RESUMO
The free fatty acids that contain one to eight carbons (C1-C8) in biodiesel would affect the quality of biodiesel. It is still a matter of challenge to simultaneously determine the composition of C1-C8 fatty acids in seed oil and seed oil-based biodiesel. Herein, a novel method of charge derivatization coupling with direct infusion mass spectrometry (CD-DIMS) was developed for the determination of the C1-C8 fatty acids in biodiesels. A fixed-charge derivatization reagent, 1-cyclohexyl-3-(2-morpholinoethyl)carbodiimide (CMCT), was used to convert fatty acids into their cationic derivatives, which significantly improved the sensitivity and selectivity of detection. Good linearity was observed with the limits of detection (LODs) in the range of 0.0002-0.001 µg mL-1 for the investigated fatty acids. The recovery was in the range of 85.1%-101.9% and the matrix effect was within the range of 75.5-93.2%. The developed method was carried out to analyze C1-C8 fatty acids in rubber seed oil (RSO) and RSO-based biodiesels produced by different catalysts, including NaOH, TiO2, and carbodiimide. It was also applied to the dynamic monitoring of C1-C8 fatty acids in RSO and produced RSO biodiesels during the oxidation process. As results, formic acid, acetic acid, and propionic acid were detected in aged RSO and biodiesel samples. The contents of formic acid, acetic acid, and propionic acid all increased in aged RSO and biodiesels, but with different growth rates. These results demonstrated that the developed CD-DIMS method can provide a quick, accurate, and sensitive analysis of C1-C8 fatty acids in seed oil and biodiesel samples.
