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 MS3 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 MS3 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.

中文翻译：

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通过模拟 MS3 产物离子模式测定生物系统中的单不饱和脂肪酸异构体。

不饱和游离脂肪酸以异构体的形式天然存在于生物样品中，其中双键可以位于酰基链中的不同碳原子上。虽然这些异构体可以对生物系统产生不同的作用和影响，但它们本质上很难单独通过质谱来识别和区分。为了应对这一挑战，一些衍生化双键或羧基金属阳离子化的技术已经在串联质谱法中产生了相应异构体的诊断产物离子。然而，诊断产物离子不一定反映定量异构体比率，因为脂肪酸异构体具有不同的电离和碎裂效率。在这里，我们介绍了一种简单快速的方法来预测异构单不饱和脂肪酸的数量比。具体来说，使用逐步线性模型对来自脂肪酸银加合物的经验得出的 MS3 产物离子模式进行建模。然后应用该模型来预测化学复杂样品中油酸和异油酸的比例，浓度在 0.45 到 5.25 μM 之间，平均准确度和精密度分别低于 2 和 5 mol%。我们表明，通过在电喷雾溶剂中简单地包含银离子，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率。此外，我们使用该方法使用纳米喷雾解吸电喷雾电离 MS3 成像直接绘制组织切片中的异构体比率，而无需任何样品制备或对仪器设置的修改。最终，