Department of Chemistry - BMC, Uppsala University, 751 24 Uppsala, Sweden.
J Am Soc Mass Spectrom. 2020 Dec 2;31(12):2479-2487. doi: 10.1021/jasms.0c00194. Epub 2020 Aug 5.
Unsaturated free fatty acids are natively present in biological samples as isomers, where double bonds can be situated on different carbons in the acyl chain. While these isomers can have different actions and impacts on biological systems, they are inherently difficult to identify and differentiate by mass spectrometry alone. To address this challenge, several techniques for derivatization of the double bond or metal cationization at the carboxylic group have yielded diagnostic product ions for the respective isomer in tandem mass spectrometry. However, diagnostic product ions do not necessarily reflect quantitative isomeric ratios since fatty acid isomers have different ionization and fragmentation efficiencies. Here, we introduce a simple and rapid approach to predict the quantitative ratio of isomeric monounsaturated fatty acids. Specifically, empirically derived MS product ion patterns from fatty acid silver adducts are modeled using a stepwise linear model. This model is then applied to predict the proportion oleic and vaccenic acid in chemically complex samples at individual concentrations between 0.45 and 5.25 μM, with an average accuracy and precision below 2 and 5 mol %, respectively. We show that by simply including silver ions in the electrospray solvent, isomeric ratios are rapidly predicted in neat standards, rodent plasma, and tissue extract. Furthermore, we use the method to directly map isomeric ratios in tissue sections using nanospray desorption electrospray ionization MS imaging without any sample preparation or modification to the instrumental setup. Ultimately, this approach provides a simple and rapid solution to differentiate monounsaturated fatty acids using commonly available commercial mass spectrometers without any instrumental modifications.
不饱和游离脂肪酸以同分异构体的形式存在于生物样本中，其中双键可以位于酰基链上的不同碳原子上。虽然这些异构体可能对生物系统有不同的作用和影响，但仅凭质谱法很难识别和区分它们。为了解决这个挑战，已经有几种技术可以对双键进行衍生化，或者在羧酸基团处进行金属阳离子化，从而在串联质谱中得到相应异构体的诊断产物离子。然而，诊断产物离子并不一定反映定量的异构体比例，因为脂肪酸异构体的电离和碎裂效率不同。在这里，我们介绍了一种简单快速的方法来预测单不饱和脂肪酸的定量异构体比例。具体来说，使用逐步线性模型对脂肪酸银加合物的经验衍生 MS 产物离子模式进行建模。然后，将该模型应用于预测化学复杂样品中单点浓度在 0.45 至 5.25 μM 之间的油酸和蓖麻油酸的比例，平均准确度和精密度分别低于 2%和 5%。我们表明，通过在电喷雾溶剂中简单地加入银离子，可以快速预测纯标准品、啮齿动物血浆和组织提取物中的异构体比例。此外，我们使用该方法直接在组织切片中使用纳米喷雾解吸电喷雾电离 MS 成像来映射异构体比例，而无需对仪器设置进行任何样品制备或修改。最终，该方法为使用常见的商业质谱仪区分单不饱和脂肪酸提供了一种简单快速的解决方案，而无需对仪器进行任何修改。
