Chemical ionization process

In brief, the method consists of introducing small amounts (partial pressures of 10 3-10 4 torr) of the substance to be investigated into the ionization chamber of a mass spectrometer which contains a high pressure (1 torr) of methane, the reactant gas. Ionization is effected by electron impact, and because the methane is present in such an overwhelming preponderance, all but a negligibly small amount of the initial ionization occurs in the methane. The methane ions then undergo ion-molecule reactions to produce a set of ions which serve as reactant ions in the chemical ionization process. The important reactant ions formed from... 

It is illuminating to examine the energetics of the chemical ionization processes in normal paraffins even though approximate energy values must be used. For n-octadecane we write the following thermochemical relations ... 

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. 

Another popular and efficient inlet system for the LC/MS combination is the atmospheric pressure chemical ionization process. This system has some similarity to the electrospray interface and can also cope with flow rates of up to 2 ml/min. and thus the total column eluent can be utilized without splitting the flow. 

Gas-phase electrolyte ions, e.g., NH4", may be formed dnring these processes as well. These ions may act as reagent gas ions in ion-molecule reactions taking place in the gas-phase, i.e., chemical ionization processes, or at the liquid-gas interface of the microdroplets. 

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). 

In chemical ionization, gaseous analyte molecules are ionized by ion-molecule reactions with reagent ions, formed by electron ionization from the appropriate reagent gas. The Cl ion source is similar to the El source, but is operated at a higher pressure (0.1-2 Torr). The chemical ionization process is illustrated for a proton transfer reaction, which is the most common... 

For all except one of the chemical ionization studies reported so far, the reactants initiated an acid-base, even-electron chemistry. For such reactants there are several reasons to anticipate that the spectra produced by chemical ionization will differ significantly from the spectra produced by electron impact, photon impact, or field ionization. The chemical ionization is effected by the transfer of massive entities such as protons, hydride ions, or alkyl carbonium ions. Consequently the chemical ionization processes are not governed by Frank-Condon considerations, and the ions produced by the transfer of even-electron entities are, for even-electron molecules, even-electron ions. The amounts of energy which are involved in chemical ionization processes tend to be low by mass spectrometric standards, although the exact amount involved depends upon the identity of the reactant used. 

These distinguishing characteristics of chemical ionization mass spectrometry have been discussed previously, but as a result of recent studies (to be discussed in more detail later) we have come to recognize another characteristic aspect of the chemical ionization process when the high-pressure mass spectrometric technique is used. When chemical ionization is effected at a pressure in the mass spectrometer ionization chamber on the order of 1 Torr, the ions comprising the mass spectra are produced by collision processes, and after formation the ions undergo a number of collisions with molecules of reactant gas before they pass out of the ionization chamber. Thus, unlike the conditions which obtain in conventional (low-pressure) electron-impact and photon-impact ionization, the ions produced by chemical ionization are not isolated. That is to say, they are not formed... 

This analysis of temperature effects and kinetics in chemical ionization must be considered as tentative and in need of further verification. However, if it is correct, or substantially correct, the activation energies for comparable reactions provide measures of the relative energies of product ions produced by the thermal decompositions, and the frequency factors provide information about entropies of activation. It is clear that reactions such as those producing benzyl ion, (28) and (29), are analogous to A lI acid-catalyzed solvolysis reactions in solution, and we suggest that the significance of the kinetic quantities in the gas-phase chemical ionization process is analogous with that in condensed-phase solvolysis. In the gas phase, solvent interactions are absent, and the prospect is that much new and valuable information will be obtained from this technique. 

Next to ESI, APCI is an alternative, especially for less polar compounds. The APCI interface consists of a different solvent inlet probe, the so-called heated nebulizer, and a discharge electrode, but uses exactly the same atmospheric-pressure ion source and vacuum transition region as in ESI (see Figure 1). In a heated nebulizer, the LC column effluent is first pneumatically nebulized into an aerosol, which subsequently is evaporated in a heated vaporizer zone. Solvent and analyte vapor meet the discharge needle, where a potential of a few kilovolts initiates a continuous discharge that results in the ionization of solvent molecules, which in turn ionize the analyte molecules by gas-phase ion-molecule reactions (common chemical ionization processes). 

The chemical ionization process produces both positive (cations) and negative (anions) ions and thus, an alternative to positive-ion chemical ionization mass... 

Because the ionization mechanism for ASAP is a chemical ionization process there is the potential to generate analyte ions by either charge transfer, in the case of a dry nitrogen atmosphere in the source, or by proton transfer if there is residual... 

Here only the first reaction step of [(H20)n+H] cluster ion formation is explicitly formulated. As the PA of H2O (697 kJ mor ) is actually below that of any analyte (Table 2.6), water cluster ions protonate the analyte molecules by a chemical ionization process at atmospheric pressure [29-31] ... 

Instead of relying on one of the above ionization processes to occur, a chemical ionization process where the reactant ions are formed by photoionization of a reagent gas is normally preferred. If the solvent (S) cannot be photoionized to play this role, a UV-absorbing dopant (D) may be added [185,187-189]. The dopant ions may then react with the analyte by charge transfer... 

The PTR-MS technique has been extensively discussed in a series of review papers [4 6,8-13]. Briefly, it combines a soft, sensitive, and efficient mode of chemical ionization, adapted to the analysis of trace VOCs, with a quadrupole mass filter. The gas to be analyzed is continuously introduced into the chemical ionization cell (drift tube) and ionized by proton transfer from H3O+. The protonated VOCs are extracted by a small electrical field from the drift tube and mass analyzed by a quadrupole mass spectrometer. The specific aspect of the chemical ionization scheme in PTR-MS is that the generation of the primary H30 ions, and the chemical ionization process, VOC + H30 V0CH" " + H2O, are spatially and temporally separated and can therefore be individually optimized. 

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