Li Hou-I, Prabhu Gurpur Rakesh D, Buchowiecki Krzysztof, Urban Pawel L
Department of Chemistry, National Tsing Hua University, 101, Section 2, Kuang-Fu Rd., Hsinchu 300044, Taiwan.
J Am Soc Mass Spectrom. 2024 Feb 7;35(2):244-254. doi: 10.1021/jasms.3c00345. Epub 2024 Jan 16.
Previous mechanistic descriptions of electrosprays mostly focused on the dynamics of Taylor cones, initial droplets, and progeny droplets. However, vapor formation during droplet desolvation in an electrospray plume has not been discussed to a great extent. Here, we implement a double-pass on-axis schlieren high-speed imaging system to observe generation and propagation of vapors in an offline electrospray source under different conditions. Switching between turbulent and laminar vapor flow was observed for all of the scanned conditions, which may be attributed to randomly occurring disturbances in the sample flow inside the electrospray emitter. Calculation of mean vapor flow velocity and analysis of vapor flow patterns were performed using in-house developed image processing programs. Experiments performed at different electrospray voltages (0-6 kV), solvent flow rates (100-600 μL min), and methanol concentrations (50-100%), indicate only a weak dependency between electrospray voltage and mean vapor velocity, implying that the vapor is mostly neutral; thus, the vapor is not accelerated by electric field. On the other hand, electrospraying solutions of analytes (with mass 151 Da or 12 kDa) did not remarkably increase the overall vapor flow velocity. The source of vapor's velocity is attributed to the inertia of the electrospray droplets. Although there are some differences between a modern electrospray ionization (ESI) setup and the setup used in our experiment (.., using a higher flow rate and larger emitter), we believe the findings of our study can be projected to a modern ESI setup.
先前对电喷雾的机理描述大多集中在泰勒锥、初始液滴和子代液滴的动力学上。然而，电喷雾羽流中液滴去溶剂化过程中的蒸汽形成在很大程度上尚未得到讨论。在这里，我们采用双程同轴纹影高速成像系统，观察不同条件下离线电喷雾源中蒸汽的产生和传播。在所有扫描条件下均观察到了湍流和层流蒸汽流之间的切换，这可能归因于电喷雾发射器内部样品流中随机出现的扰动。使用内部开发的图像处理程序对平均蒸汽流速进行了计算，并对蒸汽流模式进行了分析。在不同的电喷雾电压（0 - 6 kV）、溶剂流速（100 - 600 μL/min）和甲醇浓度（50 - 100%）下进行的实验表明，电喷雾电压与平均蒸汽速度之间仅存在微弱的相关性，这意味着蒸汽大多呈中性；因此，蒸汽不会被电场加速。另一方面，对分析物（质量为151 Da或12 kDa）进行电喷雾并未显著提高整体蒸汽流速。蒸汽速度的来源归因于电喷雾液滴的惯性。尽管现代电喷雾电离（ESI）装置与我们实验中使用的装置（……，使用更高的流速和更大的发射器）之间存在一些差异，但我们相信我们的研究结果可以推广到现代ESI装置上。
