Mechanism of ion formation

When a neutral molecule interacts with an electron of high kinetic energy, the positive radical ion is generated by El. If the electrons have less energy than the IE of the respective neutral, El is prohibited. As the electrons approach thermal energy, EC can occur instead. Under EC conditions, there are three different mechanisms of ion formation [65,75-77]... 

The mechanisms of ion formation in MALDI are a subject of continuing research. [30-34] The major concerns are the relationship between ion yield and laser flu-ence, [28,35] the temporal evolution of the desorption process and its implications upon ion formation, [36] the initial velocity of the desorbing ions, [29,37,38] and the question whether preformed ions or ions generated in the gas phase provide the major source of the ionic species detected in MALDI. [39,40]... 

A practical application coming out of field ion emission is the liquid metal ion source. Ion sources of a wide variety of chemical elements, most of them low melting point metals, can be produced by using either liquid metals131,132 or liquid alloys.133 The idea of extracting charged droplets out of liquid by application of an electrostatic field is perhaps older than field ion microscopy. But the development of liquid metal ion sources from liquid capillaries, from slit shaped emitter modules and from wetted field emission tips, etc., as well as the understanding of the mechanisms of ion formation in terms of field evaporation and field ionization theories,... 

The mechanisms of ion formation are usually studied with measurements of mass separated ion energy distributions using a magnetic sector mass spectrometer.144,145 For a liquid metal ion source, both atomic ions and cluster ions of all sizes are emitted. If the total ion current is large, neutral atoms and small droplets may also be emitted. There is little question that most of the atomic ions in a liquid metal ion source at low... 

Progress in molecular secondary ion mass spectrometry (SIMS) is presented, with emphasis on applications and the mechanism of ion formation. The mechanism involves three processes (1) energy conversion at the surface,... 

This paper contains a discussion of the most important features of laser-induced-ion spectra and mechanisms of ion formation, some of them experimentally proven, some of them still under discussion and investigation. Special attention will be given to the LAMMA technique, because this is the authors own field of work, but reference to other systems will be included where appropriate. 

The exact mechanism of ion formation in air at ambient pressure should be considered as incompletely understood for compounds of high proton affinity or compounds for which protons can be released. For example, product ions for... 

The mechanisms of ion formation in MALDI-MS are still not fully imder-stood. Two main possibilities exist ions may either be "pre-formed" in the solid state and simply be liberated upon laser irradiation, or they may be formed by ion-molecule reactions initiated by the laser pulse. ... 

The solution of perchloric acid in glacial acetic acid [Eq. (3)] shows the typical two-step mechanism of ion formation of ionogenes. An ionization (equilibrium constant ATi) process is followed by a dissociation (equilibrium constant ATd) process. The overall constant K is given by the relationship... 

Chapter 1 has been amended by the results of Karas and JaskoUa on new design matrices and the mechanisms of ion formation in M ALDI, which has substantially added to our understanding of the processes and how to optimize them for practical applications. They have also influenced our view of the method as such and have led to revision of other parts of the chapter. 

The mechanism of ion formation in MALDI is not completely understood however, it is known that ionization suppression effects must be taken into account if MALDI is to be used for quantitation. Also, above a certain concentration analyte signal cannot be increased... 

Rogers, D.E. Derrick, P.J. Mechanisms of Ion Formation in Field Desorption of Oligosaccharides. Org. Mass Specirom. 1984,19,490-495. 

The mechanisms of ion formation in MALDI are a subject of continuing research [31-35]. The major concerns are the relationship between ion yield and laser flu-ence [26,36], the temporal evolution of the desorption process and its implications... 

Hi) Gas phase charge transfer means that ion formation occurs after volatilization or desorption of neutral species from the surface into the gas phase through ionization via proton/electron transfer or other ion-molecule reactions at atmospheric pressure. Indeed, the assumption of ion-molecule reactions, eventually purely in the gas phase above the sample, has led to the development of an DAPCI source (Chap. 13.2), in the first place to prove this mechanism of ion formation [7]. The solvent pH can be used to positively affect the vapor pressure of the analyte, e.g., the vapor pressure of volatile plant alkaloids is increased by addition of abase. 

In negative ion mode, thermal electrons are presumed to generate mostly O2"" ions from air that serve as reagent ions. Of course, direct electron capture by the analyte as well as dissociative electron capture, deprotonation, or anion attachment are also feasible [58]. Thus, it has been inferred from the close similarity of the negative-ion products of DART and APPI that the same group of mechanisms of ion formation are effective. 

Beiminghoven, A. Mechanism of Ion Formation and Ion Emission During Sputtering. Zeitschrift fuer Physik 1969, 220,159-180. 

Fig. 10. A simplified mechanism of ion formation in the electrospray ionization process.

Egorov, S. E., Letokhov, V. S., and Shibanov, A. N. (1984a). Mechanism of ion formation by irradiation of molecular crystal surface with laser pulses. Soviet Journal of Quantum Electronics, 14, 940-946. 

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