Vetter Walter, Gaul Simon, Thurnhofer Saskia, Mayer Kathrin
Institute of Food Chemistry, University of Hohenheim, Garbenstr. 28, 70593 Stuttgart, Germany.
Anal Bioanal Chem. 2007 Sep;389(2):597-604. doi: 10.1007/s00216-007-1438-1. Epub 2007 Jul 17.
Methyl-branched fatty acids (MBFAs) are the dominant form of fatty acid found in many bacteria. They are also found at low levels in a range of foodstuffs, where their presence has been linked to bacterial sources. In this study we evaluated the potential of compound-specific isotope analysis to obtain insights into the stable carbon isotope ratios (delta(13)C values in per thousand) of individual MBFAs and to compare them to the stable carbon isotope ratios of straight-chain fatty acids in food. Due to their low abundance in foodstuffs, the MBFAs were enriched prior to gas chromatography coupled to isotope ratio mass spectrometric (GC-IRMS) analysis. After transesterification, urea complexation was used to suppress the 16:0 and 18:0 methyl esters that were dominant in the samples. Following that, silver-ion high performance liquid chromatography was used to separate the saturated from the unsaturated fatty acids. The resulting solutions of saturated fatty acids obtained from suet, goat's milk, butter, and human milk were studied by GC-IRMS. The delta(13)C values of fatty acids with 12-17 carbons ranged from -25.4 per thousand to -37.6 per thousand. In all samples, MBFAs were most depleted in carbon-13, followed by the odd-chain fatty acids 15:0 and 17:0. 14:0 and 16:0 contained the highest proportions of carbon-13. The results from this study illustrate that MBFAs have distinctive delta(13)C values and must originate from other sources and/or from very different substrates. These measurements support the initial hypothesis that delta(13)C values can be used to attribute MBFAs to particular sources.
甲基支链脂肪酸（MBFAs）是许多细菌中脂肪酸的主要形式。它们在一系列食品中也有少量存在，其存在与细菌来源有关。在本研究中，我们评估了化合物特异性同位素分析的潜力，以深入了解单个MBFAs的稳定碳同位素比率（以千分比表示的δ(13)C值），并将其与食品中直链脂肪酸的稳定碳同位素比率进行比较。由于它们在食品中的丰度较低，在与同位素比率质谱联用的气相色谱（GC-IRMS）分析之前，对MBFAs进行了富集。酯交换反应后，使用尿素络合来抑制样品中占主导地位的16:0和18:0甲酯。在此之后，使用银离子高效液相色谱将饱和脂肪酸与不饱和脂肪酸分离。通过GC-IRMS对从羊油、山羊奶、黄油和人乳中获得的饱和脂肪酸溶液进行了研究。含12 - 17个碳的脂肪酸的δ(13)C值范围为千分之-25.4至千分之-37.6。在所有样品中，MBFAs的碳-13含量最低，其次是奇数链脂肪酸15:0和17:0。14:0和16:0的碳-13比例最高。本研究结果表明，MBFAs具有独特的δ(13)C值，并必定源自其他来源和/或非常不同的底物。这些测量结果支持了最初的假设，即δ(13)C值可用于将MBFAs归因于特定来源。
