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. Additionally and arguably more importantly, ESI is advantageous over other atmospheric pressure ionisation processes (e.g. MALDI) since it may produce multiply charged ions, effectively extending the mass range of the analyser to accommodate the KDa-MDa orders of magnitude observed in proteins and their associated polypeptide fragments.
Mass spectrometry using ESI is called electrospray ionization mass spectrometry (ESI-MS) or, less commonly, electrospray mass spectrometry (ES-MS). ESI is a so-called 'soft ionization' technique, since there is very little fragmentation. This can be advantageous in the sense that the molecular ion (or more accurately a pseudo molecular ion) is always observed, however very little structural information can be gained from the simple mass spectrum obtained. This disadvantage can be overcome by coupling ESI with tandem mass spectrometry (ESI-MS/MS). Another important advantage of ESI is that solution-phase information can be retained into the gas-phase.
The development of electrospray ionization for the analysis of biological macromolecules was rewarded with the attribution of the Nobel Prize in Chemistry to John Bennett Fenn in 2002. One of the original instruments used by Dr. Fenn is on display at the Chemical Heritage Foundation in Philadelphia, Pennsylvania.
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- IRIBARNE & Thomson
- John Strutt, 3rd Baron Rayleigh
- John Zeleny
- Malcolm Dole
- Geoffrey Ingram Taylor
The liquid containing the analyte(s) of interest is dispersed by electrospray, into a fine aerosol. Because the ion formation involves extensive solvent evaporation (also termed desolvation), 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. These species also act to provide a source of protons to facilitate the ionisation process. Large-flow electrosprays can benefit from additional nebulization by an inert gas such as nitrogen or carbon dioxide. The aerosol is sampled into the first vacuum stage of a mass spectrometer through a capillary carrying a p.d. of approximately 3000V, 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 as the electrostatic repulsion of like charges, in an ever-decreasing droplet size, becomes more powerful than the surface tension holding the droplet together. At this point the droplet undergoes Coulomb fission, whereby the original droplet 'explodes' creating many smaller, more stable droplets. The new droplets undergo desolvation and subsequently further Coulomb fissions. 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%).
There are two major theories that explain the final production of gas-phase ions:
- The Ion Evaporation Model (IEM) suggests that as the droplet reaches a certain radius the field strength at the surface of the droplet becomes large enough to assist the field desorption of solvated ions.
- The Charge Residue Model (CRM) suggests that electrospray droplets undergo evaporation and fission cycles, eventually leading progeny droplets that contain on average one analyte ion or less. The gas-phase ions form after the remaining solvent molecules evaporate, leaving the analyte with the charges that the droplet carried.
A large body of evidence, which is consider either direct or indirect that small ions are liberated into the gas phase through the ion evaporation mechanism,  while larger ions form by charged residue mechanism 
A third model invoking combined charged residue-field emission has been proposed.
The ions observed by mass spectrometry may be quasimolecular ions created by the addition of a hydrogen cation and denoted [M + H]+, or of another cation such as sodium ion, [M + Na]+, or the removal of a hydrogen nucleus, [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. 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. The latter was renamed nano-electrospray (nanospray) in 1996. Currently the name nanospray is also in use for electrosprays fed by pumps at low flow rates, not only for self-fed electrosprays. Although there may not be a well-defined flow rate range for electrospray, microspray, and nano-electrospray, studied "changes in analyte partition during droplet fission prior to ion release" . In this paper, they compare results obtained by three other groups. and then measure the signal intensity ratio [Ba2+ + Ba+]/[BaBr+] at different flow rates.
Cold spray ionization is a form of electrospray in which the solution containing the sample is forced through a small cold capillary (10-80 °C) into an electric field to create a fine mist of cold charged droplets. Applications of this method include the analysis of fragile molecules and guest-host interactions that cannot be studied using regular electrospray ionization.
Liquid chromatography–mass spectrometry (LC-MS)
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. Among the numerous operating parameters in ESI-MS, the electrospray voltage has been identified as an important parameter to consider in ESI LC/MS gradient elution. The effect of various solvent compositions  (such as TFA or ammonium acetate, or supercharging reagents, or derivitizing groups ) or spraying conditions on Electrospray-LCMS spectra and/or nanoESI-MS spectra. have been studied.
