McMahon William P, Jorabchi Kaveh
Department of Chemistry, Georgetown University, Washington, DC 20057, USA.
Analyst. 2020 Nov 9;145(22):7357-7371. doi: 10.1039/d0an01198c.
Nano-ESI is a commonly used ionization technique with continually expanding analytical advantages. Here, we report a facile way for high-frequency (500-3800 Hz) pulsing of nano-ESI, providing a high flux of mobility-selected ions. The pulsing is accomplished using a relatively low-voltage modulation (80 V peak-to-peak) of an electrode placed <1 cm downstream of a nano-ESI emitter biased to a constant potential. Configuring the electrode as an ion gate enables mobility-based ion selection by scanning the modulation frequency. Our investigations indicate that the electrode modulation perturbs continuous nano-ESI, resulting in solution accumulation at the emitter tip between spray pulses. Selective transmission of ions occurs at frequencies corresponding to harmonics of a fundamental frequency determined by the travel time of each ion from the emitter to the ion gate (pulsing electrode). Remarkably, the intensities of ions selected in this fashion are similar across the harmonics, suggesting that the ionization efficiencies of analytes have minimal dependence on the accumulated volume at the emitter tip. Moreover, intensities of ion-mobility-selected analytes using this technique reach >50% of those in continuous nano-ESI without ion selection, underscoring efficient ion generation via high-frequency pulsing. These findings indicate the potential of the pulsed nano-ESI for enhanced analytical utility, such as a high-flux selected-reagent-ion supplier at atmospheric pressure, and chart new avenues to further enhance the analytical performance of nano-ESI.
纳米电喷雾电离（Nano-ESI）是一种常用的电离技术，其分析优势在不断扩展。在此，我们报告了一种实现纳米电喷雾电离高频（500 - 3800赫兹）脉冲化的简便方法，可提供高通量的迁移率选择离子。该脉冲化通过对置于纳米电喷雾电离发射器下游小于1厘米处的电极施加相对低电压调制（峰 - 峰值80伏）来实现，该发射器被偏置到恒定电位。将该电极配置为离子门，通过扫描调制频率可实现基于迁移率的离子选择。我们的研究表明，电极调制会干扰连续的纳米电喷雾电离，导致在喷雾脉冲之间发射器尖端出现溶液积累。离子的选择性传输发生在与由每个离子从发射器到离子门（脉冲电极）的传输时间所确定的基频谐波相对应的频率处。值得注意地是，以这种方式选择的离子在各谐波处的强度相似，这表明分析物的电离效率对发射器尖端积累的体积依赖性最小。此外，使用该技术经离子迁移率选择的分析物强度达到无离子选择的连续纳米电喷雾电离强度的50%以上，这突出了通过高频脉冲实现高效离子生成。这些发现表明脉冲纳米电喷雾电离在增强分析实用性方面的潜力，例如作为大气压下的高通量选择反应离子供应源，并为进一步提高纳米电喷雾电离的分析性能开辟了新途径。
