Identification of Furan Fatty Acids in Nutritional Oils and Fats by Multidimensional GC-MSD

Identification of furan fatty acids as minor components in oils and fats require several pre-analytical separation steps in order to obtain sufficient resolution and sensitivity in single column gaschromatography. After extraction and transesterification hydrogenation, urea complex precipitation and silicagel column chromatography or Ag+-TLC were applied prior to GC-analysis. By using a multidimensional GCMSD System with cooled injection and flow controlled column switching with cold trapping in between, the methyl esters of furan fatty acids can be identified directly without any further preanalytical separations. Butter, milk and ten nutritional oils were investigated. Different transesterification methods were used for the characterization of the oils and compared with each other. Electron impact and chemical ionization were applied to identify the fatty acid methyl esters by GC-MS. Fourteen furan fatty acids were identified in fish oil, two in butter and one in milk. The same furan fatty acid were found in peanut, thistle, sunflower, hazelnut and olive oil, whereas none were detected so far in sesame, corn, walnut or grape seed oil. INTRODUCTION As one of the first furan fatty acids (F-acids, Figure 1) the disubstituted 9,12-epoxy-octadeca-9,11dienoic acid was found in seed oil of exocarpus cupressiformis in 1966 [1]. A series of triand tetrasubstituted F-acids were later demonstrated to be present in different species of fish [2-8], in soft corals [9], different plants [10,11], amphibians [12], reptilians [12] and in mammals [13,14], including man [15]. Elaborate studies on the hepatopancreatic lipids of the crayfish Procambarus clarkii revealed a total of 30 F-acids [12,16]. 3-methyl-2,4-nonanedione, one of the major compounds that causes the light-induced offflavour of soya-bean oil [17,18] was shown to be a photooxidation product of furan fatty acids [19]. Further studies on different vegetable oils revealed the presence of F-acids 5, 7 and 10 (Table I) in soyabean, wheat germ, rapeseed and corn oil whereas none were detected in olive or sunflower oil. In butter and butter oil nine furan fatty acids were found [20]. Figure 1. Furan fatty acids. O (CH2)m (CH2)n R2 R1 COOH H3C R 1 , R 2 : H,CH 3 m: 2,4,6-12,14 n: 2-6 This report describes a method for the analysis of minor components in complex sample matrices. After transesterification furan fatty acid methyl esters can directly be identified by the use of a multidimensional GC-MSD System with cooled injection and flow controlled column switching. Butter, milk and 10 nutritional oils were investigated and their fatty acid composition characterized by different methods of transesterification. EXPERIMENTAL Sample preparation. 500 mg of oil were transesterified to the corresponding methylesters by sequential saponification and esterification [21] under a nitrogen atmosphere to avoid oxidation of the polyunsaturated fatty acids. Transesterification were also achieved by use of Trimethylsulfonium Hydroxide (TMSH): 100 mg of oil were dissolved in 5 ml tert.-butylmethylether. 50 μl TMSH were added to 100 μl of the solution and injected into the single column GC [22]. GC-MS System. A Hewlett-Packard HP 5890/5971 GC-MS system equipped with a HP 7673 automatic sampler (Hewlett Packard, Avondale PA, USA) and a 25 m x 0.2 mm i.d. HP-1 (dimethylpolysiloxane) column was used for analysis with electron-impact ionization. The column head pressure was set to 60 kPa and the injection volume was 1 μl with a split ratio of 1:10. The injector and the transfer line temperatures were 280 and 290°C, respectively; after injection the column temperature was programmed from 130 to 260°C at 2°C/min and then at 40°C/min to 300°C, held for 20 min. A model TSQ 70 GCMS system (Finnigan MAT, Bremen, Germany) was used for the chemical ionization analysis, methane being the reagent gas. Multidimensional GC-MSD. Figure 2 represents a scheme of the various components used to configure the system employed for this work. The apparatus consists of a temperature programmable cold injection system with a septumless sampling head (CIS-3, Gerstel GmbH, Mülheim an der Ruhr, Germany), two HP 5890 GC ovens (Hewlett Packard, Avondale PA, USA), connected via a heated transferline incorporating a cryotrap (CTS-1, Gerstel GmbH, Mülheim an der Ruhr, Germany). The second oven is equipped with a mass selective detector (HP 5971 A, Hewlett Packard, Avondale PA, USA). Figure 2. Schematic diagram of the applied system which consists of a temperature programmable cold injection system with a septumless sampling head (1), a GC (2) configured with a monitor FID (3), column switching device (4) and pneumatics, connected via a heated transfer line incorporating a cryotrap (5) to a second GC (6) which has a second switching device (7) installed after the the transfer line with the main column to the msd (8). 8 1 3 4