Other Chemical Ionization Processes

Reactions of O2 + with organic molecules are dominated by charge exchange, since O2 has a relatively high first ionization energy of 12.1 eV [53]. However, given that most 

Harrison, A. G. (1992) Chemical Ionization Mass Spectrometry, 2nd edn, CRC Press, Boca Raton. 

Hunter, E. P. L., Lias, S. G. (1998) Evaluated gas phase basicities and proton affinities of molecules an update. J. Phys. Chem. Ref. Data 27,413. 

Schiff, H. I., Bohme, D. K. (1975) Flowing afterglow studies at York University. Int. J. Mass Spectrom. Ion Phys. 16, 167. 

Maitland, G. C., Rigby, M., Smith, E. B., Wakeham, W. A. (1987) Intermolecular Forces Their Origin and Determination, Clarendon Press. 

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Rosen, R.T. Hartmann, T.G. Rosen, J.D. Ho, C.-T. Fast-Atom-Bombardment Mass Spectra of Low-Molecular-Weight Alcohols and Other Compounds. Evidence for a Chemical-Ionization Process in the Gas Phase. Rapid Commun. Mass Spectrom. 1988, 2, 21-23. 

In the work described here the utility of solvent adduct ions in TSP LC-MS which consist in the use of novel additives in the chromatographic eluent, such as ammonium formate or chloroacetonitrile, will be demonstrated for confirmation of structure of a variety of herbicides including triazines, phenylurea and chlorinated phenoxyacids. Complementary adduct ion information to the conventional TSP LC-MS mode of operation will be obtained. Because TSP LC-MS involves mainly a chemical ionization process where the vaporized eluent acts as chemical ionization gas, it will be of interest to compare the different adduct ions obtained here with those using other interfacing systems such as direct liquid introduction (DLI) (13-18). 

Chemical ionization (Cl) The formation of new ionized species when gaseous molecules interact with ions. This process may involve the transfer of an electron, proton, or other charged species between the reactants in an ion-molecule reaction. Cl refers to positive ions, and negative Cl is used for negative ions. 

Chemical and other physical methods of ionization were also employed for the structural determination of dienes and polyenes. Such is the case for the recent investigation of aliphatic dienes and trienes by chemical ionization with nitric oxide (NO+)25. It has been known since 1975 that olefins can be chemically ionized by NO+ [CI(NO)]26. Two distinct processes may apparently occur (i) electrophilic addition of NO+ to the ene leading to [M + NO]+ ion and (ii) an oxidative cleavage (possibly catalysed by the... 

Usually when a new chemical or process tool is developed, one expects that it will do something unique, something no other reagent was able to accomplish effectively. This is particularly true of an expensive reagent like ionizing radiation. It behooves us therefore to examine closely the unique properties of ionizing radiation. 

One obvious way to improve ionization efficiency is to make sure the sample is as clean as possible. A heated filament provides a constant amount of energy, and any devoted to evaporating or forming ions of contaminant species is lost to the desired process. Sodium, potassium, calcium, and other readily ionized elements are bad actors the fact they are also ubiquitous makes the problem just that much more difficult. Every element presents its own challenges, and much effort has been invested in purifying target elements of interest. Loading a chemically pure sample on the filament is one way to improve ion emission. 

Strege summarized the technique of high-performance liquid chromatography-electrospray ionization mass spectrometry (HPLC-ESI-MS) in dereplication of natural products. In contrast to earlier electron impact ionization (El), ESI technique is applicable to virtually any ion in solution with a soft ionization process. A comparison of ESI with fast atom bombardment (FAB), matrix assisted laser desorption ionization (MALDI), atmospheric pressure chemical ionization (APCI) and other techniques demonstrates its superior sensitivity, compatibility and reliability when coupled with HPLC [51]. 

In the ion sources, the analysed samples are ionized prior to analysis in the mass spectrometer. A variety of ionization techniques are used for mass spectrometry. The most important considerations are the internal energy transferred during the ionization process and the physico-chemical properties of the analyte that can be ionized. Some ionization techniques are very energetic and cause extensive fragmentation. Other techniques are softer and only produce ions of the molecular species. Electron ionization, chemical ionization and field ionization are only suitable for gas-phase ionization and thus their use is limited to compounds sufficiently volatile and thermally stable. However, a large number of compounds are thermally labile or do not have sufficient vapour pressure. Molecules of these compounds must be directly extracted from the condensed to the gas phase. 

The mass spectrometric analysis starts with an ionization process (see also Section 3.5). This ionization takes place in the ion source of the MS instrument, where the analyte is introduced as gas phase. There are two common ionization procedures used for GC/MS electron ionization (El) and chemical Ionization (Cl). Other ionization procedures are also used in mass spectroscopy (see below and Section 5.4). The El process consists of an electron bombardment, which is commonly done with electrons having an energy of 70 eV. The electrons are usually generated by thermoionic effect from a heated filament and accelerated to the required energy. A schematic diagram of an El source is shown in Figure 5.3.1. 

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