Hung Min-Min, Elpa Decibel P, Ochirov Ochir, Urban Pawel L
Department of Chemistry, National Tsing Hua University, 101, Section 2, Kuang-Fu Road, Hsinchu 300044, Taiwan.
J Am Soc Mass Spectrom. 2025 May 7;36(5):1191-1199. doi: 10.1021/jasms.5c00082. Epub 2025 Apr 15.
In electrospray ionization (ESI) mass spectrometry (MS), an electric DC potential is often applied to a metal capillary used to infuse a liquid sample. However, in some cases, especially when employing nanoelectrospray ionization (nanoESI), it is convenient to use a nonconducting capillary for sample delivery and spraying. In these cases, the potentials can be applied, for example, using a metal union placed in the proximity of the capillary outlet or to an electrode located in the sample reservoir near the capillary inlet. The optimum potential values, which warrant high MS signals, are different in these two operational conditions. A higher potential needs to be applied when the electrode is placed further away from the capillary outlet. Moreover, sample conductivity has a strong influence on the optimum potential values. Lower potentials must be used with highly conductive electrolytes. Thus, DC voltage scans are required to determine the optimum potentials. Applying electric potential to the electrode located in the sample reservoir, rather than metal union, significantly decreases the appearance of oxidized analyte peaks. We also show that a single-polarity square AC waveform can be applied to the union or sample reservoir electrode, and if its frequency is sufficiently high, it has a similar effect as decreasing DC voltage, allowing for digital control of electrospray with square waves (by varying duty cycle). Interestingly, the liquid meniscus oscillation frequency is independent of the AC signal frequency if the frequency is sufficiently high. Applying the AC signal in certain conditions stabilizes the electrospray plume. These observations reveal the resemblance of the ESI sample line to an circuit.
在电喷雾电离（ESI）质谱（MS）中，直流电势通常施加于用于注入液体样品的金属毛细管。然而，在某些情况下，特别是采用纳米电喷雾电离（nanoESI）时，使用非导电毛细管进行样品输送和喷雾较为方便。在这些情况下，可以通过例如在毛细管出口附近放置金属接头或在靠近毛细管入口的样品储存器中的电极来施加电势。在这两种操作条件下，保证高MS信号的最佳电势值是不同的。当电极离毛细管出口较远时，需要施加更高的电势。此外，样品电导率对最佳电势值有很大影响。对于高导电电解质必须使用较低的电势。因此，需要进行直流电压扫描来确定最佳电势。将电势施加于位于样品储存器中的电极而非金属接头，可显著减少氧化分析物峰的出现。我们还表明，可以将单极性方波交流波形施加于接头或样品储存器电极，并且如果其频率足够高，它具有与降低直流电压类似的效果，从而允许通过方波（通过改变占空比）对电喷雾进行数字控制。有趣的是，如果频率足够高，液体弯月面振荡频率与交流信号频率无关。在某些条件下施加交流信号可稳定电喷雾羽流。这些观察结果揭示了ESI样品线与一个电路的相似之处。
