Flexing the Electrified Meniscus: The Birth of a Jet in Electrosprays

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Abstract
Spraying of liquids through an electrified meniscus has become a method of choice to produce ions from large biomolecules. Using mass spectrometry, the generated ions can be analyzed to provide detailed information on their composition and structure. This technique enables high-throughput protein analysis that is a prerequisite for answering the questions presented by proteomics. In this report, Taylor cone deformations are shown to play a central role in the mechanism of electrostatic spraying. Spontaneous spray current oscillations are known to exist in most electrospray regimes and affect the stream of ions introduced into the mass spectrometer. Fast time-lapse imaging of the Taylor cone throughout its evolution indicates the presence of a nodal line and standing waves on its surface. Four phases of the cone pulsation cycle (liquid accumulation, cone formation, ejection of a jet, relaxation) are established. Based on image analysis, apex velocities, curvatures, and opening angles are determined. During jet ejection, the apex velocity and the curvature exhibit singularities. Furthermore, the pulsation frequencies of the Taylor cone deformations are determined using Fourier analysis of light refraction measurements. The oscillation frequency of the electrospray current collected by the counter electrode shows close correlation to the cone deformations, providing the first direct evidence that links spray current oscillations to Taylor cone pulsation. Thus, monitoring the oscillation frequency throughout the spraying process and adjusting the spray parameters can be used to stabilize the spray. Furthermore, synchronizing the injection of ions in time-of-flight systems with the spontaneous spray oscillations may improve the signal-to-noise ratio in the collected mass spectra.

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