Feedback Control of Electrospray with and without an External Liquid Pump Using the Spray Current and the Apex Angle of a Taylor Cone for ESI-MS

Abstract

An electrospray operated in the steady cone-jet mode is highly stable but the operating state can shift to pulsation or multijet modes owing to changes in flow rate, surface tension, and electrostatic variables. Here, a simple feedback control system was developed using the spray current and the apex angle of a Taylor cone to determine the error signal for correcting the emitter voltage. The system was applied to lock the cone-jet mode operation against external perturbations. For a pump-driven electrospray at a regulated flow rate, the apex angle of the Taylor cone decreased with increasing voltage. In contrast, for a voltage-driven electrospray with low flow resistance, the angle was found to increase with the emitter voltage. A simple algorithm based on iterative learning control was formulated and implemented using a personal computer to automatically correct the emitter voltage in response to the error signal. For voltage-driven electrospray ionization (ESI), the feedback control of the spray current can also be used to regulate the flow rate to an arbitrary value or pattern. Electrospray ionization-mass spectrometry (ESI-MS) with feedback control was demonstrated to produce ion signal acquisition with long-term stability that was insusceptible to the emulated external disturbances.

Formation of Alternating Surfactant-Enriched and Surfactant-Depleted Phases in the Taylor Cone of a Nanoelectrospray

“…To ensure that the change in ion intensities was not caused by the instability in the spray current, a feedback control was imposed on the emitter voltage using the spray current as the feedback signal (Figure ). The feedback control algorithm was described in the previous report . In short, the emitter voltage V was corrected iteratively by the updating rule: V → V + A Gain ( I Target – I spray ), where I Target and I Spray are target and measured spray currents, respectively, and A Gain is an empirically selected constant.…”

“…Spray current was acquired by measuring the voltage drop across a 1 MΩ resistor connected in series with the liquid charging electrode. To evaluate the geometry of the Taylor cone, a real-time image processing system previously developed for measuring its apex angle was used . In brief, selected coordinates of the conic geometry were retrieved in real time by using a camera and computer.…”

High-Speed Schlieren Imaging of Vapor Formation in Electrospray Plume

“…In order to unravel its mechanism, the spraying process and modes of electrospray have been visualized and characterized since the early 20th century. , With the development of improved imaging techniques, nowadays clearer images of electrospray can be captured at high resolutions and frame rates. − Besides, with the aid of image treatment techniques, electrospray images can be processed: to provide clearer images as compared with raw images, to calculate velocity map, , or even to identify spraying modes automatically by a computer program. , In addition to the above-mentioned direct observation methods, involving adoption of image sensors, various indirect approaches for investigation of charged droplet size, number of charges, droplet evaporation and discharge hydrodynamics, and ionization mechanisms have also been proposed, including: scanning electron microscopy, phase Doppler interferometry, ,− or differential mobility analysis . Indirect methods do not provide the spraying image of electrospray optically, but they do provide other valuable information that cannot be obtained using the direct methods.…”