Extractive electrospray ionization (EESI) has been a well-known technique for high-throughput online molecular characterization of chemical reaction products and intermediates, detection of native biomolecules, in vivo metabolomics, and environmental monitoring with negligible thermal and ionization-induced fragmentation for over two decades. However, the EESI extraction mechanism remains uncertain. Prior studies disagree on whether particles between 20 and 400 nm diameter are fully extracted or if the extraction is limited to the surface layer. Here, we examined the analyte extraction mechanism by assessing the influence of particle size and coating thickness on the detection of the molecules therein. We find that particles are extracted fully: organics-coated NH₄NO₃ particles with a fixed core volume (156 and 226 nm in diameter without coating) showed constant EESI signals for NH₄NO₃ independent of the shell coating thickness, while the signals of the secondary organic molecules comprising the shell varied proportionally to the shell volume. We also found that the EESI sensitivity exhibited a strong size dependence, with an increase in sensitivity by 1–3 orders of magnitude as particle size decreased from 300 to 30 nm. This dependence varied with the electrospray (ES) droplet size, the particle size and the residence time for coagulation in the EESI inlet, suggesting that the EESI sensitivity was influenced by the coagulation coefficient between particles and ES droplets. Overall, our results indicate that, in the EESI, particles are fully extracted by the ES droplets regardless of the chemical composition, when they are collected by the ES droplets. However, their coalescence is not complete and depends strongly on their size. This size dependence is especially relevant when EESI is used to probe size-varying particles as is the case in aerosol formation and growth studies with size ranges below 100 nm.

中文翻译：

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气溶胶大小和涂层厚度对萃取电喷雾电离分子检测的影响

萃取电喷雾电离 (EESI) 一直是一种众所周知的技术，用于化学反应产物和中间体的高通量在线分子表征、天然生物分子的检测、体内代谢组学和环境监测，热和电离诱导的碎裂可忽略不计超过两个几十年。然而，EESI 提取机制仍然不确定。先前的研究不同意直径在 20 到 400 nm 之间的颗粒是否被完全提取，或者提取是否仅限于表面层。在这里，我们通过评估粒径和涂层厚度对其中分子检测的影响来检查分析物提取机制。我们发现颗粒被充分提取：有机物包覆的 NH ₄ NO ₃具有固定核心体积（直径为 156 和 226 nm，无涂层）的颗粒对 NH ₄ NO ₃显示出恒定的 EESI 信号与壳涂层厚度无关，而构成壳的次级有机分子的信号与壳体积成比例变化。我们还发现 EESI 灵敏度表现出强烈的尺寸依赖性，随着粒径从 300 减小到 30 nm，灵敏度增加 1-3 个数量级。这种依赖性随电喷雾 (ES) 液滴尺寸、粒径和在 EESI 入口凝固的停留时间而变化，这表明 EESI 灵敏度受颗粒和 ES 液滴之间的凝固系数的影响。总的来说，我们的结果表明，在 EESI 中，当颗粒被 ES 液滴收集时，无论化学成分如何，它们都会被 ES 液滴完全提取。然而，它们的合并并不完全，很大程度上取决于它们的大小。当 EESI 用于探测尺寸变化的颗粒时，这种尺寸依赖性尤其重要，就像在尺寸范围低于 100 nm 的气溶胶形成和生长研究中一样。