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Electrospray ionization

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Electrospray ionization

Electrospray (nanoSpray) ionization source

Electrospray ionization (ESI) is a technique used in mass spectrometry to produce ions using an electrospray in which a high voltage is applied to a liquid to create an aerosol. It is especially useful in producing ions from macromolecules because it overcomes the propensity of these molecules to fragment when ionized. ESI is different than other atmospheric pressure ionization 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.[1][2]

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 electrospray ionization technique was first reported by Masamichi Yamashita and John Fenn in 1984.[3] The development of electrospray ionization for the analysis of biological macromolecules[4] was rewarded with the attribution of the Nobel Prize in Chemistry to John Bennett Fenn in 2002.[5] One of the original instruments used by Dr. Fenn is on display at the Chemical Heritage Foundation in Philadelphia, Pennsylvania.

Contents

  • History 1
  • Ionization mechanism 2
  • Variants 3
    • Ambient ionization 3.1
  • Applications 4
    • Liquid chromatography–mass spectrometry (LC-MS) 4.1
    • Capillary electrophoresis-mass spectrometry (CE-MS) 4.2
    • Noncovalent gas phase interactions 4.3
  • See also 5
  • References 6
  • Further reading 7
  • External links 8

History

Diagram of Electrospray Ionization. (1) Under high voltage, the Taylor Cone emits a jet of liquid drops (2) The solute from the droplets progressively evaporates, leaving them more and more charged (3) When the charge exceeds the Rayleigh limit the droplet explosively dissociates, leaving a stream of charged ions

In 1882, Lord Rayleigh theoretically estimated the maximum amount of charge a liquid droplet could carry before throwing out fine jets of liquid.[6] This is now known as the Rayleigh limit.

In 1914, John Zeleny published work on the behaviour of fluid droplets at the end of glass capillaries and presented evidence for different electrospray modes.[7] Wilson and Taylor [8] and Nolan investigated electrospray in the 1920s[9] and Macky in 1931.[10] The electrospray cone (now known as the Taylor cone) was described by Sir Geoffrey Ingram Taylor.[11]

The first use of electrospray ionization with mass spectrometry was reported by Malcolm Dole in 1968.[12][13] John Bennett Fenn was awarded the 2002 Nobel Prize in Chemistry for the development of electrospray ionization mass spectrometry in the late 1980s.[14]

Ionization mechanism

Fenn's first electrospray ionization source coupled to a single quadrupole mass spectrometer

The liquid containing the analyte(s) of interest is

  • Electrospray Ionization Primer National High Magnetic Field Laboratory
  • Electrospray Ionization Mass Spectrometry at the US National Library of Medicine Medical Subject Headings (MeSH)

External links

  • Cole, Richard (1997). Electrospray ionization mass spectrometry: fundamentals, instrumentation, and applications. New York: Wiley.  
  • Gross, Michael; Pramanik, Birendra N.; Ganguly, A. K. (2002). Applied electrospray mass spectrometry. New York, N.Y: Marcel Dekker.  
  • Snyder, A. Peter (1996). Biochemical and biotechnological applications of electrospray ionization mass spectrometry. Columbus, OH: American Chemical Society.  
  • Alexandrov, M. L.; L. N. Gall; N. V. Krasnov; V. I. Nikolaev; V. A. Pavlenko; V. A. Shkurov (July 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].  
  • Alexandrov, M. L.; L. N. Gall; N. V. Krasnov; V. I. Nikolaev; V. A. Pavlenko; V. A. Shkurov (2008) [July 1984]. "Extraction of ions from solutions under atmospheric pressure as a method for mass spectrometric analysis of bioorganic compounds". Rapid Communications in Mass Spectrometry 22 (3): 267–270.  

