No edit summary
No edit summary
Line 1: Line 1:
<!--
<!--
My current timeline for developing this project is as follows:
# Pore over the literature collected so far and classify it according to where it will contribute to this page (for ease of use and portability, I will probably do this in an independent file kept on my flash drive).
# Keeping my goals for each heading in mind, begin to further flesh out each area, starting with the introduction and working through to later sections, updating the references in lockstep with the text.
# Upon achieving a satisfactory amount of information to constitute a complete core page, I will then seek to elaborate upon a number of areas and incorporate multimedia sources (links to external pages like Wikipedia for supporting info, extra generated images/schematics/videos, etc., possibly splitting off information onto separate pages if size and content dictate that it is necessary)
# Use this as a foundation for undergraduate research in the area, that may later be usable as a senior project.
# Continue to update this sight with additional information obtained through continuing literature review and (hopefully) my own personal undergraduate research.
An image (right now, I'm thinking one of a Taylor Cone) will be included here once it is generated.
An image (right now, I'm thinking one of a Taylor Cone) will be included here once it is generated.
[[Image:YOURPHOTO.gif|thumb|left| YOUR CAPTION]]
[[Image:YOURPHOTO.gif|thumb|left| YOUR CAPTION]]
Line 17: Line 10:
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.<ref name="Tools">Salata OV. 2005. Tools of nanotechnology: electrospray. Curr Nanosci 1(1):25-33.</ref> 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."<ref name="Tools" /> 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.<ref name="Tools" /><sup>,</sup><ref name="EPandP">Gaskell SJ. 1997. Electrospray: principles and practice. J Mass Spectrom 32:677-688.</ref>  
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.<ref name="Tools">Salata OV. 2005. Tools of nanotechnology: electrospray. Curr Nanosci 1(1):25-33.</ref> 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."<ref name="Tools" /> 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.<ref name="Tools" /><sup>,</sup><ref name="EPandP">Gaskell SJ. 1997. Electrospray: principles and practice. J Mass Spectrom 32:677-688.</ref>  
   
   
<!--
Basically this section will be electrospray in a nutshell: it will delve into the history of electrospray, and follow its development as a technological tool in modern day scientific circles. The basics of the apparatus will be described, and a simplistic schematic will be generated to represent the core concepts. A very brief overview of its applications will be provided to demonstrate the value and versatility of this piece of equipment. The section will conclude with an iteration of the aims of this article ... to make electrospray more widely available and raise awareness of its remarkable capabilities.
-->
==How it Works==
==How it Works==


Line 84: Line 74:
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 {{WP|vapor pressure}} due to the {{WP|Kelvin equation}}.<ref name="Theory" /> Instead, the remaining ions are thought to be caught in solvent molecule bulges from which the ions, not the solvent, eventually evaporate via IEM.<ref name="Thesis" />
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 {{WP|vapor pressure}} due to the {{WP|Kelvin equation}}.<ref name="Theory" /> Instead, the remaining ions are thought to be caught in solvent molecule bulges from which the ions, not the solvent, eventually evaporate via IEM.<ref name="Thesis" />


<!--
Describe the nitty gritty physics behind the electrospray process. Such a discussion, from what I have read so far, will have to include an introduction to the basics like ion formation, electric potentials, and surface tension, then progress from these to describe the three main steps (Droplet formation, droplet shrinkage, gaseous ion formation) as identified in [http://www.wiley.com/legacy/wileychi/ms/articles/677_a.pdf 1]. Depending upon time constraints, etc., this theory section may be expanded to describe phenomena resulting from usage of electrospray, such as microencapsulation and its use in facilitating DNA entry through the cell membrane in gene therapy.
-->
==Making Electrospray a Reality==
==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.
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.