Noncovalent gas phase interactions
Electrospray ionization is also utilized in studying noncovalent gas phase interactions. The electrospray process is thought to be capable of transferring liquid-phase noncovalent complexes into the gas phase without disrupting the noncovalent interaction. Problems such as non specific interactions have been identified when studying ligand substrate complexes by ESI-MS or nanoESI-MS. An interesting example of this is studying the interactions between enzymes and drugs which are inhibitors of the enzyme. Competition studies between STAT6 and inhibitors have used ESI as a way to screen for potential new drug candidates.
- Protein mass spectrometry
- Taylor cone
- Desorption electrospray ionization
- Sonic spray ionization
- Ho, CS; Chan MHM, Cheung RCK, Law LK, Lit LCW, Ng KF, Suen MWM, Tai HL (February 2003). "Electrospray Ionisation Mass Spectrometry: Principles and Clinical Applications". Clin Biochem Rev 24 (1): 3–12. PMC 1853331. PMID 18568044.
- Pitt, James J (February 2009). "Principles and Applications of Liquid Chromatography-Mass Spectrometry in Clinical Biochemistry". Clin Biochem Rev 30 (1): 19–34. PMC 2643089. PMID 19224008.
- Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. (1989). "Electrospray ionization for mass spectrometry of large biomolecules". Science 246 (4926): 64–71. Bibcode:1989Sci...246...64F. doi:10.1126/science.2675315. PMID 2675315.
- Markides, K; Gräslund, A. "Advanced information on the Nobel Prize in Chemistry 2002" (PDF).
- J Chem Phys 49:2240-2249, 1968
- Zeleny, J. (1914). "The Electrical Discharge from Liquid Points, and a Hydrostatic Method of Measuring the Electric Intensity at Their Surfaces". Physical Review 3 (2): 69. Bibcode:1914PhRv....3...69Z. doi:10.1103/PhysRev.3.69.
- Dole M, Mack LL, Hines RL, Mobley RC, Ferguson LD, Alice MB (1968). "Molecular Beams of Macroions". Journal of Chemical Physics 49 (5): 2240–2249. Bibcode:1968JChPh..49.2240D. doi:10.1063/1.1670391.
- Alexandrov, M. L.; L. N. Gall, N. V. Krasnov, V. I. Nikolaev, V. A. Pavlenko, and V. A. Shkurov (1984). "Экстракция ионов из растворов при атмосферном давлении – Метод масс-спектрометрического анализа биоорганических веществ" [Extraction of ions from solutions at atmospheric pressure - A method for mass spectrometric analysis of bioorganic substances]. Doklady Akademii Nauk SSSR (in Russian) 277 (2): 379–383.
- Current Measurements within the Electrospray Emitter JASMS 18: 737-748, 2007
- Olumee et al., Droplet Dynamics Changes in Electrostatic Sprays of Methanol-Water Mixtures J. Phys. Chem. A 102:9154-9160, 1998
- Fernández De La Mora J (2007). "The Fluid Dynamics of Taylor Cones". Annual Review of Fluid Mechanics 39: 217. Bibcode:2007AnRFM..39..217F. doi:10.1146/annurev.fluid.39.050905.110159.
- Cole, Richard B (2010). Electrospray and MALDI Mass Spectrometry: Fundamentals, Instrumentation, Practicalities, and Biological Applications (2 ed.). Wiley. p. 4. ISBN 978-0471741077.
- Li KY, Tu H, Ray AK (April 2005). "Charge limits on droplets during evaporation". Langmuir 21 (9): 3786–94. doi:10.1021/la047973n. PMID 15835938.
- Kebarle P, Verkerk UH (2009). "Electrospray: from ions in solution to ions in the gas phase, what we know now". Mass Spectrom Rev 28 (6): 898–917. doi:10.1002/mas.20247. PMID 19551695.
