Electrospray: Difference between revisions

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 charged 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 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. Gas-Phase Ion Generation

Below, each of these steps will be discussed individually and the governing roles that various mechanical and electrochemical factors play will be described. An overview of the apparatus can be found in Figure 1.

Onset and Emission

At rest, no activity is witnessed in an electrospray system due to the lack of a sufficiently strong electric field to drive the vaporization of solvent at the emitter tip. Droplets are This value has been characterized by the relationship:^[3]

Einitial ≈ √((2γcosθ0) / (ε0rc))

Where:

γ = the W of the solvent
θ0; = the cone half angle (see Figure 2)
ε0 = the W of the solvent
rc = the radius of the emitter orifice

Initiation of electrospray via formation of a ^W is achieved by applying a ^W to liquid housed in a capillary (see Figure 1). The magnitude of the voltage needed, V_onset, is dependent upon the following relationship:^[1]

Vonset ∝ √(γrc)

Where:

γ = the surface tension of the solvent
rc = the radius of the emitter orifice

By varying this applied voltage, the electric field at the emitter, E_ES, can be manipulated and eventually increased to levels that create the Taylor cone. E_ES can be calculated via the following equation:^[3]

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

Where:

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

The resulting plume of charged airborne droplets are accelerated towards the counter electrode due to the electric field, and subsequently undergo a series of droplet fission events.

Droplet Fission

Once airborne, the liquid droplets' structural integrity becomes dependent upon the struggle of surface tension with the electrostatic repulsion that results from the solvated ions. Up to a point, known as the Rayleigh limit, surface tension will hold the repulsive forces in check and prevent droplet fragmentation. Due to evaporation, however, continuous shrinkage in droplet size gradually brings the charges closer together, increasing repulsion proportionally. Eventually, the Rayleigh limit is overcome and the droplet undergoes Coulombic explosion, splitting into progeny droplets in which the process is reset (see Figure 3). The amount of charge, q_R, at which the Rayleigh limit is exceeded and fission occurs has been described by the mathematical relationship:^[4]

qR = 8π√(ε0γr3)

Where:

ε0 = the vacuum permitivity of the solvent
γ = the surface tension of the solvent
r = the radius of the droplet

This is only a general guideline, however, as a number of labs have reported Rayleigh discharge (a.k.a. particle fission) at 70% to 120% of this value.^[4]

Gas-Phase Ion Generation

Two models have been put forward to explain how ions eventually enter the gas phase. The first, known as the Ion Evaporation Model (IEM), postulates that the electrostatic repulsion present in very small (tens of nm in diameter) droplets is strong enough to actually force the ions to desorb from the surface.^[4] The charge residue model (CRM), on the other hand, simply states that the cycle of evaporation and coulombic explosion continues until it terminally results in the generation of gas-phase ions.^[4] It has not yet been determined which one predominates, and it is indeed possible that both may viable models under different circumstances (droplet size, charge density, etc.), or even intermingle.^[3]

In IEM, the potential for ion evaporation from the solvent surface can be determined by examining the related change in ^W. If the overall value is negative and the ^W barrier is overcome, then the reaction can spontaneously occur. A model by Iribarne and Thomson, as well as one by Born, both seek to mathematically explain this event, however the former fails to take into account a number of factors and the latter has been shown experimentally to severely underestimate free energies.^[3]

The CRM view that progressive fragmentation due to fission and solvent evaporation leads to the generation of gas-phase ions has its own difficulties, however. Once droplets reach a size of only a few nm, the Rayleigh equation breaks down due to the loss of (assumed) equal charge distribution and shifts in ^W due to the ^W.^[3] Instead, the remaining ions are thought to be caught in solvent molecule bulges from which the ions, not the solvent, eventually evaporate via IEM.^[4]

Making Electrospray a Reality

Though the central design and components used to construct an electrospray apparatus are essentially the same, customization is often neccessary depending upon the application. Below, the process of constructing a basic ES device will be described, with notes detailing modifications that may be necessary for various uses.

Material Requirements

Tools

Construction

Operation

Working and Innovating with Electrospray

References