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 RC circuit.

在电喷雾离子化（ESI）质谱法（MS）中，通常会在用于注入液体样品的金属毛细管上施加直流电压。然而，在某些情况下，特别是在使用纳升电喷雾离子化（nanoESI）时，使用非导电毛细管输送和喷射样品会更为方便。在这种情况下，可以通过在靠近毛细管出口处放置金属接头或将电极置于毛细管入口附近的样品池中来施加电压。这两种操作条件下所需的最优电压值不同。如果电极远离毛细管出口，则需要施加更高的电压。此外，样品的导电性对最佳电压值有显著影响。使用高导电性的电解质时必须使用较低的电压。因此，需要进行直流电压扫描以确定最佳电压值。将电压施加到位于样品池中的电极上（而非金属接头），可以大大减少氧化产物峰的出现。我们还展示了单极性方波交流信号可以应用于接头或样品池电极，并且如果其频率足够高，它会产生类似于降低直流电压的效果，允许通过改变占空比来用方波控制电喷雾。有趣的是，当频率足够高时，液体液滴表面的振荡频率与AC信号频率无关。在某些条件下应用AC信号可以稳定电喷雾羽流。这些观察结果揭示了ESI样品线与RC电路之间的相似性。