Heintz Chris, Kersten Hendrik, Benter Thorsten, Wißdorf Walter
Department of Physical and Theoretical Chemistry, University of Wuppertal, Gaussstraße 20, 42119 Wuppertal, Germany.
J Am Soc Mass Spectrom. 2025 Apr 2;36(4):839-849. doi: 10.1021/jasms.4c00508. Epub 2025 Mar 21.
Electrospray ionization (ESI) is the most widely used technique for the ionization of liquid samples, for example, from liquid chromatography (LC) coupled to mass spectrometry. Recent experiments demonstrate the penetration of charged droplets or at least large clusters into the high-vacuum region of different ESI mass spectrometers. Using a Bruker micrOTOF equipped with a standard Bruker Apollo ESI source, we demonstrated that time-of-flight (TOF) MS can detect signatures of these droplets by analyzing the statistics of individual TOF spectra, resulting from a single orthogonal acceleration (oa) stage pulse. A custom experimental setup allows additional online monitoring of the ion current in the oa-stage by coupling an oscilloscope to an auxiliary secondary electron multiplier (SEM). The results obtained with ESI are compared to mass spectra recorded under similar conditions using atmospheric pressure chemical ionization (APCI). Our findings reveal that the observation of droplet signatures is unique to the ESI process, with their frequency and intensity strongly determined by the ion source settings. We also report that the majority of the individual spectra obtained do not contain ion signals. The observed intensity in the summed spectra stems from a few very intense spectra, which result from single droplet fragment bursts. In contrast, APCI provides an almost continuous and stable ion current, without intense signal bursts characteristic for ESI. Additional optical monitoring strongly suggests that these signatures are not a result of spray instability, but are common even for undisturbed, continuous spray operation. The variation of ion source parameters shows that specific capillary voltages, nebulizer pressures, and dry gas flows lead to an increase in the frequency of droplet occurrence. Since these parameters are fundamental and frequently altered in analytical measurements, the results reported in this contribution underscore the significance of understanding droplet dynamics in ESI-MS and provide insights regarding droplets affecting the ESI signal intensity recorded in analytical runs.
电喷雾电离（ESI）是用于液体样品电离的最广泛使用的技术，例如，用于与质谱联用的液相色谱（LC）中的液体样品。最近的实验表明带电液滴或至少大的团簇能够穿透到不同ESI质谱仪的高真空区域。使用配备标准布鲁克Apollo ESI源的布鲁克微型飞行时间质谱仪（micrOTOF），我们证明了飞行时间（TOF）质谱可以通过分析由单个正交加速（oa）阶段脉冲产生的单个TOF谱的统计数据来检测这些液滴的特征。一种定制的实验装置通过将示波器连接到辅助二次电子倍增器（SEM），可以对oa阶段的离子电流进行额外的在线监测。将ESI获得的结果与在类似条件下使用大气压化学电离（APCI）记录的质谱进行比较。我们的研究结果表明，液滴特征的观察是ESI过程所特有的，其频率和强度在很大程度上由离子源设置决定。我们还报告说，获得的大多数单个谱不包含离子信号。在总和谱中观察到的强度来自少数非常强烈的谱，这些谱是由单个液滴碎片爆发产生的。相比之下，APCI提供几乎连续且稳定的离子电流，没有ESI特有的强烈信号爆发。额外的光学监测有力地表明，这些特征不是喷雾不稳定性的结果，即使在未受干扰的连续喷雾操作中也是常见的。离子源参数的变化表明，特定的毛细管电压、雾化器压力和干燥气体流量会导致液滴出现频率增加。由于这些参数在分析测量中是基本的且经常改变，本论文报道的结果强调了理解ESI-MS中液滴动力学的重要性，并提供了有关影响分析运行中记录的ESI信号强度的液滴的见解。
