 |
In the most general sense, electrospray is a method of generating a very
fine liquid aerosol through electrostatic charging, rather than the more
familiar gas (pneumatic) methods. Remember those old perfume bottles
with the little bulb attached? You squeeze the bulb and a fine mist of
perfume would effuse from the nozzle. In the form of air pumped through
a tube by squeezing the bulb, you are using gas to generate droplets.
Electrospray, as the name implies, uses electricity instead of gas to
form the droplets. In electrospray, a liquid is passing through a
nozzle. The plume of droplets is generated by electrically charging the
liquid to a very high voltage. The charged liquid in the nozzle becomes
unstable as it is forced to hold more and more charge. Soon the liquid
reaches a critical point, at which it can hold no more electrical charge
and at the tip of the nozzle it blows apart into a cloud of tiny, highly
charged droplets.
These tiny droplets are less than 10 µm in diameter, (1 µm = 1 millionth
of a meter!), and fly about searching for a potential surface to land on
that is opposite in charge to their own. As they fly about, they rapidly
shrink as solvent molecules evaporate from their surface. Since it is
difficult for charge to evaporate, the distance between electrical
charges in the droplet dramatically decreases. If the droplet can't find
a home in which to dissipate its charge in time, the electrical charge
reaches a critical state and the droplet will violently blow apart
again. Originally observed by the physist John Zeleny in 1914, (Zeleny,
J. The Physical Review 3 (1914):69-91), the electrospray process has
profoundly effected the field of mass spectrometry by allowing
structural analysis of unlimited molecular weight, e.g., large
biomolecules, and being directly compatible with liquid chromatography
methods.
On the industrial scale, electrospray is used in the application of
paints and coatings to metal surfaces. The fine spray results in very
smooth even films, with the paint actually attracted to the metal, so
the paint material is used more efficiently. This lowers the cost, cuts
down on the amount of organic solvents required, and reduces
environmental impact! Miniaturized versions of electrospray are even
finding their way into the next generation of micro-satellites. The
electrostatic plume makes an efficient, although very low power, ion
propulsion engine.
When electrospray is used as a soft ionization method for chemical
analysis, the more generally accepted term is "electrospray ionization"
(ESI). Ionization is the process of generating a gas phase ion from a
typically solid or liquid chemical species. It is called "soft" since
the molecule being ionized does not fall apart or break-up during the
process. Ionization is a critical event in mass spectrometry as only
ions can be can be accurately measured. Once we know an ion's mass, the
chemical composition can be determined. When combined with mass
spectrometry, the value of ESI is unparalleled, especially in the
analysis of large biological molecules such as proteins and DNA.
The most common electrospray apparatus used by mass spectrometrists
employs a sharply pointed hollow metal tube, such as a syringe needle,
with liquid pumped through the tube. A high-voltage power supply is
connected to the outlet of the tube and the tube is positioned in front
of a plate, called a counter-electrode, commonly held at ground
potential.
When the power supply is turned on and adjusted for the proper voltage,
the liquid being pumped through the tube transforms into a fine
continuous mist of droplets that fly rapidly toward the
counter-electrode.
The electrospray process, which occurs at the tip of the emitter, can be
viewed through a high-powered microscope. In general, as the liquid
begins to exit the needle, it charges up and assumes a conical shape,
referred to as the Taylor cone, in honor of Mr. G.I. Taylor who
described the phenomena in 1964. The liquid assumes this shape because
when charged up, a cylindrical shape can hold more charge than a sphere.
At the tip of the cone, the liquid again changes shape into a fine jet.
This jet however, then becomes unstable, breaking up into the mist of
fine droplets. Since these droplets are all highly charged with the same
electrical charge they repel each other very strongly. Thus the droplets
fly apart from each other and cover a wide surface area. You can see
actual photographs of the process in action by taking a look at this
animation.
New Objective is a leading supplier of specialty hollow tubes, that we
call PicoTips™, and apparatus for the analytical applications of
ESI-MS. We manufacture a variety of PicoTip emitters to optimize the
sensitivity and efficiency of ESI-MS while minimizing the amount of
sample required for analysis. These devices find application in the
interfacing of powerful separation methods such as capillary
electrophoresis and liquid chromatography with mass spectrometry. Many
of our products are useful at very low-flow rates, generally referred to
as micro-electrospray or nanospray. Many of our product applications
involve the study and discovery of novel proteins and peptides, now
creating the new field of proteomics. If you have a mass spectrometer
and need to optimize your analysis, New Objective may have the right
products for you. Please take a thorough visit through our web site. It
features an introduction to our core products, useful technical and
application notes, a downloadable catalog, and other goodies. For more
technical information about electrospray, electrospray ionization, and
the applications of ESI-MS take a look at our ESI Resources Section.
 |
