Electrospray ionization
From Freepedia
Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. The invention of electrospray ionization was rewarded with the attribution of the Nobel Prize in Chemistry to John Fenn in 2002.
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How it works
In electrospray ionization a liquid is pushed through a very small charged, usually metal, capillary. The liquid contains the substance which is to be studied, the analyte, as well as a large amount of solvent, which is usually much more volatile than the analyte. The charge contained in the capillary transfers to the liquid and the analyte molecule becomes charged, often by the addition of a positively charged hydrogen ion (proton). As like charges repel, the liquid pushes itself out of the capillary and forms a mist or an aerosol of small droplets about 10<math>\mu</math>m across. This jet of aerosol droplets is at least partially produced by a process involving the formation of a Taylor cone and a jet from the tip of this cone. A neutral carrier gas, such as nitrogen gas, is sometimes used to help nebulize the liquid and to help evaporate the neutral solvent in the small droplets. As the small droplets, now suspended in air, evaporate the charged analyte molecules are forced closer together. The proximity of the molecules becomes unstable as the similarly charged molecules come closer together and the droplets once again explode. This is referred to as Coulombic fission because it is the repulsive Coulombic forces between charged analyte molecules that drive it. This process repeats itself until the analyte is free of solvent and is a lone ion. There remains debate as to the exact mechanisms of electrospray processes particularly in the later part of the process as the lone ion is formed. The lone ion then continues along to the mass analyzer of a mass spectrometer.
In electrospray processes the ions observed are quasimolecular ions that are ionized by the addition of a proton (hydrogen ion) to give [M+H] (M=analyte molecule, H=hydrogen ion), or other cation such as sodium ion [M+Na], or the removal of a proton [M-H] for example. In electrospray multiply charged ions such as [M+2H] are often observed. For large macromolecules there will often be a distribution of many charge states and the charge on the ions can be great such as [M+25H]. Note that these are all even-electron species. Electrons themselves (alone) have neither been added or removed as with some other ionizations. The formation of ions in electrospray is somewhat homologous to acid-base reactions. Redox reactions do occur and a circuit with measurable current flow exists but atomic and molecular ions are the primary carriers of charge in the solution and gas phases.
Issues of debate
There are two major competing theories about the final production of lone ions, the charged residue model (CRM) and the ion evaporation model (IEM). The CRM proports that columbic fission (explosions) continues until a lone ion is formed. The IEM proports that ions are evaporated (usually surrounded by a layer of solvent) from the surface of the small droplets produced later in the cascade of Coulomic fissions. It has been suggested that both models probably occur for different analytes/solvents and in the limit of both models they have a tendency to converge. That is to say that the distinction between a droplet containing an analyte molecule and an analyte molecule surrounded by a layer of solvent eventually disappears and columbic fission looks a lot like ion evaporation. The real question is scale and timing and the techniques to definitively determine this are not available.
The use of the word "ionization" in "electrospray ionization" is criticized by some due to that many of the ions observed are thought to be preformed in solution before the electrospray process or created by simple changes in concentrations as the aerosolized droplets shrink. It is argued that electrospray is simply an interface for transferring ions from the solution phase to the gas phase.
Variants
There exist many variations on the basic electrospray technique. Two important ones are microspray (µ-spray) and nanospray. The primary difference is in the reduced flow rate of the analyte containing liquid; however many other differences, such as the reduced internal diameter of the tubing or lack of nebulization gas, exist because of the difference in flow rate. These variants are important because they generally offer better sensitivity over traditional electrospray. The µ and nano designations refer to the flow rate of the analyte containing liquid; µLiters/minute and nanoLiters/minute respectively.
Applications
Liquid chromatography-mass spectrometry
- see also the main article on liquid chromatography-mass spectrometry
Electrospray ionization is the primary ion source used in liquid chromatography-mass spectrometry. This is due to being a liquid-gas interface that is capable of coupling liquid chomatography with mass spectrometry.
Noncovalent gas phase interactions
Electrospray ionization is also ideal in studying noncovalent gas phase interactions. The electrospray process is capable of transferring liquid-phase noncovalent complexes into the gas phase without disrupting the noncovalent interaction. This means that a cluster of two molecules can be studied in the gas phase by other mass spectrometry techniques. An interesting example of this is studying the interactions between enzymes and drugs which are inhibitors of the enzyme. Because inhibitors generally work by noncovalently binding to its target enzyme with reasonable affinity the noncovalent complex can be studied in this way. Competition studies have been done in this way to screen for potential new drug candidates.
External links
- Electrospray Ionization from Cambridge University
- A thesis with significant background on electrospray ionization
