Electrospray ionization mechanism

Two relatively new techniques, matrix assisted laser desorption ionization-lime of flight mass spectrometry (MALDI-TOF) and electrospray ionization (FS1), offer new possibilities for analysis of polymers with molecular weights in the tens of thousands. PS molecular weights as high as 1.5 million have been determined by MALDI-TOF. Recent reviews on the application of these techniques to synthetic polymers include those by Ilantoif54 and Nielen.555 The methods have been much used to provide evidence for initiation and termination mechanisms in various forms of living and controlled radical polymerization.550 Some examples of the application of MALDI-TOF and ESI in end group determination are provided in Table 3.12. The table is not intended to be a comprehensive survey. 

We have previously considered the mechanism of electrospray ionization in terms of the charging of droplets containing analyte and the formation of ions as the charge density on the surface of the droplet increases as desolvation progresses. The electrospray system can also be considered as an electrochemical cell in which, in positive-ion mode, an oxidation reaction occurs at the capillary tip and a reduction reaction at the counter electrode (the opposite occurs during the production of negative ions). This allows us to obtain electrospray spectra from some analytes which are not ionized in solution and would otherwise not be amenable to study. In general terms, the compounds that may be studied are therefore as follows ... 

Charge-residue mechanism One of the two mechanisms used to account for the production of ions by electrospray ionization. 

Zhang and Nunes [92] studied the structure and the generation mechanism of a novel degradation product formed by oxidatively induced coupling of miconazole nitrate with butylated hydroxy toluene in a topical ointment studied by high performance liquid chromatography-electrospray ionization mass spectrometry and organic synthesis. 

Z. Takats, J. M. Wiseman and R. G. Cooks, Ambient mass spectrometry using desorption electrospray ionization (DESI) instmmentation, mechanisms and applications in forensics, chemistry, and biology, J. Mass Spectrom., 40, 1261 1275 (2005). 

The mechanism of zinc deprivation by 3-nitrosobenzamide was elucidated most recently. When the reconstituted nucleocapsid protein p7 of HIV-1 (15 i-M) was incubated with 3-nitrosobenzamide (300 iM) at pH 7.5, three disulfide bonds per protein molecule were formed while 3-nitrosobenzamide was reduced to the hydroxylamine. Molecular masses of p7 adducts augmented by one or two 3-nitrosobenzamide residues were observed by electrospray ionization MS, consistent with covalent bond formation between cysteine sulfur and the nitroso nitrogen atom127. 

Y. Yang, K. Janota, K. Tabei, N. Huang, M. M. Siegel, Y. I. Lin, B. A. Rasmussen, D. M. Shlaes, Mechanism of Inhibition of the Class A beta-Lactamases PCI and TEM-1 by Tazobactam. Observation of Reaction Products by Electrospray Ionization Mass Spectrometry , J. Biol. Chem. 2000, 275, 26674-26682. 

Electrospray ionization (ESI) refers to the overall process by which an intense electric field disperses a sample liquid into a bath gas as a fine spray of highly charged droplets. Evaporation of those charged droplets produces gas-phase ions by mechanisms that remain the subject of much argument and debate. The ESI is a complex of independent component processes, the two most important of which are electrospray dispersion, the electrostatic dispersion of sample liquid into charged droplets, and ionization, the transformation of solute species in those droplets to free ions in the gas phase. 

Rundlett, K. L., and Armstrong, D. W. (1996). Mechanism of signal suppression by anionic surfactants in capillary electrophoresls-electrospray ionization mass spectrometry. Anal. Chem. 68, 3493-3497. 

Experimental support for an Ir(I)-Ir(III) mechanism was provided by Chen and Dietiker [30]. They reported an elegant experimental investigation of the hydrogenation of styrene with [Ir(PHOX)(COD)]BArp in the gas phase by means of electrospray ionization tandem mass spectrometry. By means of reversible deuterium labeling, the investigators found masses corresponding only to intermediates with a mass corresponding to a dihydride complex with no presence of a trihydride species and concluded that no Ir(V) species with PHOX could be present in the catalytic reaction. 

The high throughput capability of MALDI and the different ionization mechanisms make this technique also an attractive alternative to electrospray ionization for the analysis of low relative molecular mass compounds (LRMM) [44]. However, interferences of matrix ions and the ionization of the low relative molecular mass compounds are the challenges of this technique [45, 46]. 

R. G. Ambient mass spectrometry using desorption electrospray ionization (DESI) instrumentation, mechanisms and applications in forensics, chemistry, and biology. 

P Kebarle, Y Ho. On the mechanism of electrospray mass spectrometry. In RB Cole, ed. Electrospray Ionization Mass Spectrometry. New York Wiley, 1997, pp 3-63. 

Electrospray ionization (ESI) was first employed more than 20 years ago, but it is fairly recently that it became a routine technique for the soft ionization of a wide range of polar analytes, including biomolecules. For this technique, the analyte is usually dissolved in a mixture of an organic solvent (most commonly acetonitrile or methanol) and water with a pH modifier [e.g. formic (methanoic) or acetic (ethanoic) acid for positive ion mode]. The presence of the pH modifier ensures that ionization takes place in the solution state. This is the only common case where ionization occurs before ion vaporization the exact mechanism of the vaporization (Figure 5.6) is still not clearly understood in ESI. 

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