We investigated the electrospray ionization inside the narrow channel of the ion inlet tube. An insulating emitter capillary made of fused silica with a 0.2 mm outer diameter was inserted into the ion inlet tubes with a 0.5 and 0.6 mm inner diameter to aspirate all the charged droplets. A custom-made ion inlet tube with two side holes near its entrance is used to observe the spraying condition. The spray current is measured and monitored during the MS acquisition using isolation amplifiers. Because the emitter is cylindrically surrounded in close proximity by the metallic inner wall, it is difficult to obtain a stable and symmetric Taylor cone with its apex at the center of the emitter. Instead, a stable operation under a flow rate of 1-4 μL/min is found to be in the form of a multicone-jet mode with two or more Taylor cones anchoring around the rim of the emitter. The emitted charged droplet jets are dragged from hitting the wall by the fast-flowing air inside the inlet tube. Comparison with the typical cone-jet and multijet mode operated several millimeters outside the inlet capillary shows signal enhancements for protein standards.

中文翻译：

---

离子入口管内的电喷雾电离：多喷射模式操作。

我们研究了离子入口管窄通道内的电喷雾电离。将外径为 0.2 毫米的熔融石英制成的绝缘发射毛细管插入内径为 0.5 和 0.6 毫米的离子入口管中，以吸入所有带电液滴。一个定制的离子入口管在其入口附近有两个侧孔，用于观察喷雾情况。在 MS 采集过程中使用隔离放大器测量和监测喷雾电流。由于发射器被金属内壁紧密环绕成圆柱形，因此很难获得稳定且对称的泰勒锥，其顶点位于发射器的中心。反而，发现在 1-4 μL/min 流速下的稳定运行是多锥喷射模式，其中两个或多个泰勒锥锚定在发射器的边缘。喷出的带电液滴射流被入口管内快速流动的空气拖着撞到壁上。与在入口毛细管外几毫米处操作的典型锥形喷射和多喷射模式相比，蛋白质标准品的信号增强。