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Revision as of 22:40, 24 March 2008
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]
- Onset and Emission
- Droplet Fission
- 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
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. 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 3). The magnitude of the voltage needed, Vonset, 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, EES, can be manipulated and eventually increased to levels that create the Taylor cone. EES 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 airborne droplets full of charged ions proceed to enter the second phase, droplet fission.
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. The amount of charge, qR, at which the Rayleigh limit is exceeded and fission occurs has been described by the mathematical relationship:[4]
qR = 8π√(εγr3)
Where:
ε = the vacuum permitivity of the solvent γ = the surface tension of the solvent r = the radius of the droplet
Ion Evaporation
Making Electrospray a Reality
Material Requirements
Tools
Construction
Operation
Working and Innovating with Electrospray
References
- ↑ 1.0 1.1 1.2 1.3 Salata OV. 2005. Tools of nanotechnology: electrospray. Curr Nanosci 1(1): 25-33.
- ↑ Gaskell SJ. 1997. Electrospray: principles and practice. J Mass Spectrom 32:677-688.
- ↑ 3.0 3.1 3.2 Rohner TC, Lion N, Girault HH. 2004. Electrochemical and theoretical aspects of electrospray ionisation. Phys Chem Chem Phys 6:3056-3068.
- ↑ Grimm RL. 2006. Fundamental studies of the mechanisms and applications of field-induced droplet ionization mass spectrometry and electrospray mass spectrometry. Thesis.