Further reading

  1. ^ 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.  
  2. ^ Pitt, James J (February 2009). "Principles and Applications of Liquid Chromatography-Mass Spectrometry in Clinical Biochemistry". Clin Biochem Rev 30 (1): 19–34.  
  3. ^ Yamashita, Masamichi; Fenn, John B. (September 1984). "Electrospray ion source. Another variation on the free-jet theme". The Journal of Physical Chemistry 88 (20): 4451–4459.  
  4. ^ Fenn, J. B.; Mann, M.; Meng, C. K.; Wong, S. F.; Whitehouse, C. M. (1989). "Electrospray ionization for mass spectrometry of large biomolecules".  
  5. ^ Markides, K; Gräslund, A. "Advanced information on the Nobel Prize in Chemistry 2002" (PDF). 
  6. ^ Rayleigh, L. (1882). "On the Equilibrium of Liquid Conducting Masses charged with Electricity".  
  7. ^ Zeleny, J. (1914). "The electrical discharge from liquid points, and a hydrostatic method of measuring the electric intensity at their surfaces.".  
  8. ^ Wilson, C. T.; G. I Taylor (1925). "The bursting of soap bubbles in a uniform electric field". Proc. Cambridge Philos. Soc. 22 (5): 728.  
  9. ^ Nolan, J. J. (1926). Proc. R. Ir. Acad. Sect. A 37: 28. 
  10. ^ Macky, W. A. (October 1, 1931). "Some Investigations on the Deformation and Breaking of Water Drops in Strong Electric Fields".  
  11. ^ Sir Geoffrey Taylor (1964). "Disintegration of Water Droplets in an Electric Field".  
  12. ^ a b Dole M, Mack LL, Hines RL, Mobley RC, Ferguson LD, Alice MB (1968). "Molecular Beams of Macroions".  
  13. ^ Birendra N. Pramanik; A.K. Ganguly; Michael L. Gross (28 February 2002). Applied Electrospray Mass Spectrometry: Practical Spectroscopy Series. CRC Press. pp. 4–.  
  14. ^ "Press Release: The Nobel Prize in Chemistry 2002". The Nobel Foundation. 2002-10-09. Retrieved 2011-04-02. 
  15. ^ Pozniak BP, Cole RB (2007). "Current Measurements within the Electrospray Emitter". JASMS 18 (4): 737–748.  
  16. ^ Olumee; et al. (1998). "Droplet Dynamics Changes in Electrostatic Sprays of Methanol-Water Mixtures". J. Phys. Chem. A 102 (46): 9154–9160.  
  17. ^ Fernández De La Mora J (2007). "The Fluid Dynamics of Taylor Cones". Annual Review of Fluid Mechanics 39: 217.  
  18. ^ Cole, Richard B (2010). Electrospray and MALDI Mass Spectrometry: Fundamentals, Instrumentation, Practicalities, and Biological Applications (2 ed.). Wiley. p. 4.  
  19. ^ Li KY, Tu H, Ray AK (April 2005). "Charge limits on droplets during evaporation". Langmuir 21 (9): 3786–94.  
  20. ^ a b c 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.  
  21. ^ Iribarne JV, Thomson BA (1976). "On the evaporation of small ions from charged droplets". Journal of Chemical Physics 64 (6): 2287–2294.  
  22. ^ a b 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.  
  23. ^ Gamero-Castaño M (2000). "Direct measurement of ion evaporation kinetics from electrified liquid surfaces". J. Chem. Phys. 113 (2): 815.  
  24. ^ de la Mora Fernandez (2000). "Electrospray ionization of large multiply charged species proceeds via Dole's charged residue mechanism". Analytica Chimica Acta 406: 93–104.  
  25. ^ de la Mora Fernandez (2000). "Electrospray ionization of large multiply charged species proceeds via Dole's charged residue mechanism". Analytica Chimica Acta 406: 93–104.  
  26. ^ de la Mora Fernandez (2000). "Electrospray ionization of large multiply charged species proceeds via Dole's charged residue mechanism". Analytica Chimica Acta 406: 93–104.  
  27. ^ de la Mora Fernandez (2000). "Electrospray ionization of large multiply charged species proceeds via Dole's charged residue mechanism". Analytica Chimica Acta 406: 93–104.  
  28. ^ 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.  
  29. ^ Unraveling the Mechanism of Electrospray Ionization. Lars Konermann, Elias Ahadi, Antony D. Rodriguez and Siavash Vahidi, Anal. Chem., 2013, 85 (1), pages 2–9, doi:10.1021/ac302789c
  30. ^ Li, Anyin; Qingjie Luo; So-Jung Park; R. Graham Cooks (17 March 2014). "Synthesis and Catalytic Reactions of Nanoparticles formed by Electrospray Ionization of Coinage Metals". Angewandte Chemie International Edition 53 (12): 3147–3150.  
  31. ^ Gale DC, Smith RD (1993). "Small Volume and Low Flow Rate Electrospray Ionization Mass Spectrometry for Aqueous Samples". Rapid Commun. Mass Spectrom. 7: 1017–1021.  
  32. ^ 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.  
  33. ^ 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.  