<!--
Before proceeding to dive right in to the business of building an electrosprayer, this section will raise considerations that potential electrospray-builders should be made aware of prior to starting i.e. proper work environments, material availabilities, power requirements, cost, etc. Ways of circumventing difficulties in these areas, depending on time, may also be presented, such as the availability of grants/venture capital, alternative energy (photovoltaics, wind, etc.), etc.
The organization of this heading's subsections may also be modified ... if things get to unwieldy, I might use the <nowiki>"===xyz==="</nowiki> headers for each general type of setup, and then include <nowiki>"====xyz===="</nowiki> subheaders in each of those apparatuses for material requirements, tools, construction, operation.
-->
===Material and Tool Requirements===
===Material and Tool Requirements===


Line 103: Line 85:
* A {{WP|ground (electricity)}} electrode to be placed in opposition to the emitter. This can be as simple as a metal plate in contact with the earth.
* A {{WP|ground (electricity)}} electrode to be placed in opposition to the emitter. This can be as simple as a metal plate in contact with the earth.
* Any chemicals / reagents needed for the desired use.
* Any chemicals / reagents needed for the desired use.


In addition, some applications may require other components, such as:
In addition, some applications may require other components, such as:
* A finely controllable solvent pump for feeding the emitter.
* A finely controllable solvent pump for feeding the emitter.
* A high voltage output transformer, RF amplifier, and waveform generator (if pursuing {{WP|alternating current}} as opposed to {{WP|direct current}}).<ref name="AC">Yeo LY, Lastochkin D, Wang S-C, Chang H-C. 2004. A new AC electrospray mechanism by Maxwell-Wagner polarization and capillary resonance. Phys Rev Lett 92:133902.</ref><sup>, </sup><ref name="AC synth">Yeo LY, Gagnon Z, Chang H-C. 2005. AC electrospray biomaterials synthesis. Biomaterials 26:6122-6218.</ref>
* A high voltage output transformer, RF amplifier, and waveform generator (if pursuing {{WP|alternating current}} as opposed to {{WP|direct current}}).<ref name="AC">Yeo LY, Lastochkin D, Wang S-C, Chang H-C. 2004. A new AC electrospray mechanism by Maxwell-Wagner polarization and capillary resonance. Phys Rev Lett 92:133902.</ref><sup>, </sup><ref name="AC synth">Yeo LY, Gagnon Z, Chang H-C. 2005. AC electrospray biomaterials synthesis. Biomaterials 26:6122-6218.</ref>
* A closed chamber with a {{WP|nebulizer}} feeding nitrogen may be required to prevent {{WP|corona discharge}}.
* Characterization equipment, such as {{WP|voltmeters}}, {{WP|ammeters}}, telescopic equipment, and cameras.
* Characterization equipment, such as {{WP|voltmeters}}, {{WP|ammeters}}, telescopic equipment, and cameras.


With respect to tools, nothing special is required for the basic design herein described. In some cases, more specialized devices, such as micropipette pullers, may be needed, depending upon the type of project being explored.<ref name="Analytical">Wilm M, Mann M. 1996. Analytical properties of the nanoelectrospray ion source. Anal Chem 68:1-8.</ref><sup>, </sup><ref name="Low-flow">Barnidge DR, Nilsson S, Markides KE. 1999. A design for low-flow sheathless electrospray emitters. Anal Chem 71:4115-4118.</ref>
With respect to tools, nothing special is required for the basic design herein described. In some cases, more specialized devices, such as micropipette pullers, may be needed, depending upon the type of project being explored.<ref name="Analytical">Wilm M, Mann M. 1996. Analytical properties of the nanoelectrospray ion source. Anal Chem 68:1-8.</ref><sup>, </sup><ref name="Low-flow">Barnidge DR, Nilsson S, Markides KE. 1999. A design for low-flow sheathless electrospray emitters. Anal Chem 71:4115-4118.</ref>
<!--
List necessary components for constructing an electrospray system and alternatives. These will largely depend upon the type of device being constructed and its intended uses. It will also describe the types of reagents, etc. that will commonly be worked with.
-->
<!--
List tools for constructing an electrospray system and alternatives. Again, depending upon the type of system being constructed and modifications users might want to make, actually needs can vary widely. Though the core set is relatively basic, micropipette pullers and other arcane paraphernalia may be desired for those looking for increased experimental flexibility in their setups.
-->


===Construction===
===Construction===


<!--
The actual assembly of an electrospray system is simple, and the plans are relatively straightforward (see Figure 4). Depending upon your application, a few things may vary. A few examples include:
Describe different ways of making a functional electrospray system. These will include generated blueprints, that will, as mentioned above, vary based upon the type of setup being pursued. Recommendations for maintaining safety and optimizing the final product will be brought forward to help streamline the process and ensure the best possible result.  
* A reservoir of solvent in conjunction with a finely controllable solvent pump may have to be used to supplement the emitter.
-->
* Distance from the emitter tip to the target / ground will vary by application type (i.e. in mass spectrometry it may only be a few mm, while for gene therapy it's typically closer to 2 cm).
* The need for a closed chamber environment and a nitrogen nebulizer.