- Iribarne JV, Thomson BA (1976). "On the evaporation of small ions from charged droplets". Journal of Chemical Physics 64 (6): 2287–2294. Bibcode:1976JChPh..64.2287I. doi:10.1063/1.432536.
- Nguyen S, Fenn JB (January 2007). "Gas-phase ions of solute species from charged droplets of solutions". Proc. Natl. Acad. Sci. U.S.A. 104 (4): 1111–7. Bibcode:2007PNAS..104.1111N. doi:10.1073/pnas.0609969104. PMC 1783130. PMID 17213314.
- M. Gamero-Castaño and J. Fernández de la Mora Direct measurement of ion evaporation kinetics from electrified liquid surfaces J. Chem. Phys. 113, 815 (2000); http://dx.doi.org/10.1063/1.481857
- de la Mora , Electrospray ionization of large multiply charged species proceeds via Dole’s charged residue mechanism, Analytica Chimica Acta 406:93–104, 2000 at page 104, "An evaluation of the electric field on the drop surface at the point when it just ceases to be spherical (yet carries the total ion charge z) indicates that small PEG ions may be formed by ion evaporation. The break observed in the charge distribution may perhaps mean that the shift from the Dole to the ion evaporation mechanism arises at m�104[clarification needed], though this inference is highly hypothetical."
- de la Mora, Electrospray ionization of large multiply charged species proceeds via Dole’s charged residue mechanism, Analytica Chimica Acta 406:93–104, 2000
- de la Mora Analytica Chimica Acta 406:93–104, 2000, at page 93 "For most published data examined, zmax is between 65% and 110% of zR, providing strong support in favor of Dole’s charged residue mechanism, at least for masses from 3.3 kD up to 1.4 MD. Other large but less compact ions from proteins and linear chains of polyethylene glycols (PEGs) have zmax values considerably larger than zR, apparently implying that they also formas charged residues, though from non-spherical drops held together by the polymer backbone. "
- (de la Mora Analytica Chimica Acta 406:93–104, 2000at page 100, "The data do show a nearly discontinuous jump in the observed m/z for a mass somewhere between 20,000 and 50,000, and it is tempting to conclude that this is due to a corresponding transition where the ionization mechanism shifts from one type to the other. This would correspond to a critical value of z in the vicinity of 50, with a corresponding electric field of 2.6 V/nm. Of course, this is entirely hypothetical, and there is yet no compelling evidence of any kind indicating that an ion with as many as 30 charges can be formed by field evaporation."
- Hogan CJ, Carroll JA, Rohrs HW, Biswas P, Gross ML (January 2009). "Combined charged residue-field emission model of macromolecular electrospray ionization". Anal. Chem. 81 (1): 369–77. doi:10.1021/ac8016532. PMC 2613577. PMID 19117463.
- Emmett MR, Caprioli RM (1994). "Micro-electrospray mass spectrometry: ultra-high-sensitivity analysis of peptides and proteins". J. Am. Soc. Mass Spectrom. 5 (7): 605–613. doi:10.1016/1044-0305(94)85001-1.
- Wilm MS, Mann M (1994). "Electrospray and Taylor-Cone theory, Dole's beam of macromolecules at last?". Int. J. Mass Spectrom. Ion Proc. 136 (2–3): 167–180. Bibcode:1994IJMSI.136..167W. doi:10.1016/0168-1176(94)04024-9.
- Wilm M, Mann M (1996). "Analytical properties of the nanoelectrospray ion source". Anal. Chem. 68 (1): 1–8. doi:10.1021/ac9509519. PMID 8779426.
- Gibson et al.; Mugo, Samuel M.; Oleschuk, Richard D. (2009). "Nanoelectrospray emitters: Trends and perspective". Mass Spectrometry Reviews 28 (6): 918–936. doi:10.1002/mas.20248. PMID 19479726.
- Page JS, Marginean I, Baker ES, Kelly RT, Tang K, Smith RD (December 2009). "Biases in ion transmission through an electrospray ionization-mass spectrometry capillary inlet". J. Am. Soc. Mass Spectrom. 20 (12): 2265–72. doi:10.1016/j.jasms.2009.08.018. PMC 2861838. PMID 19815425.
