Konermann Lars, McAllister Robert G, Metwally Haidy
Department of Chemistry, The University of Western Ontario , London, Ontario N6A 5B7, Canada.
J Phys Chem B. 2014 Oct 16;118(41):12025-33. doi: 10.1021/jp507635y. Epub 2014 Oct 2.
Electrospray ionization (ESI) produces desolvated ions from solution phase analytes for mass spectrometric detection. The final steps of gas phase ion formation from nanometer-sized solvent droplets remain a matter of debate. According to the ion evaporation model (IEM), analytes are ejected from the droplet surface via field emission, whereas the charged residue model (CRM) envisions that ions are released upon droplet evaporation to dryness. Exposure of salt solutions to ESI conditions produces a range of cluster ions. Despite the rich literature on these systems, it is still unclear if these salt clusters form via the CRM or the IEM. The current study explores the formation of Na(n)Cl(m)((n-m)+) clusters from aqueous sodium chloride solution under positive and negative polarity conditions. Molecular dynamics (MD) methods are used for simulating the temporal evolution of charged NaCl-containing water droplets. A trajectory stitching approach is developed for continuously removing evaporated moieties from the simulation, thereby dramatically reducing computational cost. In addition, this procedure ensures adequate temperature control and eliminates evaporative cooling that would otherwise slow down the process. Continuous water evaporation leads to progressive droplet shrinkage, while the emission of solvated single ions ensures that the system remains at ca. 90% of the Rayleigh limit. Early during the process all ions in the droplet behave as freely dissolved species, but after a few nanoseconds at 370 K the systems gradually morph into amorphous wet salt aggregates. Ultimately, free Na(n)Cl(m)((n-m)+) clusters form as the last solvent molecules evaporate. Our data therefore provide direct evidence that sodium chloride cluster formation during ESI proceeds via the CRM. The IEM nonetheless plays an ancillary role, as it allows the system to shed charge (mostly in the form of hydrated Na(+) or Cl(-)) during droplet shrinkage. It appears that this study marks the first successful MD simulation of complete CRM processes.
电喷雾电离（ESI）从溶液相分析物中产生去溶剂化离子用于质谱检测。从纳米尺寸的溶剂液滴形成气相离子的最终步骤仍是一个有争议的问题。根据离子蒸发模型（IEM），分析物通过场发射从液滴表面喷射出来，而带电残基模型（CRM）则设想离子在液滴蒸发至干时释放。将盐溶液暴露于ESI条件下会产生一系列簇离子。尽管关于这些体系有丰富的文献，但仍不清楚这些盐簇是通过CRM还是IEM形成的。当前的研究探索了在正负极性条件下，从氯化钠水溶液中形成Na(n)Cl(m)((n - m)+)簇的过程。分子动力学（MD）方法用于模拟含氯化钠的带电水滴的时间演化。开发了一种轨迹拼接方法，用于从模拟中连续去除蒸发的部分，从而显著降低计算成本。此外，该程序确保了适当的温度控制，并消除了否则会减缓该过程的蒸发冷却。连续的水蒸发导致液滴逐渐收缩，而溶剂化单离子的发射确保系统保持在约90%的瑞利极限。在过程早期，液滴中的所有离子都表现为自由溶解的物种，但在370 K下经过几纳秒后，系统逐渐演变成无定形湿盐聚集体。最终，随着最后溶剂分子的蒸发，形成了游离的Na(n)Cl(m)((n - m)+)簇。因此，我们的数据提供了直接证据，表明ESI过程中氯化钠簇的形成是通过CRM进行的。然而，IEM起到了辅助作用，因为它允许系统在液滴收缩过程中释放电荷（主要以水合Na(+)或Cl(-)的形式）。看来这项研究标志着首次成功地对完整的CRM过程进行MD模拟。