The literature itself is actually vague as to the details of optimally assembling an electrospray apparatus and ensuring that it works, so if anyone has personal experience in this area your contributions would be greatly appreciated.
<!--I will continue to expand this section with more personal advice once I get started on trying to do this myself.-->
===Operation===
===Operation===



Revision as of 18:33, 6 April 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 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, 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 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, qR, 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 and Tool Requirements

The absolute bare-bone requirements for setting up an electrospray apparatus are:

  • A W capable of producing at least 1 kV of potential difference (in most cases, current output is a secondary concern). The exact values needed will vary widely depending upon application. It is possible that a high W may be used as a cheaper alternative, though this remains to be substantiated.
  • An emitter, typically a steel capillary with a diameter on the order of 0.1 mm or less (remember, the smaller the diameter, the lower the required voltage to create the spray).
  • A W electrode to be placed in opposition to the emitter. This can be as simple as a metal plate in contact with the earth.
  • Any chemicals / reagents needed for the desired use.

In addition, some applications may require other components, such as:

  • A finely controllable solvent pump for feeding the emitter.
  • A high voltage output transformer, RF amplifier, and waveform generator (if pursuing W as opposed to W).[5], [6]
  • A closed chamber with a W feeding nitrogen may be required to prevent W.
  • Characterization equipment, such as W, W, telescopic equipment, and cameras.

With respect to tools, nothing special is required for the basic design herein described. In some cases, more specialized devices, such as micropipette pullers, may be needed, depending upon the type of project being explored.[7], [8]

Construction

The actual assembly of an electrospray system is simple, and the plans are relatively straightforward (see Figure 4). Depending upon your application, a few things may vary. A few examples include:

  • A reservoir of solvent in conjunction with a finely controllable solvent pump may have to be used to supplement the emitter.
  • Distance from the emitter tip to the target / ground will vary by application type (i.e. in mass spectrometry it may only be a few mm, while for gene therapy it's typically closer to 2 cm).
  • The need for a closed chamber environment and a nitrogen nebulizer.

The literature itself is actually vague as to the details of optimally assembling an electrospray apparatus and ensuring that it works, so if anyone has personal experience in this area your contributions would be greatly appreciated.

Operation

Working and Innovating with Electrospray

References

  1. 1.0 1.1 1.2 1.3 Salata OV. 2005. Tools of nanotechnology: electrospray. Curr Nanosci 1(1):25-33.
  2. Gaskell SJ. 1997. Electrospray: principles and practice. J Mass Spectrom 32:677-688.
  3. 3.0 3.1 3.2 3.3 3.4 3.5 Rohner TC, Lion N, Girault HH. 2004. Electrochemical and theoretical aspects of electrospray ionisation. Phys Chem Chem Phys 6:3056-3068.
  4. 4.0 4.1 4.2 4.3 4.4 Grimm RL. 2006. Fundamental studies of the mechanisms and applications of field-induced droplet ionization mass spectrometry and electrospray mass spectrometry. Thesis.
  5. Yeo LY, Lastochkin D, Wang S-C, Chang H-C. 2004. A new AC electrospray mechanism by Maxwell-Wagner polarization and capillary resonance. Phys Rev Lett 92:133902.
  6. Yeo LY, Gagnon Z, Chang H-C. 2005. AC electrospray biomaterials synthesis. Biomaterials 26:6122-6218.
  7. Wilm M, Mann M. 1996. Analytical properties of the nanoelectrospray ion source. Anal Chem 68:1-8.
  8. Barnidge DR, Nilsson S, Markides KE. 1999. A design for low-flow sheathless electrospray emitters. Anal Chem 71:4115-4118.
Cookies help us deliver our services. By using our services, you agree to our use of cookies.