Gas phase ionization reaction

Although the product ion in Eq. (3) is indistinguishable from that in Eq. (1), the supporting atmospheres and reactant ions differ. Because commercial analyzers are operated with purified air, the formation of product ions will occur via Eq. (3). 

Decomposition of the adduct ion can also lead to the loss of NO2, which can be accompanied by the retention of charge to NO2 (an ion with m/z 46 and a high mobihty usually faster in drift time than the reactant ion peak) as shown in Eq. (4) or to (M-NO2). data not shown. 

Another pathway is for NO2, formed in Eq. (4), to attach to excess sample neutrals to form an adduct ion, M-N02 as shown in Eq. (5). Although this can be a distinctive and pronounced pathway for some explosives, such as nitrogylcerin, the reaction comes at a cost — two molecules are consumed for each product ion, degrading quantitative response. One molecule provides N02, which clusters with the second molecule. 

The adduct ions as found with nitroglycerin (NG) are sensitive to temperature and can be seen at 100°C and below. However, the lifetime for the adduct ion is decreased as the temperature is increased and only the fragment NO , per Eq. (4) is observed with NG in analyzers where drift tube temperatures are 125°C or greater. Other ions seen in IMS from explosives include NO3 or M-NOs and may arise from reactions between NO2 with 02 or N02 with O2. 

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Ionization interference can be a problem in the analysis of alkali metals at relatively low temperature and in the analyses of other elements at higher temperature. For any element, we can write a gas-phase ionization reaction ... 

For most purposes, it suffices to have relative electrode potentials in order to gain the thermodynamic information being sought. This is a fortunate situation since the electrode potential for a half reaction cannot be measured. There are instances where it would be very desirable to be able to estimate the energy of such a reaction. Since the gas phase ionization reactions (39) and (40) can be studied, it is possible to relate the gas phase energetics, the ioniza-... 

A wide variety of desorption ionization methods is available [7] desorption chemical ionization (DCI), secondary-ion mass spectrometry (SIMS), fast-atom bombardment (FAB), liquid-SIMS, plasma desorption (PD), matrix-assisted laser desorption ionization (MALDI), and field desorption (FD). Two processes are important in the ionization mechanism, i.e., the formation of ions in the sample matrix prior to desorption, and rapid evaporation prior to ionization, which can be affected by very rapid heating or by sputtering by high-energy photons or particles. In addition, it is assumed that the energy deposited on the sample surface can cause (gas-phase) ionization reactions to occur near the interface of the solid or liquid and the vacuum (the so-called selvedge) or provide preformed ions in the condensed phase with sufficient kinetic energy to leave their environment. 

Use bond enthalpies (Table 8.4), electron affinities (Figure 7.11), and the ionization energy of hydrogen (1312 kJ/mol) to estimate AH for the following gas-phase ionization reactions ... 

The bond dissociation energy for the gas phase ionization reaction (equation 16.23) can be obtained by using the following thermodynamic cycle ... 

A mixture of water/pyridine appears to be the solvent of choice to aid carbenium ion formation [246]. In the Hofer-Moest reaction the formation of alcohols is optimized by adding alkali bicarbonates, sulfates [39] or perchlorates. In methanol solution the presence of a small amount of sodium perchlorate shifts the decarboxylation totally to the carbenium ion pathway [31]. The structure of the carboxylate can also support non-Kolbe electrolysis. By comparing the products of the electrolysis of different carboxylates with the ionization potentials of the corresponding radicals one can draw the conclusion that alkyl radicals with gas phase ionization potentials smaller than 8 e V should be oxidized to carbenium ions [8 c] in the course of Kolbe electrolysis. This gives some indication in which cases preferential carbenium ion formation or radical dimerization is to be expected. Thus a-alkyl, cycloalkyl [, ... 

Atmospheric pressure chemical ionization (APCI) is a gas phase ionization process based on ion-molecule reactions between a neutral molecule and reactant ions [31]. The method is very similar to chemical ionization with the difference that ionization occurs at atmospheric pressure. APCI requires that the liquid sample is completely evaporated (Fig. 1.12). Typical flow rates are in the range 200-1000 xL min , but low flow APCI has also been described. First, an aerosol is formed with the help of a pneumatic nebulizer using nitrogen. The aerosol is directly formed in a heated quartz or ceramic tube (typical temperatures 200-500 °C) where the mobile phase and the analytes are evaporated. The temperature of the nebulized mobile phase itself remains in the range 120-150 °C due to evapo-... 

