Ionization methods used

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Chemical ionization An ionization method used to maximize the production of intact molecular species. Used for volatile, thermally stable analytes. 

Fast-atom bombardment An ionization method used for involatile and thermally labile materials. In this technique, the sample is dissolved in a matrix material and bombarded with a high-energy atom or ion beam. 

FIGURE 15.2 Common protein ionization methods used for MS-based proteomics. Two common ionization technologies are currently available for protein analysis. Top ESI volatilizes and ionizes peptides and proteins in solution. Bottom MALDI uses analytes that are co-crystallized in a matrix composed of organic acid on a solid support. A pulse of ultraviolet laser evaporates the matrix and analyte into gas phase, resulting in generation of single charge ions. 

Electrospray ionization occurs by the same four steps as listed above for thermospray (see Section 4.6). In contrast to thermospray, and most other ionization methods used in mass spectrometry, it should be noted that electrospray ionization unusually takes place at atmospheric pressure. A similar process carried out under vacuum is known as electrohydrodynamic ionization and gives rise to quite different analytical results. This technique has not been developed into a commercial LC-MS interface and will not be considered further. 

Whatever the ionization method used, the mass spectra of oligosaccharides analysed by ESI, MALDI or FAB display intense ions of the molecular species resulting from protonation (M -(- H)+ or cationization by an alkali metal ion (M + alkali metal)"1" in the positive ion mode or from deprotonation (M — II) in the negative ion mode. In ESI, multiply charged ions also are produced. 

Because the soft ionization methods used for oligosaccharides produce few fragments, collision-induced dissociation (CID) or post-source decay (PSD) must be used for structural study. These two techniques have been applied to deprotonated, protonated or alkaliated... 

Because fatty acids derived from natural sources are present in a mixture, an ideal analysis method for these molecules should be applicable to mixtures without requiring a prior separation or derivatization. Mass spectrometry is an excellent tool for determining the structure of fatty acids present in a mixture. It is possible to determine not only the molecular weight and thus the elemental composition but also, in most cases, the nature and position of the branching and the other substituents on the carbon chain. [268,269] Furthermore, such an analysis requires low quantities ranging from 10 pg to 100 ng of total lipid, depending upon the analysed sample, the ionization method used and the configuration of the spectrometer. [270,271]... 

J. 2.2.2 Moving Belt System. Initially developed by McFadden et al. [13], the moving belt system was based on the physical method of evaporation of the mobile phase through heat and vacuum that leave analytes as a thin coating on a continuously cycling polyimide belt. The analytes were transported from atmospheric pressure region to the vacuum of the ion source through differentially pumped vacuum locks. Ionization methods used... 

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. 

In the past decade, mass spectrometry (MS) has become the method of choice for quality control of synthetic peptides. Historically, plasma desorption (PD) and fast atom bombardment (FAB) were the first ionization methods used for the mass analysis of nonderivatized peptides. More recently, electrospray ionization (ESI) MS and matrix-assisted laser desorption ionization (MALDI) MS have found widespread utility for peptide analysis. Both of the latter methods yield protonated molecules and, thus, provide direct molecular weight information. As will be covered later, ESI can be employed with a variety of mass analyzers, including quadrupole, magnetic sector, ion trap, and time-of-flight (TOF) analyzers. On the other hand,... 

Table 17.1. Ionization methods used for mass spectrometry...

There is no doubt that the most important ionization method used for studying CyDs and other complex and heavy biomolecules is electrospray. 

It may at first appear at variance with the rest of the handbook to discuss derivatization for fast atom/ ion bombardment mass spectrometry, where chromatography may be only incidentally rather than directly involved. Nevertheless, it seems appropriate to include this aspect because these techniques are complementary to the electron impact and chemical ionization methods used with gas chromatography—mass spectrometry (GC-MS) as described in Chapter 14, and thus extend the coverage to include derivatization methods for the majority of mass spectrometric techniques. 

The analysis of MF resins as well as alkoxylated MF resins by chromatographic methods has been described vastly in the literature (119,120) and allows the separation and identification of oligomeric methylolated melamines by conventional UV- or refractive index based detection. The most direct method for the determination of methylolated melamines within a MF resin is the coupling between HPLC methods and Mass Spectrometry (qv), enabling the direct structural analysis by HPLC-MS methods on methoxylated (121-124) and butoxylated MF resins (see Chromatography, HPLC) (125). Favored ionization methods use ESI-MS ionization methods to achieve an efficient ionization process. The detection of up to pentameric melamine units can be achieved by this method. 

Fast atom bombardment (FAB) In MS, an ionization method using fast atoms to transfer ions from a liquid matrix to the gas phase. 

A serious problem with the Knudsen cell is that it may not give an accurate picture of a vapor s composition (M, M2, M3, etc.) emitting from an open surface of hot metal, which is not in thermal equilibrium. More research is certainly needed in this area. A second problem is that mass spectrometry requires that the metal particle be ionized before mass analysis. Just the process of ionization may cause fragmentation of M2, M3, etc. Further research using very gentle ionization methods (using energetically controlled photons instead of electrons) is needed in this area as well. 

This is the same technique used to create ions in an electron capture detector (ECD). There are other ionization methods used in IMS, but i currently is used in most field-deployable instruments. 

Central to any measurements in mass spectrometry is the ionization of the sample molecules, and transfer of those ions into the vacuum required for operation of the mass spectrometer. The choice of ionization methods available to analytical and organic mass spectrometrists has expanded greatly in the past ten years, and now includes means for the ionization of nonvolatile as well as volatile molecules. Any of several methods might be chosen to address a particular problem, and each might provide satisfactory results. Since there is no single ionization method used exclusively with TLC/MS, this section of the chapter contains an overview of the most common methods of sample molecule ionization in mass spectrometry. 

ESI (Section 10.4) Electrospray ionization, a soft ionization method used for mass spectrometry of biological samples of very high molecular weight. 

Table 1. Summary of ionization methods used in the analyses of crude oil.

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