Chemical ionization reagents, liquid,

With the ion traps that offer the possibility of performing negative chemical ionization with liquid reactants, methanol is a preferred reactant in negative mode. The reagent ions are then methanolate ions CHjO formed according to reactions 6 and 7 in Figure 9.63. 

However, the spatial inhomogeneity in the distribution of reagents is not the only reason why the radiolysis of substances in the condensed state is different from that of gases. As we have already mentioned in Section VIII, as we pass from the gaseous state to the condensed one, at the primary stage of radiolysis we already observe a redistribution of yields of primary active particles (resulting in the increase of the yield of ionized states). Also different are the subsequent relaxation processes, as well as the processes of decay of excited and ionized states.354 Another specific feature of processes in a condensed medium is the cage effect, which slows down the decay of a molecule into radicals.355 Finally, the formation of solvated electrons is also a characteristic feature of radiation-chemical processes in liquids.356... 

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. 

It has become painfully obvious that most of the excellent approaches and techniques that have been developed for use in liquid chromatography are not applicable to liquid chromatography/mass spectrometry (LC/MS) with atmospheric pressure ionization. Chapter 5 described the reagents and the range of mobile-phase compositions that are compatible with electrospray and atmospheric pressure chemical ionization (APCI), and these are limited to volatile components that do not cause significant ion suppression. Certain problems that are not significant with standard LC separations become difficult to deal with because of the limitations placed on the mobile phase by atmospheric pressure ionization (API) LC/MS. 

Ion Trap Analysis with Liquid Chemical Ionization (Cl) Reagents USEPA Method 521... 

In chemical ionization-mass spectrometry (CI-MS), the sample molecules are combined with a stream of ionized reagent gas that is present in great excess relative to the sample. When the sample molecules collide with the preionized reagent gas, some of the sample molecules are ionized by various mechanisms, including proton transfer, electron nansfer, and adduct formation. Almost any readily available gas or highly volatile liquid can be used as a reagent gas for CI-MS. 

The APCI probe consists of a liquid introduction capillary surrounded by a coaxial tube for pneumatic nebulization. After pneumatic nebulization of the column effluent, the droplets are swept through a heated zone in order to achieve evaporation of the droplets. The solvent vapours are subsequently used as reagent gas in chemical ionization, initiated by electrons from a corona discharge electrode kept at a few kV. In positive-ion mode, a series of gas-phase ion-molecule reactions leads to the formation of protonated solvent molecules which may react with the analyte molecules having a higher proton affinity than the solvent. The protonated analytes are sampled and subsequently mass analysed. 

Table 6.1 shows that all the devices mentioned allow electron ionization and chemical ionization in positive mode (Cl in positive mode is often optional). The internal ionization ion trap offers the double advantage of allowing the use of liquid reagents and rapid switching from one ionization mode to another. This clearly makes the internal ionization ion trap the perfect device for structural elucidation that requires easy access to both ionization modes. However, this analyzer is the only one that does not operate in negative chemical ionization for the reasons covered in Chapter 4. 

Indirect effects in nonaqueous media have received very little attention. It is likely that semiselective chemical reactions will occur in organic solvents containing active solutes. Liquid ammonia and sulfur dioxide also might offer some possibilities as solvents. The direct action of densely ionizing radiation on ammonia to produce hydrazine might yield reasonable quantities of this important reagent. 

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