Principle of Electrospray Ionization

Schematic diagram of the principle of electrospray ionization in the positive-ion 

There are three major theories that explain the final formation of gas-phase ions  

It is generally accepted that low molecular weight ions are liberated into the gas phase through the ion evaporation mechanism [5], while larger ions form by charged residue mechanism [8]. The third model appears well demonstrated in the ionization of lipid species. 

Since the presence of volatile organic solvents) (e.g., methanol, chloroform, isopropanol) is favorable for desolvation, it enhances ionization based on the 

For the charge-neutral, but polar, compounds, the ion source can generate gas-phase ions based on the availability of counterions present in the mobile phase according to the third model. Specifically, in the high electric field, the compounds, which are present in the sprayed solution and carry no separable charge(s) but 

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Figure 14.2 Principle of electrospray ionization, (a) The analyte is dissolved in an appropriate solvent and sprayed via a capillary into an electric field. Here, the liquid filament finally forms charged droplets, (b) The solvent of the charged droplets evaporates, resulting in an increase of the surface charge up to a critical boundary, at which a Coulomb explosion occurs. The newly formed droplets undergo the same process. The final products are the desolvated, naked ions, which are then entering the mass spectrometer.

Wilm, M. 2011. Principles of electrospray ionization. Mol. Cell Proteomics 10 1. El-Aneed, A., Cohen, A., and Banoub, J. 2009. Mass spectrometry, review of the basics Electrospray, MALDl, and commonly used mass analyzers. Appl. Spectrosc. Rev. 44 210-230. 

Figure 5 Principle of electrospray ionization inside an atmospheric pressure ion source.

FIGURE 18.5 Principle of electrospray ionization and a mass-to-charge ratio spectrum of a protein. Values given are protonation charge states [41],... 

Wilm M. Principles of electrospray ionization. Mol Cell Proteomics 2011 10(7) M111.009407-Mll 1.009407. 

Figure 1 The principle of electrospray ionization (ESI). (Reproduced from Ref. 6. John Wiley Sons, Inc., 2009.)...

SMITH, R.D., LOO, J.A, OGORZALEK-LOO, R.R., BUSMAN, M., UDSETH, H.R., Principles and practice of electrospray ionization-mass spectrometry for large polypeptides and proteins, Mass Spec. Rev., 1991,31,472-485. 

Smith, R. D., Loo, J. A., Ogorzalek-Loo, R. R., Busman, M., and Udseth, H. R. Principles and practice of electrospray ionization—Mass-sp>ectrometry for large pol3 p>eptides and proteins. Moss Spectrom. Rev., 10, 359,1991. 

Figure 2.16. Basic components of electrospray ionization. (Reproduced from C. Dass, Principles and Practice of Biological Mass Spectrometry, Wiley-Interscience, 2001.)...

In the past 15 years, electrospray ionization has become one of the most important ionization methods in mass spectrometry. It is the method of choice in the coupling of liquid chromatography (LC) and mass spectrometry (MS). By estimation, it is used in over 90% of all LC-MS applications. It is especially useful in the analysis of highly polar, ionic, and macromolecular analytes. In addition, electrospray ionization plays an important role in the characterization of biomacromolecules, especially peptides and proteins. History, principle, instrumentation, practical aspects, and application of electrospray ionization are discussed in this article. 

The combination of the electrospray ion source with HPLC has without a doubt become the LC/MS interface in recent years. It is a particularly powerful combination, since this ionization technique covers a wide range of samples [38] that are commonly separated by HPLC [39] or electrophoresis [40]-[44]. The ESI source exhibits concentration-dependent behavior and thus gives optimal signals at most flow rates. The principle of the ionization process is discussed in Section 20.4.9. The most important feature of this interface is aspray needle which can be connected directly to the separation column, if the flow rates are compatible. Initially the major limitation was that only low flow rates (a few pL/min) could be used, but now flow rates of 1 mL/min or more are possible by using heated sprayers or ultrasonic devices. Splitting of the flow is possible as well, allowing two detectors to be used simultaneously. Since buffers can be used as long as they are volatile and not too concentrated, a sheath flow... 

Actually, the development of electrospray ionization itself by the Dole group [45] preceded API, TSP, and EHI by several years [46]. The underlying principle of ESI, which it shares with EHI, even dates back to work by Zeleny [39] and Taylor... 

In the following chapters, the basic principles of HPLC and MS, in as far as they relate to the LC-MS combination, will be discussed and seven of the most important types of interface which have been made available commercially will be considered. Particular attention will be paid to the electrospray and atmospheric-pressure chemical ionization interfaces as these are the ones most widely used today. The use of LC-MS for identification and quantitation will be described and appropriate applications will be discussed. 

Electrospray ionization. Although the electrospray phenomenon was known since the beginning of the twentieth century, and the first description of its principle was published in 1968,[14] the breakthrough for ESI came in the mid 1980s from work by John Fenn[4] and a Russian research group. [15] (Fenn was jointly awarded the Nobel Prize for Chemistry in 2002.)... 

Figure 14.1 Schematic view of a mass spectrometer. Its basic parts are ion source, mass analyzer, and detector. Selected principles realized in modern mass spectrometers are assigned El—electron impact. Cl—chemical ionization, FAB—fast atom bombardment, ESI—electrospray ionization, MALDI—matrix-assisted laser desorption/ionization. Different combinations of ion formation with mass separation can be realized.

A new ionization method called desorption electrospray ionization (DESI) was described by Cooks and his co-workers in 2004 [86]. This direct probe exposure method based on ESI can be used on samples under ambient conditions with no preparation. The principle is illustrated in Figure 1.36. An ionized stream of solvent that is produced by an ESI source is sprayed on the surface of the analysed sample. The exact mechanism is not yet established, but it seems that the charged droplets and ions of solvent desorb and extract some sample material and bounce to the inlet capillary of an atmospheric pressure interface of a mass spectrometer. The fact is that samples of peptides or proteins produce multiply charged ions, strongly suggesting dissolution of the analyte in the charged droplet. Furthermore, the solution that is sprayed can be selected to optimize the signal or selectively to ionize particular compounds. 

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