  34. ^ Wilm M, Mann M (1996). "Analytical properties of the nanoelectrospray ion source". Anal. Chem. 68 (1): 1–8.  
  35. ^ Gibson; Mugo, Samuel M.; Oleschuk, Richard D.; et al. (2009). "Nanoelectrospray emitters: Trends and perspective". Mass Spectrometry Reviews 28 (6): 918–936.  
  36. ^ 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.  
  37. ^ a b 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.  
  38. ^ Wilm M. S., Mann M. (1994). "Electrospray and Taylor-Cone Theory, Dole's Beam of Macromolecules at Last?". Int. J. Mass Spectrom. Ion Processes 136 (2–3): 167–180.  
  39. ^ Fernandez de la Mora J., Loscertales I. G. (2006). "The Current Emitted by Highly Conducting Taylor Cones". J. Fluid Mech. 260: 155–184.  
  40. ^ Pfeifer RJ, Hendricks (1968). "Parametric Studies of Electrohydrodynamic Spraying". Aiaa J. 6 (3): 496–502.  
  41. ^ RSC Chemical Methods Ontology, Cold-spray ionisation mass spectrometry
  42. ^ Page JS, Tang K, Kelly RT, Smith RD, (2008). "A subambient pressure ionization with nanoelectrospray (SPIN) source and interface for improved sensitivity in mass spectrometry". Analytical Chemistry 80: 1800–1805.  
  43. ^ I. Marginean, J. S. Page, A. V. Tolmachev, K. Tang and R. D. Smith (2010). "Achieving 50% Ionization Efficiency in Subambient Pressure Ionization with Nanoelectrospray". Analytical Chemistry 82: 9344–9349.  
  44. ^ Cooks, R. Graham; Ouyang, Zheng; Takats, Zoltan; Wiseman, Justin M. (2006). "Ambient Mass Spectrometry". Science 311 (5767): 1566–70.  
  45. ^ a b c Monge, María Eugenia; Harris, Glenn A.; Dwivedi, Prabha; Fernández, Facundo M. (2013). "Mass Spectrometry: Recent Advances in Direct Open Air Surface Sampling/Ionization". Chemical Reviews 113 (4): 2269–2308.  
  46. ^ Huang, Min-Zong; Yuan, Cheng-Hui; Cheng, Sy-Chyi; Cho, Yi-Tzu; Shiea, Jentaie (2010). "Ambient Ionization Mass Spectrometry". Annual Review of Analytical Chemistry 3 (1): 43–65.  
  47. ^ Z. Takáts, J.M. Wiseman, B. Gologan, R.G. Cooks (2004). "Mass Spectrometry Sampling Under Ambient Conditions with Desorption Electrospray Ionization". Science 306 (5695): 471–473.  
  48. ^ Takáts Z, Wiseman JM, Cooks RG (2005). "Ambient mass spectrometry using desorption electrospray ionization (DESI): instrumentation, mechanisms and applications in forensics, chemistry, and biology". Journal of mass spectrometry : JMS 40 (10): 1261–75.  
  49. ^ 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.  
  50. ^ Nemes; Goyal, Samita; Vertes, Akos; et al. (2008). "Conformational and Noncovalent Complexation Changes in Proteins during Electrospray Ionization". Analytical Chemistry 80 (2): 387–395.  
  51. ^ Sobott; Robinson (2004). "Characterising electrosprayed biomolecules using tandem-MS—the noncovalent GroEL chaperonin assembly". International Journal of Mass Spectrometry 236 (1–3): 25–32.  
  52. ^ 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.  
  53. ^ 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.  
  54. ^ Iavarone; Jurchen, John C.; Williams, Evan R.; et al. (2000). "Effects of solvent on the maximum charge state and charge state distribution of protein ions produced by electrospray ionization". JASMS 11 (11): 976–985.  
  55. ^ 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.  
  56. ^ Teo CA, Donald WA (May 2014). "Solution additives for supercharging proteins beyond the theoretical maximum proton-transfer limit in electrospray ionization mass spectrometry". Anal. Chem. 86 (9): 4455–62.  
  57. ^ 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.  
  58. ^ 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.  
  59. ^ 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.  
  60. ^ 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.  
  61. ^ Nemes P, Goyal S, Vertes A (January 2008). "Conformational and noncovalent complexation changes in proteins during electrospray ionization". Anal. Chem. 80 (2): 387–95.  
  62. ^ 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.  
  63. ^ 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.  
  64. ^ 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.  
  65. ^ Rosu F, De Pauw E, Gabelica V (July 2008). "Electrospray mass spectrometry to study drug-nucleic acids interactions". Biochimie 90 (7): 1074–87.  
  66. ^ 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.  
  67. ^ a b 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.  
  68. ^ 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.  
  69. ^ 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.  

