What is Electrospray?

Advantages of ESI   | Advantages of NanoESI

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.

Diagram of ionized droplets in electrospray

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.

Diagram of nanospray experimental setup

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.

Magnified image of Taylor cone and plume

See it in action

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 emitters, 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 useful items.