Development of a Method for the Identification and Quantitation of Trace Chlorine-Containing Free Fatty Acids in Waste Cooking Oil Samples

Abstract Waste cooking oil is a potentially renewable substitute for crude oil for the generation of transportation fuels. Unfortunately, compared with the compounds found in crude oil, the compounds in waste cooking oil contain more heteroatoms, such as nitrogen, sulfur, and chlorine. Chlorinated compounds, for example, may arise upon reactions of unsaturated fatty acids, a major component of cooking oil, and hypochlorous acid, which is a frequently used cleaning agent. The goal of this study was to develop a methodology based on tandem mass spectrometry for the detection, identification, and quantitation of chlorinated fatty acids in waste cooking oil. Due to the often low concentrations of chlorine-containing compounds in waste cooking oil samples, the method involved the initial separation of polar compounds, such as fatty acids, from nonpolar compounds, such as glycerides, by using reversed-phase solid-phase extraction (SPE). A reversed-phase high-performance liquid chromatography (HPLC)/negative-ion-mode electrospray ionization (ESI) high-resolution tandem mass spectrometry (HRMS/MS) method was used to identify and quantify the chlorinated fatty acids. Monochloro-, dichloro-, and chlorohydroxy fatty acids were identified in the waste cooking oil samples by determining their elemental compositions via accurate mass measurements and by comparing their HPLC retention times to those of model compounds. Further, the collision-activated dissociation (CAD) patterns of their deprotonated forms were measured using MS/MS experiments and compared to those of deprotonated model compounds, thus acquiring valuable information about the fragmentation behavior of deprotonated chlorinated fatty acids. The accuracy of the quantitation method was determined to be good (greater than 95%) by doping a waste cooking oil sample with a known amount of an internal standard. For all identified chlorinated free fatty acids, a coefficient of variation (a measure of precision) was found to be <10% for the quantitation results.