Electrospray

Introduction

Electrospray is a phenomenon that results from the application of an electric field to fluid contained in a small capillary. The driving electrostatic force incites the emission of droplets that cycle through phases of evaporation and coulombic explosion, ideally resulting in the formation of gas-phase ions or a very fine liquid aerosol. Though this technique has found widespread use in the area of mass spectrometry, it has also been documented to function in a wide range of other applications such as industrial painting, particle deposition, and gene therapy.

This wide array of modern uses, however, belies the fact that the basic science behind electrospray is anything but new. Indeed, it can trace its origins all the way back to Lord Rayleigh's article, "On the equilibrium of liquid conducting masses charged with electricity" published in 1882.^[1] A little over 30 years thereafter, John Zeleny became the first man to witness an electrospray event, and subsequently published his observations in "The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their surfaces."^[1] Since then, continuing research by Taylor, Fenn, Dole, and a number of other researchers have continued to push forward science's understanding and range of applications for electrospray.^[1],[2]

How it Works

As a process, the literature segregates the electrospray event into a series of 3 unique phases:^[3]

1. Onset and Emission
2. Droplet Fission
3. Ion Evaporation

Below, each of these steps will be discussed individually and the governing roles that various mechanical and electrochemical factors play will be described. Figure 1 gives an overview of the apparatus setup while actively spraying.

Onset and Emission

Initiation of an electrospray is achieved by applying an electric field to liquid housed in a capillary (see Figure 2). The magnitude of the electric field needed, E_onset, is dependent upon the following relationship:^[3]

Eonset ≈ √((2γcosθ0) / (ε0rc))

Where:

γ = the W of the solvent
θ0; = the cone half angle (see Figure 3)
ε0 = the W of the solvent
rc = the radius of the emitter orifice

It is worth noting that this relationship is only true for the system prior to the application of a voltage to generate the electric field. Once this has occurred, the electric field at the emitter, E_ES, can be calculated via the following:^[3]

EES = (2VES) / (rcln(4d/rc))

Where:

VES = the applied W
rc = the radius of the emitter orifice
d = the distance between the emitter orifice and the counter electrode

Droplet Fission

Ion Evaporation

Making Electrospray a Reality

Material Requirements

Tools

Construction

Operation

Working and Innovating with Electrospray

References

1. Electrospray: Principles and Practice