Electrospray ionization principle

JB Fenn, M Mann, CK Meng, SF Wong, CM Whitehouse. Electrospray ionization—Principles and practice. Mass Spec Rev 9 37, 1990. 

Fenn JB, Mann M, Men CK, Wong SF, and Whitehouse CM (1990) Electrospray ionization - principles and practice. Mass Spectrometry Reviews 9 37-70. 

Fenn, J.B., Mann, M., Meng, C.K., et aL (1990) Electrospray ionization Principles and practice. Mass Spectrometry Reviews, 9,37-70. 

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.)... 

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. 

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.

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.

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. 

For detection of carbohydrates in principle, ultraviolet (UV), laser-induced fluorescence, refractive index, electrochemical, amperometric, and mass spec-trometric detection can be used. Mass spectrometry, with its various ionization methods, has traditionally been one of the key techniques for the structural determination of proteins and carbohydrates. Fast-atom bombardment (FAB) and electrospray ionization (ESI) are the two on-line ionization methods used for carbohydrate analysis. The ESI principle has truly revolutionized the modern mass spectrometry of biological molecules, due to its high sensitivity and ability to record large-molecule entities within a relatively smaU-mass scale. 

Three popular ionization techniques are electrospray ionization (ESI), atmospheric pressure chemical ionization (APCI) and matrix-assisted laser desorption (MALDI). Electrospray is the most widely used ionization technique when performing LC-MS, and has proved to be a most versatile tool for soft ionization [72] of a large variety of analytes such as them described in paper I. Figure 6 shows the principle of the ESI. 

Techniques for the Ionization of Molecules The measurability of molecules by MSI is enabled through the local desorption and ionization of the molecules from a surface. In theory, all types of molecules that can undergo these two chemical processes can be measured. Many techniques have been developed or adapted to achieve desorption and ionization of molecules from surfaces, but three different desorption/ioniza-tion techniques made their way to commercially available products. The acronyms of these technologies are desorption electrospray ionization (DESI), MALDI, and secondary ion mass spectrometry (SIMS). The principles of these three methods and a comparison of their possibilities and limitations are outlined throughout this section and summarized in Figure 1 and Table 2, respectively. 

In principle, electrospray ionization is a concentration-sensitive technique. This means that the signal response should be independent of the injection volume. It is often true, however, that the chromatographic separation concentrates the analyte at the head of the column and some mass-sensitive response is observed. 

Over the years, many different approaches based on these two basic principles have been developed.4 9 We decided to focus on developing an approach to transfer analytes by coupling capillary tubing with electrospray ionization devices. From this basic design principle, we were able to develop a simple three-position device for the analysis of proteomic samples by mass spectrometry.7,10 We developed this principle further into an automated nine-position device,6 and to perform frontal analysis separations of peptides.11 This chapter reviews these early developments in coupling microfabricated devices to mass spectrometers. 

Other Techniques. A growing technique related to lc/ms and regarded as complementary to it is that of capillary zone electrophoresis/mass spectrometry (cze/ms) (22). Using cze/ms, high resolution separation of water-soluble compounds is accompHshed by the principles of electrophoresis (qv). The sample is then coupled to the mass spectrometer by electrospray ionization (23) or a fast atom bombardment interface (fab) to produce molecular ions (24). Biotechnology applications of cze/ms have great potential (25). 

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