GC×GC) and By comparison with gas chromatography-mass spectrometry (GC-MS), GC×GC was found to have far more compounds than GC-MS, thanks to the strong separation and qualitative capabilities of GC×GC.

The crude oil samples from this study were mainly from the northern slope belt of the Biyang Depression, including normal crude oil and heavy oil (Heavy oil is mainly biodegradable oil). According to Wenger's standard^[5], the crude oil degradation grade is divided into 2, 3, 4, 5, 6, 7, and 8 grades, and two mature normal crude oil samples in the same area are compare. According to research on biomarker compounds such as tricyclosterane, crude oil is derived from the same horizon source rock. In this work, these crude oils with different degrees of biodegradation were studied, and the carboxylic acid compounds were analyzed to explore the distribution of carboxylic acid compounds in biodegradable oils. The results show that the biodegradable crude oil in Biyang Depression is rich in carboxylic acid compounds, including linear, branched fatty acids, monocyclic, polycyclic carboxylic acids, benzene-containing carboxylic acids, decanoic acids, and decanoic acids. Dicarboxylic acids, etc., various compounds in the GC × GC-TOFMS chromatogram in a two-dimensional space regularly distributed, which provides an important basis for the identification of unknown compounds. By comparing the distribution of carboxylic acid compounds in crude oils with different degradation grades, it is found that the distribution and relative content of carboxylic acid compounds show obvious regularity with the increase of biodegradation.