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

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.