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 development of electrospray ionization for the analysis of biological macromolecules[1] was rewarded with the attribution of the Nobel Prize in Chemistry to John Bennett Fenn in 2002.[2] One of the original instruments used by Dr. Fenn is on display at the Chemical Heritage Foundation in Philadelphia, Pennsylvania.
Mass spectrometry using ESI is called electrospray ionization mass spectrometry (ESI-MS) or, less commonly, electrospray mass spectrometry (ES-MS).
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Early researchers:
The liquid containing the analyte(s) of interest is dispersed by electrospray into a fine aerosol. Because the ion formation involves extensive solvent evaporation, the typical solvents for electrospray ionization are prepared by mixing water with volatile organic compounds (e.g. methanol, acetonitrile). To decrease the initial droplet size, compounds that increase the conductivity (e.g. acetic acid) are customarily added to the solution. Large-flow electrosprays can benefit from additional nebulization by an inert gas such as nitrogen. The aerosol is sampled into the first vacuum stage of a mass spectrometer through a capillary, which can be heated to aid further solvent evaporation from the charged droplets. The solvent evaporates from a charged droplet until it becomes unstable upon reaching its Rayleigh limit. At this point, the droplet deforms and emits charged jets in a process known as Coulomb fission. During the fission, the droplet loses a small percentage of its mass (1.0-2.3%) along with a relatively large percentage of its charge (10-18%).[6][7]
There are two major theories that explain the final production of gas-phase ions:
While there is no definite scientific proof, a large body of indirect evidence suggests that small ions are liberated into the gas phase through the ion evaporation mechanism, while larger ions form by charged residue mechanism.
The ions observed by mass spectrometry may be quasimolecular ions created by the addition of a proton (a hydrogen ion) and denoted [M + H]+, or of another cation such as sodium ion, [M + Na]+, or the removal of a proton, [M − H]−. Multiply charged ions such as [M + nH]n+ are often observed. For large macromolecules, there can be many charge states, resulting in a characteristic charge state envelope. All these are even-electron ion species: electrons (alone) are not added or removed, unlike in some other ionization sources. The analytes are sometimes involved in electrochemical processes, leading to shifts of the corresponding peaks in the mass spectrum.
The electrosprays operated at low flow rates generate much smaller initial droplets, which ensure improved ionization efficiency. In 1994, two research groups coined the name micro-electrospray (microspray) for electrosprays working at low flow rates. Emmett and Caprioli demonstrated improved performance for HPLC-MS analyses when the electrospray was operated at 300-800 nL/min.[9] Wilm and Mann demonstrated that a capillary flow of ~ 25 nL/min can sustain an electrospray at the tip of emitters fabricated by pulling glass capillaries to a few micrometers.[10] The latter was renamed nano-electrospray (nanospray) in 1996.[11][12] Currently the name nanospray is also in use for electrosprays fed by pumps at low flow rates, not only for self-fed electrosprays. There are no well-defined flow rate ranges for electrospray, microspray, and nano-electrospray.
Electrospray is used to study protein folding [13][14][15]
Electrospray ionization is the ion source of choice to couple liquid chromatography with mass spectrometry. The analysis can be performed online, by feeding the liquid eluting from the LC column directly to an electrospray, or offline, by collecting fractions to be later analyzed in a classical nanoelectrospray-mass spectrometry setup. The effect of various ion-pairing agents (such as TFA [16] on Electrospray-LCMS has been studied.
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 between STAT6 and inhibitors [17][18] have been done in this way to screen for potential new drug candidates.[19]
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