- Schmidt A, Karas M, Dülcks T (May 2003). "Effect of different solution flow rates on analyte ion signals in nano-ESI MS, or: when does ESI turn into nano-ESI?". J. Am. Soc. Mass Spectrom. 14 (5): 492–500. doi:10.1016/S1044-0305(03)00128-4. PMID 12745218.
- Schmidt,Karas & Dulcks, JASMS 14:492–500 2003, at page 492
- Wilm, M. S.; Mann, M. Electrospray and Taylor-Cone Theory, Dole’s Beam of Macromolecules at Last? Int. J. Mass Spectrom. Ion Processes 136:167–180, 1994; Fernandez de la Mora, J.; Loscertales, I. G. The Current Emitted by Highly Conducting Taylor Cones. J. Fluid Mech. 260:155–184, 1994 ;. . Pfeifer & Hendricks Parametric Studies of Electrohydrodynamic Spraying. AIAA J. 1968, 6:496–502, 1968.
- RSC Chemical Methods Ontology, Cold-spray ionisation mass spectrometry
- Konermann, L; Douglas, DJ (1998). "Equilibrium unfolding of proteins monitored by electrospray ionization mass spectrometry: Distinguishing two-state from multi-state transitions". Rapid Communications in Mass Spectrometry 12 (8): 435–442. doi:10.1002/(SICI)1097-0231(19980430)12:8<435::AID-RCM181>3.0.CO;2-F. PMID 9586231.
- Nemes et al.; Goyal, Samita; Vertes, Akos (2008). "Conformational and Noncovalent Complexation Changes in Proteins during Electrospray Ionization". Analytical Chemistry 80 (2): 387–395. doi:10.1021/ac0714359. PMID 18081323.
- Sobott; Robinson (2004). "Characterising electrosprayed biomolecules using tandem-MS—the noncovalent GroEL chaperonin assembly". International Journal of Mass Spectrometry 236 (1–3): 25–32. Bibcode:2004IJMSp.236...25S. doi:10.1016/j.ijms.2004.05.010.
- for proteins: Vaidyanathan S., Kell D.B., Goodacre R. (2004). "Selective detection of proteins in mixtures using electrospray ionization mass spectrometry: influence of instrumental settings and implications for proteomics". Analytical Chemistry 76 (17): 5024–5032. doi:10.1021/ac049684. PMID 15373437.
- Marginean I, Kelly RT, Moore RJ, Prior DC, LaMarche BL, Tang K, Smith RD (April 2009). "Selection of the optimum electrospray voltage for gradient elution LC-MS measurements". J. Am. Soc. Mass Spectrom. 20 (4): 682–8. doi:10.1016/j.jasms.2008.12.004. PMC 2692488. PMID 19196520.
- Iavarone et al., JASMS 11:976–985, 2000
- Garcia (2005). "The effect of the mobile phase additives on sensitivity in the analysis of peptides and proteins by high-performance liquid chromatography–electrospray mass spectrometry". Journal of Chromatography B 825 (2): 111–123. doi:10.1016/j.jchromb.2005.03.041.
- Lomeli SH, Peng IX, Yin S, Loo RR, Loo JA (January 2010). "New reagents for increasing ESI multiple charging of proteins and protein complexes". J. Am. Soc. Mass Spectrom. 21 (1): 127–31. doi:10.1016/j.jasms.2009.09.014. PMC 2821426. PMID 19854660.
- Lomeli SH, Yin S, Ogorzalek Loo RR, Loo JA (April 2009). "Increasing charge while preserving noncovalent protein complexes for ESI-MS". J. Am. Soc. Mass Spectrom. 20 (4): 593–6. doi:10.1016/j.jasms.2008.11.013. PMC 2789282. PMID 19101165.
- Yin S, Loo JA (March 2011). "Top-Down Mass Spectrometry of Supercharged Native Protein-Ligand Complexes". Int J Mass Spectrom 300 (2–3): 118–122. doi:10.1016/j.ijms.2010.06.032. PMC 3076692. PMID 21499519.
