Exclusive quantification of methyl-branched fatty acids and minor 18:1-isomers in foodstuff by GC/MS in the SIM mode using 10,11-dichloroundecanoic acid and fatty acid ethyl esters as internal standards

摘要

Minor fatty acids (iso- and anteiso-fatty acids, vaccenic acid, elaidic acid) in foodstuff (seafood, milk, and dairy products) were analyzed by gas chromatography with electron ionisation mass spectrometry in the selected ion monitoring mode (GC/EI-MS-SIM). For this purpose, lipids were obtained by accelerated solvent extraction (ASE) and the fatty acid constituents were converted into methyl esters. Instead of the determination of the relative contribution of the minor fatty acids to the sum of all fatty acids detected (the so-called 100% method), we exclusively quantified the minor fatty acids, which was possible by using two types of internal standards (IS-1 and IS-2). For recovery checks during the extraction and/or the transesterification step we added the novel 10,11-dichloroundecanoic acid (DC-11:0) as IS-1. DC-11:0, which has never been detected in foodstuff, was synthesized by electrophilic addition of chlorine to 10-undecenoic acid (11:1n-10). The novel IS eluted in the range of 23:0 from the polar GC column used and showed the same properties as fatty acids in foodstuff during sample preparation. Recovery rate of DC-11:0 was generally >96% in the various samples analyzed. Ethyl esters (FAEE) of a12:0, a14:0, a15:0, a16:0, a17:0, and a18:0 (IS-2) were added to both the external standard (a quantitative mixture of methyl esters of methyl-branched fatty acids and 18:1n-9trans) in order to determine their response factors relative to FAEE and to the food samples. With this technique, (only) methyl-branched fatty acids (MBFAs) as well as vaccenic acid (18:1n-7) and elaidic acid (18:1n-9trans) were quantified in a range of dairy products (including twelve cheeses) and seafood. All samples were analyzed in triplicates, and good standard deviations (concentrations 0.002–5 g/100 g; standard deviations 0.00–0.03) were obtained in all cases. MBFAs were detected in all samples analyzed. The highest content of MBFAs (3.0 g/100 g) was determined in red-smear of romadur cheese. In all except two cheeses, i17:0 was the most abundant MBFA. The highest amount of 18:1n-9trans was found in feta (2.84 g/100 g) whereas 0.03 g/100 g in big eye snapper (Pricanthus tayenus) marked the lowest record of this minor fatty acid. Seal oil contained the highest amount of 18:1n-7 with 5.00 g/100 g, whereas emmental cheese was the sample with the lowest content of this monoenoic fatty acid. The combination of suitable IS and a sensitive GC/EI-MS-SIM method proved to be well suited for the quantification of minor fatty acids in foodstuff. When only a set of fatty acids is going to be analyzed, this method is less time consuming compared to “100% methods” and less prone to false results due to the higher selectivity of GC/MS compared to GC in combination with flame ionisation detection (GC/FID).