Institute of Chemistry, University of Tartu, Ravila 14a, 50411, Tartu, Estonia.
CNRS, Université Claude Bernard Lyon 1, Ens de Lyon, Institut des Sciences Analytiques, University of Lyon, UMR 5280, 5 rue de la Doua, F-69100, Villeurbanne, France.
J Am Soc Mass Spectrom. 2017 Oct;28(10):2124-2131. doi: 10.1007/s13361-017-1737-5. Epub 2017 Jul 25.
Ionization efficiency and mechanism in ESI is strongly affected by the properties of mobile phase. The use of mobile-phase properties to accurately describe droplets in ESI source is convenient but may be inadequate as the composition of the droplets is changing in the plume due to electrochemical reactions occurring in the needle tip as well as continuous drying and fission of droplets. Presently, there is paucity of research on the effect of the polarity of the ESI mode on mobile phase composition in the droplets. In this paper, the change in the organic solvent content, pH, and droplet size are studied in the ESI plume in both ESI+ and ESI- ionization mode. We introduce a rigorous way - the absolute pH (pH) - to describe pH change in the plume that takes into account organic solvent content in the mobile phase. pH enables comparing acidities of ESI droplets with different organic solvent contents. The results are surprisingly similar for both ionization modes, indicating that the dynamics of the change of mobile-phase properties is independent from the ESI mode used. This allows us to conclude that the evolution of ESI droplets first of all proceeds via the evaporation of the organic modifier and to a lesser extent via fission of smaller droplets from parent droplets. Secondly, our study shows that qualitative findings related to the ESI process obtained on the ESI+ mode can almost directly be applied also in the ESI- mode. Graphical Abstract ᅟ.
在 ESI 中，离子化效率和机制强烈受到流动相性质的影响。利用流动相性质来准确描述 ESI 源中的液滴是方便的，但可能不够充分，因为由于在针尖处发生的电化学反应以及液滴的连续干燥和分裂，液滴中的组成在羽流中发生变化。目前，关于 ESI 模式的极性对液滴中流动相组成的影响的研究很少。在本文中，研究了在 ESI+和 ESI-电离模式下，在 ESI 羽流中有机溶剂含量、pH 值和液滴尺寸的变化。我们引入了一种严格的方法 - 绝对 pH（pH）- 来描述羽流中 pH 值的变化，该方法考虑了流动相中的有机溶剂含量。pH 值使我们能够比较具有不同有机溶剂含量的 ESI 液滴的酸度。两种电离模式的结果惊人地相似，这表明流动相性质变化的动力学与所使用的 ESI 模式无关。这使我们能够得出结论，ESI 液滴的演变首先通过有机改性剂的蒸发进行，并且在较小程度上通过从小液滴从母液滴的分裂进行。其次，我们的研究表明，在 ESI+模式下获得的与 ESI 过程相关的定性发现几乎可以直接应用于 ESI-模式。