- Krusemark CJ, Frey BL, Belshaw PJ, Smith LM (September 2009). "Modifying the charge state distribution of proteins in electrospray ionization mass spectrometry by chemical derivatization". J. Am. Soc. Mass Spectrom. 20 (9): 1617–25. doi:10.1016/j.jasms.2009.04.017. PMC 2776692. PMID 19481956.
- Nemes P, Goyal S, Vertes A (January 2008). "Conformational and noncovalent complexation changes in proteins during electrospray ionization". Anal. Chem. 80 (2): 387–95. doi:10.1021/ac0714359. PMID 18081323.
- Ramanathan R, Zhong R, Blumenkrantz N, Chowdhury SK, Alton KB (October 2007). "Response normalized liquid chromatography nanospray ionization mass spectrometry". J. Am. Soc. Mass Spectrom. 18 (10): 1891–9. doi:10.1016/j.jasms.2007.07.022. PMID 17766144.
- Gabelica V, Vreuls C, Filée P, Duval V, Joris B, Pauw ED (2002). "Advantages and drawbacks of nanospray for studying noncovalent protein-DNA complexes by mass spectrometry". Rapid Commun. Mass Spectrom. 16 (18): 1723–8. doi:10.1002/rcm.776. PMID 12207359.
- Daubenfeld T, Bouin AP, van der Rest G (September 2006). "A deconvolution method for the separation of specific versus nonspecific interactions in noncovalent protein-ligand complexes analyzed by ESI-FT-ICR mass spectrometry". J. Am. Soc. Mass Spectrom. 17 (9): 1239–48. doi:10.1016/j.jasms.2006.05.005. PMID 16793278.
- Rosu F, De Pauw E, Gabelica V (July 2008). "Electrospray mass spectrometry to study drug-nucleic acids interactions". Biochimie 90 (7): 1074–87. doi:10.1016/j.biochi.2008.01.005. PMID 18261993.
- Wortmann A, Jecklin MC, Touboul D, Badertscher M, Zenobi R (May 2008). "Binding constant determination of high-affinity protein-ligand complexes by electrospray ionization mass spectrometry and ligand competition". J Mass Spectrom 43 (5): 600–8. doi:10.1002/jms.1355. PMID 18074334.
- Jecklin MC, Touboul D, Bovet C, Wortmann A, Zenobi R (March 2008). "Which electrospray-based ionization method best reflects protein-ligand interactions found in solution? a comparison of ESI, nanoESI, and ESSI for the determination of dissociation constants with mass spectrometry". J. Am. Soc. Mass Spectrom. 19 (3): 332–43. doi:10.1016/j.jasms.2007.11.007. PMID 18083584.
- Touboul D, Maillard L, Grässlin A, Moumne R, Seitz M, Robinson J, Zenobi R (February 2009). "How to deal with weak interactions in noncovalent complexes analyzed by electrospray mass spectrometry: cyclopeptidic inhibitors of the nuclear receptor coactivator 1-STAT6". J. Am. Soc. Mass Spectrom. 20 (2): 303–11. doi:10.1016/j.jasms.2008.10.008. PMID 18996720.
- Czuczy N, Katona M, Takats Z (February 2009). "Selective detection of specific protein-ligand complexes by electrosonic spray-precursor ion scan tandem mass spectrometry". J. Am. Soc. Mass Spectrom. 20 (2): 227–37. doi:10.1016/j.jasms.2008.09.010. PMID 18976932.
- Cole, Richard (1997). Electrospray ionization mass spectrometry: fundamentals, instrumentation, and applications. New York: Wiley. ISBN 0-471-14564-5.
- Gross, Michael; Pramanik, Birendra N.; Ganguly, A. K. (2002). Applied electrospray mass spectrometry. New York, N.Y: Marcel Dekker. ISBN 0-8247-0618-8.
- Snyder, A. Peter (1996). Biochemical and biotechnological applications of electrospray ionization mass spectrometry. Columbus, OH: American Chemical Society. ISBN 0-8412-3378-0.
- Electrospray Ionization Primer National High Magnetic Field Laboratory
- Electrospray Ionization Mass Spectrometry at the US National Library of Medicine Medical Subject Headings (MeSH)