References

See also

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[20][63] such as non specific interactions[64] 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.[65][66][67] Competition studies between STAT6 and inhibitors[67][68][69] have used ESI as a way to screen for potential new drug candidates.

Noncovalent gas phase interactions

Capillary electrophoresis-mass spectrometry was enabled by an ESI interface that was developed and patented by Richard D. Smith and coworkers at Pacific Northwest National Laboratory, and shown to have broad utility for the analysis of very small biological and chemical compound mixtures, and even extending to a single biological cell.

Capillary electrophoresis-mass spectrometry (CE-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,[52] the electrospray voltage has been identified as an important parameter to consider in ESI LC/MS gradient elution.[53] The effect of various solvent compositions [54] (such as TFA[55] or ammonium acetate,[20] or supercharging reagents,[56][57][58][59] or derivitizing groups[60] ) or spraying conditions[61] on Electrospray-LCMS spectra and/or nanoESI-MS spectra.[62] have been studied.

Liquid chromatography–mass spectrometry (LC-MS)

Electrospray is used to study protein folding.[49][50][51] Additionally, ESI-MS is used to test for the presence of nano clusters, for example U-60 (citation needed).

Applications

Laser-based electrospray-based ambient ionization is a two-step process in which a pulsed laser is used to desorb or ablate material from a sample and the plume of material interacts with an electrospray to create ions.[45] For ambient ionization, the sample material is deposited on a target near the electrospray. The laser desorbs or ablates material from the sample which is ejected from the surface and into the electrospray which produces highly charged ions. Examples are electrospray laser desorption ionization, matrix-assisted laser desorption electrospray ionization, and laser ablation electrospray ionization

Extractive electrospray ionization is an spray-type, ambient ionization method that uses two merged sprays, one of which is generated by electrospray.[45]

Desorption electrospray ionization (DESI) is an ambient ionization technique in which a solvent electrospray is directed at a sample.[47][48] The electrospray is attracted to the surface by applying a voltage to the sample. Sample compounds are extracted into the solvent which is again aerosolized as highly charged droplets that evaporate to form highly charged ions. After ionization, the ions enter the atmospheric pressure interface of the mass spectrometer. DESI allows for ambient ionization of samples at atmospheric pressure, with little sample preparation.

In ambient ionization, the formation of ions occurs outside the mass spectrometer without sample preparation.[44][45][46] Electrospray is used for ion formation is a number of ambient ion sources.

Diagram of a DESI ambient ionization source.

Ambient ionization

Electrospray ionization has also been achieved at pressures as low as 25 torr and termed subambient pressure ionization with nanoelectrospray (SPIN) based upon a two-stage ion funnel interface developed by Richard D. Smith and coworkers.[42] The SPIN implementation provided increased sensitivity due to the use of ion funnels that helped confine and transfer ions to the lower pressure region of the mass spectrometer. Operation at low pressure was particularly effective for low flow rates where the smaller electrospray droplet size allowed effective desolvation and ion formation to be achieved. As a result later the researchers were later able to demonstrate achieving in excess of 50% overall ionization utilization efficiency for transfer of ions from the liquid phase, into the gas phase as ions, and through the dual ion funnel interface to the mass spectrometer.[43]

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.[41] Applications of this method include the analysis of fragile molecules and guest-host interactions that cannot be studied using regular electrospray ionization.

The electrosprays operated at low flow rates generate much smaller initial droplets, which ensure improved ionization efficiency. In 1993 Gale and Richard D. Smith reported significant sensitivity increases could be achieved using lower flow rates, and down to 200 nL/min.[31] 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.[32] 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.[33] The latter was renamed nano-electrospray (nanospray) in 1996.[34][35] Currently the name nanospray is also in use for electrosprays fed by pumps at low flow rates,[36] not only for self-fed electrosprays. Although there may not be a well-defined flow rate range for electrospray, microspray, and nano-electrospray,[37] studied "changes in analyte partition during droplet fission prior to ion release" .[37] In this paper, they compare results obtained by three other groups.[38][39][40] and then measure the signal intensity ratio [Ba2+ + Ba+]/[BaBr+] at different flow rates.

Variants

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. This effect is demonstrated in the direct ionization of noble metals such as copper, silver and gold using electrospray.[30]

A third model invoking combined charged residue-field emission has been proposed.[28] Another model called chain ejection model (CEM) is proposed for disordered polymers (unfolded proteins).[29]

A large body of evidence, which is consider either direct or indirect that small ions (from small molecules) are liberated into the gas phase through the ion evaporation mechanism,[22][23] [24] while larger ions (from folded proteins for instance) form by charged residue mechanism [25][26][27]

There are two major theories that explain the final production of gas-phase ions: the ion evaporation model (IEM) and the charge residue model (CRM). The 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.[21][22] The CRM suggests that electrospray droplets undergo evaporation and fission cycles, eventually leading progeny droplets that contain on average one analyte ion or less.[12] The gas-phase ions form after the remaining solvent molecules evaporate, leaving the analyte with the charges that the droplet carried.

[20][19] 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%).[18] The aerosol is sampled into the first vacuum stage of a mass spectrometer through a capillary carrying a potential difference 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.[17] or carbon dioxide.nitrogen by an inert gas such as nebulization 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 ionization process. Large-flow electrosprays can benefit from additional [16]

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