Electrospray ionization , degradation analysis

Early in the history of mass spectrometry (MS), large biomolecules were not analyzed because efficient methods to transport these molecules into the gas phase were unknown. Degradation typically occurred during vaporization of these nonvolatile molecules so that electron ionization of the intact molecular ion was not possible. Ionization by fast atom bombardment (FAB), field desorption (FD), secondary ionization mass spectrometry (SIMS), and plasma desorption (PD)" from the radioactive decay of Cf finally made the ionization and analysis of peptides possible. These latter techniques, although still used today, are not as popular as electrospray... 

Mass spectrometry This is often used for analysis of plastics. Thermal degradation can be carried out using an ion source, or in a pyrolyzer connected directly to a mass spectrometer or via a chromatographic column. Two soft ionization techniques, electrospray ionization and matrix-assisted laser desorption ionization (MALDI), are the most common in the analyses of large molecules. MALDI-MS is usually used offline but online connection with a... 

The ion source creates analyte ions from the neutral species in the vapor phase. Several designs of ion sources have been used for CWA degradation product analysis in recent years including thermospray ionization (TSP), atmospheric pressure chemical ionization (APCI), and electrospray ionization (ESI). These soft ionization techniques generally produce [M-H] or [M+H]" " fragments for the alkyl phosphonic acids and some other CWA degradation products. The inductively coupled plasma (ICP) is a hard ionization source and has been described in the literature. HPLC-ICP-MS has been reported for the detection of alkyl phosphonic acids.In recent years, ESI has become the most common ion source in HPLC-MS analysis in general with the APCI source a close second. The thermospray source has fallen out of favor in HPLC-MS systems since the introduction of ESI. 

Figure 4.4 LC-ICP-MS analysis of alkyl phosphonic acids in food and beverage matrices [24]. Reprinted from Kubachka, K.M., Richardson, D.D., Heitkemper, D.T., Caruso, J.A., Detection of chemical warfare agent degradation products in foods using liquid chromatography coupled to inductively coupled plasma mass spectrometry and electrospray ionization mass spectrometer. Journal of Chromatography A, 2008, 1202, (2), 124-131, with permission from Elsevier...

To achieve sufficient vapor pressure for El and Cl, a nonvolatile liquid will have to be heated strongly, but this heating may lead to its thermal degradation. If thermal instability is a problem, then inlet/ionization systems need to be considered, since these do not require prevolatilization of the sample before mass spectrometric analysis. This problem has led to the development of inlet/ionization systems that can operate at atmospheric pressure and ambient temperatures. Successive developments have led to the introduction of techniques such as fast-atom bombardment (FAB), fast-ion bombardment (FIB), dynamic FAB, thermospray, plasmaspray, electrospray, and APCI. Only the last two techniques are in common use. Further aspects of liquids in their role as solvents for samples are considered below. 

R.M. Black and R.W. Read, Analysis of degradation products of organophosphorus chemical warfare agents and related compounds by liquid chromatography-mass spectrometry using electrospray and atmospheric pressure chemical ionization, J. Chromatogr A, 794, 233-244 (1998). 

Electrospray appears to be useful especially when dealing with compounds that give poor APCI spectra due to thermal degradation (such as glucuronide conjugates). Meanwhile, electrospray and its derivative, nanoflow electrospray , have developed a niche in the analysis of proteins. They are especially useful as an ionization method combined with capillary electrophoresis, and for offline analysis of spots retrieved from chromatographic plates. 

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