Ionization in the gas phase

Table 4.13. Comparison of Substituent Contributions to Phenol Ionization in the Gas Phase and Solution" ...

In region III, the discharge is maintained only by ionization in the gas phase without electron injection from the cathode. Because of the inertial effect of ions and electrons, only small part of charged particles in the gas phase can arrive on the electrode. Therefore, polymerization may be induced principally by diffused free radicals and/or ion-electron pairs ... 

MALDI A soft (gentle) method for creating gas-phase ions that utilizes energy from a laser targeted onto a mixture of analyte and a chemical matrix. Analyte ions can be formed from a combination of vaporization of existing ions and by vaporization of neutrals followed by ionization in the gas phase (Hillenkamp et al., 1990). 

Thermal evaporation of neutral molecules from the solid followed by ionization in the gas phase. 

O. Mo et al., Spontaneous self-ionization in the gas phase A theoretical prediction. Chem. Phys. Chem. 7, 465 167 (2001)... 

In Section II, A we defined the processes of electron ionization in the gas phase and of electron transfer to the continuum in the solution phase ... 

Electron ionization is the most commonly method used for the analysis of volatile compounds. It is the case for organic molecules. This is a ionization in the gas phase that occurs in the ion source by the collision of the neutral molecules of the sample and electrons emitted from a filament by a thermoionic process (Figure 16.17). The ejection of the most weakly held electron leads to positive ions. This reproducible procedure facilitates the identification of a compound by comparing its spectrum with those collected in a spectral library, assuming the compound is registered within. 

Obviously, the close correspondence between the rather instantaneous vertical PES ionization in the gas phase and the considerably slower oxidation in solution, which allows the adiabatic relaxation of the radical cation generated in its solvent cage, are interrelated facets of the same species. Therefore, a short summary on their ESR/ENDOR spectroscopically determined spin populations, their structural changes relative to the neutral precursors and their observable molecular dynamics provides some insight into the reality of molecular radical cation ground states. 

LC-MS using atmospheric pressure ionization (LC-API-MS) has dramatically changed the analytical methods used to detect polar pollutants in water. Most API mass spectrometers offer two interfaces electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI), both of which can be operated in positive and negative ion mode. ESI transfers ions from solution into the gas phase, whereas APCI ionizes in the gas phase. Analytes occurring as ions in solution may be best analyzed by ESI, while nonionic analytes may be well suited for APCI. What must always be taken into consideration is the relations between analyte properties and the chosen method of chromatographic separation. 

In eq 5, AHjo, (gas) is the corrected gas phase ionization potential of the nucleotide, or cluster, and ASjon Cs ) e change in entropy associated with ionization in the gas phase. Compared to AHj<, ij(gas), TAS i (gas) is negligible, so that AGj (gas) AHio i2(gas). This approximation is supported by the observation that, by electron convention Fermi-Dirac statistics, TAS for the ionization of a hydrogen atom at room temperature is 0.05 eV (44). In this investigation, the values of IP employed in eq 5 were the values of IPcorr(i) obtained via eq 1. 

Comparison of substituent contributions to phenol ionization in the gas phase and solution... 

Fig. 6 Comparison of benzene ionization in the gas phase (a) and in aqueous solution (b) showing some of the common excimer laser photon energies /3,9/.

O Table 23.1 shows the ionization potentials and the mean energies needed for ionization in the gas phase (W values). The Upvalues are between 21 and 36 eV, and they are much higher than the ionization potential values. The high difference is partly because a considerable part of energy absorbed is used for excitation and also to the fact that ionization does not solely occur with the lowest energy electron. 

Hinderling C, Adlhart C, Chen P. Olefin Metathesis of a Ruthenium Carbene Complex by Electrospray Ionization in the Gas Phase. Angew Chem Int Ed. 1998 37(19) 2685-2689. 

The structural and compositional analysis of synthetic polymers by MALDI MS has developed rapidly. The MALDI technique involves embedding the analyte in a matrix that absorbs at the wavelength of the laser. The energy is then transferred from the matrix to the analyte, which is desorbed and subsequently ionized in the gas phase. The ions produced can be analyzed in a time-of-flight (TOP) mass analyzer, a natural choice for pulsed-ionization techniques. 

O 2. MALDI-TOF mass spectrometry is a technique for the determination of molar masses in which a sample is ionized in the gas phase and the mass-to-charge number ratios of aU ions are measured. 

The Space-Focus Plane. The early time-of-flight mass spectrometers utilized electron impact (El) to ionize volatile molecules in the gas phase. Ionization in the gas phase results in considerable uncertainties in the initial position of the ions in the extraction field that are less problematic in the desorption (PDMS and MALDI) instruments in use today. Sdll, it is useful to consider this problem (and the means by which it is corrected), since spatial distributions do exist when ions are desorbed from irregular or insulating surfaces. In addition, it is likely that compact EI-TOF instruments will continue to be developed (and commercialized) for environmental monitoring or as gas-chromatographic detectors. 

Sputtering by kilovolt ion beams produces far more neutral species than ions. When static methods are employed, one can assume that a large percentage of these sputtered neutrals will be intact molecules. Thus, there is the very real possibility for improving detection limits (sensitivity) if these neutrals can be observed by post-ionization in the gas phase. Such techniques are known as secondary neutral mass spectrometry (SNMS). ... 

The spark is initiated by emission of electrons from the cathode by field emission (opposite process to FI), which becomes possible due to the high local field strengths (lO -IO V m ) at edges of the microscopically rough surface. The electrons are then accelerated towards the anode. There, the energy of the impinging electrons causes evaporation of the anode material (sample) that is subsequently ionized in the gas phase by electron ionization. 

Volatile molecules can be ionized in the gas phase by colliding them with a beam of high-energy (70 eV) electrons. Typically, lower molecular mass compounds are more volatile. El is suitable for the analysis of these compounds in the molecular mass range of 1-800 Da. (Note, however, that there are compounds, such as fluo-rinated hydrocarbons or some transition metal complexes, that have MWs higher than 1000 Da and, yet, they are still volatile enough for El analysis.)... 

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