Thurnhofer Saskia, Vetter Walter
University of Hohenheim, Institute of Food Chemistry (170b), Garbenstrasse 28, D-70599 Stuttgart, Germany.
J Agric Food Chem. 2006 May 3;54(9):3209-14. doi: 10.1021/jf053022j.
Ethyl esters (FAEE) and trideuterium-labeled methyl esters (d3-FAME) of fatty acids were prepared and investigated regarding their suitability as internal standards (IS) for the determination of fatty acids as methyl esters (FAME). On CP-Sil 88, ethyl esters of odd-numbered fatty acids eluted approximately 0.5 min after the respective FAME, and only coelutions with minor FAME were observed. Depending on the problem, one or even many FAEE can be added as IS for the quantification of FAME by both GC-FID and GC-MS. By contrast, d3-FAME coeluted with FAME on the polar GC column, and the use of the former as IS requires application of GC-MS. In the SIM mode, m/z 77 and 90 are suggested for d3-methyl esters of saturated fatty acids, whereas m/z 88 and 101 are recommended for ethyl esters of saturated fatty acids. These m/z values give either no or very low response for FAME and can thus be used for the analysis of FAME in food by GC-MS in the SIM mode. Fatty acids in sunflower oil and mozzarella cheese were quantified using five saturated FAEE as IS. Gravimetric studies showed that the transesterification procedure could be carried out without of loss of fatty acids. GC-EI/MS full scan analysis was suitable for the quantitative determination of all unsaturated fatty acids in both food samples, whereas GC-EI/MS in the SIM mode was particularly valuable for quantifying minor fatty acids. The novel GC-EI/MS/SIM method using fatty acid ethyl esters as internal standards can be used to quantify individual fatty acids only, that is, without determination of all fatty acids (the common 100% method), although this is present. This was demonstrated by the exclusive quantification of selected fatty acids including methyl-branched fatty acids, erucic acid (18:1n-9trans), and polyunsaturated fatty acids in cod liver oil and goat's milk fat.
制备了脂肪酸的乙酯（FAEE）和氘代甲基酯（d3-FAME），并研究了它们作为内标（IS）用于测定脂肪酸甲酯（FAME）的适用性。在CP-Sil 88上，奇数脂肪酸的乙酯在相应的FAME之后约0.5分钟洗脱，仅观察到与少量FAME的共洗脱。根据具体问题，一种甚至多种FAEE可作为内标添加，用于通过GC-FID和GC-MS对FAME进行定量。相比之下，d3-FAME在极性GC柱上与FAME共洗脱，将前者用作内标需要使用GC-MS。在选择离子监测（SIM）模式下，建议对饱和脂肪酸的d3-甲酯使用m/z 77和90，而对饱和脂肪酸的乙酯推荐使用m/z 88和101。这些m/z值对FAME的响应要么没有要么非常低，因此可用于在SIM模式下通过GC-MS分析食品中的FAME。使用五种饱和FAEE作为内标对葵花籽油和马苏里拉奶酪中的脂肪酸进行了定量。重量法研究表明，酯交换过程可以在不损失脂肪酸的情况下进行。GC-EI/MS全扫描分析适用于定量测定两种食品样品中的所有不饱和脂肪酸，而SIM模式下的GC-EI/MS对于定量微量脂肪酸特别有价值。使用脂肪酸乙酯作为内标的新型GC-EI/MS/SIM方法仅可用于定量单个脂肪酸，即无需测定所有脂肪酸（常见的100%方法），尽管所有脂肪酸都存在。这通过对鱼肝油和山羊奶脂肪中包括甲基支链脂肪酸、芥酸（18:1n-9反式）和多不饱和脂肪酸在内的选定脂肪酸的单独定量得到了证明。
