Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization
Ion Formation from Rapidly Heated Aqueous Droplets by Droplet-Assisted Ionization
复制标题
DOI：
10.1021/acs.jpca.0c07101
复制
发表时间：
2020-09-10
影响因子：
2.9
通讯作者：
Johnston, Murray, V
中科院分区：
文献类型：
--
作者：
Apsokardu, Michael J.;Krasnomowitz, Justin M.;Johnston, Murray, V
当水性液滴从大气压行进通过温度控制的毛细管进入真空时，它们经历空气动力学和/或热分裂以产生带电的后代液滴，所述带电的后代液滴随后从原始液滴中的溶质产生气相分子离子。这种现象是液滴辅助电离的基础，液滴辅助电离是最近开发的一种通过质谱法在线表征气溶胶的方法。通过计算流体力学（CFD）和液滴蒸发模型研究了初始液滴分裂成子代液滴的条件。计算流体力学的结果，然后用来解释离子电流与毛细管壁温度的实验测量。对于低于约150 ℃的毛细管壁温度，液滴经历空气动力学或热破碎的能力强烈依赖于温度。在此温度以上，初始液滴分裂的模式变得与温度无关，并且离子信号强度的温度依赖性可以解释为与带电后代液滴的离子形成有关。离子形成的活化能分为两大类：与主要含有非离子溶质的液滴的41 kJ mol（-1）相似，这与水的蒸发焓相匹配，并表明离子形成的电荷残留过程，与含有盐的液滴的24 kJ mol（-1）相似，这表明离子蒸发过程，其中离子从溶剂分子簇内的液滴表面喷射。
When aqueous droplets travel through a temperature-controlled capillary from atmospheric pressure into a vacuum, they undergo aerodynamic and/or thermal breakup to give charged progeny droplets that subsequently produce gas-phase molecular ions from solutes that were in the original droplets. This phenomenon is the basis of droplet-assisted ionization, a method that was recently developed for online characterization of aerosols by mass spectrometry. The conditions allowing initial droplets to break up into progeny droplets were studied by computational fluid dynamics (CFD) with a droplet evaporation model. The CFD results were then used to interpret experimental measurements of ion current vs capillary wall temperature. For capillary wall temperatures below about 150 degrees C, the abilities of droplets to undergo either aerodynamic or thermal breakup are strongly temperature dependent. Above this temperature, the mode of initial droplet breakup becomes temperature independent, and the temperature dependence of the ion signal intensity can be explained in relation to ion formation from charged progeny droplets. Activation energies for ion formation fall into two main categories: similar to 41 kJ mol(-1) for droplets containing predominantly nonionic solutes, which matches the enthalpy of vaporization for water and suggests a charge residue process for ion formation, and similar to 24 kJ mol(-1) for droplets containing salts, which suggests an ion evaporation process where the ion is ejected from the droplet surface within a cluster of solvent molecules.