Redox potential data frequently correlate with parameters obtained by other spectroscopic measurements. The correlation of E° potentials with gas-phase ionization potentials has already been briefly discussed. Electronic transitions observed by UV-visible spectroscopy involve the promotion of an electron from one orbital to another and this can be viewed as an intramolecular redox reaction. If the promotion involves the displacement of an electron from the HOMO to the LUMO, then the redox potentials for the reduction of the compound, °REd, and for its oxidation, °ox, are of importance. For a closely related series of compounds, trends in oxidation and reduction potentials can be related to shifts in the absorption frequency, v. If the structural perturbation causes the HOMO and the LUMO to rise or fall in energy in tandem, then (E°RED — E°ox) will remain constant in such cases the HOMO—LUMO frequency (energy) will be essentially independent of the structural perturbation. Where there is a differential influence of the perturbation on the HOMO and the LUMO, then ( °red E°ox) will vary as will the energy of the electronic transition. In such cases a linear correlation of °red or E°0x may result. In the limit the energy of the HOMO, or more usually the LUMO, will be unaffected by structural perturbation where the acceptor orbital is pinned, direct linear correlation of E°Gx with v should be apparent. With E°ox and v in a common energy unit, the plot E°0x versus v should have a slope close to one.33-36... 

For the trifluoroacetylation of 2-substituted thiophenes, furans, and pyrroles in C2H4C12, 75°, the p values are —7.4, —10.3, and ca. —4.5, respectively.259 The value for substituted benzenes is not known. In the gas phase ionization of substituted furans, thiophenes, selenophenes, and pyrroles,264 a reaction proceeding through a positively charged molecular ion taken to be analogous to the Wheland intermediate for electrophilic substitution, the p values are reported to be —20.2, —16.5,... 

The major chemical processes in radiation chemistry are reduction and oxidation reactions, according to the following examples. In the gas phase, ionization predominates... 

With the wide range of SSE s presently available, it should be possible to get an experimental value of Ein or E for almost any substrate, except possibly for those which are extremely difficult to reduce or oxidize or tend to form films. In the rare cases where an experimental value cannot be obtained, a reasonable value can often be inter- or extrapolated using known correlations between Hiickel MO parameters and oxidation or reduction potentials, or between gas phase ionization potentials and oxidation potentials 66 A very thorough discussion of structural effects on electrode reactions is available 24 as well as a comprehensive list of oxidation potentials of organic compounds 10 ... 

Ion mobility spectrometry (IMS) is an instrumental method where sample vapors are ionized and gaseous ions derived from a sample are characterized for speed of movement as a swarm in an electric field [1], The steps for both ion formation and ion characterization occur in most analytical mobility spectrometers at ambient pressure in a purified air atmosphere, and one attraction of this method is the simplicity of instrumentation without vacuum systems as found in mass spectrometers. Another attraction with this method is the chemical information gleaned from an IMS measurement including quantitative information, often with low limits of detection [2 1], and structural information or classification by chemical family [5,6], Much of the value with a mobility spectrometer is the selectivity of response that is associated with gas-phase chemical reactions in air at ambient pressure where substance can be preferentially ionized and detected while matrix interferences can be eliminated or suppressed. In 2004, over 20000 IMS-based analyzers such as those shown in Fig. 1 are placed at airports and other sensitive locations worldwide as commercially available instruments for the determination of explosives at trace concentration [7],... 

Considerable insight about fundamental aspects of the behavior of simple Ge, Sn and Pb species can be obtained from studies aimed at characterizing the reactivity of gas-phase ions. Reactions of the primary ions obtained by electron ionization of GelTj with the neutral monogermane precursor have been characterized both by low- and high-pressure mass spectrometric techniques , and more recently by ion trap techniques (ITMS). The ability to select a particular isotopic species (usually the Ge-containing ion) in low pressure studies carried out by Fourier Transform Mass Spectrometry (FTMS) has been essential in understanding the mechanism of these processes. The main results can be summarized as follows ... 

Recently, a number of efforts have sought to use new theoretical methods, particularly density functional theory (DFT), to predict redox potentials in solution. While considerable progress has been made toward predicting gas-phase ionization potentials, the role of solvation and coupled chemical reactions are of high importance to solution chemists. Baik and Friesner have recently discussed the ability of new methods for incorporating solvation effects into DFT calculations. Using these solvation methods, DFT calculations can be used to determine solution redox potentials to within 150mV.i ... 

A radical-cation Cope rearrangement of 2,5-diphenylhexa-l,5-dienes under electron ionization conditions (by mass spectrometry at 70 eV) has been described to occur in the gas phase. The reaction directionality differs from that in a thermal transformation. The rearrangement of hexamethyl-Dewai-benzene 410 into hexamethylbenzene (equation 156) as well as the closure of the bridged hexahydrodiene 411 into the so-called birdcage hydrocarbon 412 proceed during hemin-catalyzed epoxidation via a radical cation intermediate (equation 157)224. These processes are Cope-like rearrangement because two double bonds are separated by one CHt group in 410 and by three -hybridized C-atoms in 411. 

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