Electron ionization principles

Figure 2.32 Principles of electron ionization source. Cathode (W, Re) e - emission e - energy is a function of accelerating voltage pressure (p) of gaseous sample <10 Pa. (H. Kienitz (ed.), Massenspek-trometrie (1968), Verlag Chemie, Weinheim. Reproduced by permisssion of Wiley-VCH.)...

In principle, this risk does not exist for the molecular peak. In fact, the contribution of a peak with composition (M + H)+ associated to a molecular M,+ peak should never be excluded. This artefact can have high abundance in the case of compounds such as amides in electron ionization. 

The displaced electron is generally assumed to be the electron with the lowest ionization energy. In order of probability, this will be a nonbonding electron followed by a 7t bond electron and then a a bond electron. Thus El yields, in the first instance, a molecular ion which is a radical cation with an unpaired electron. In principle, any remaining energy will then be dissipated by bond cleavages that result in the formation of the most stable cation with a paired electron (even-electron ion). These even-electron ions may be formed by homolytic or heterolytic cleavages. This whole process happens very rapidly (<10-8s) and is the reason for the close similarity of El spectra produced across all different instruments. It is important to remember that mass spectral reactions in the El source are unimolecular. This is because the pressure in the El source is too low for bimolecular (ion-molecule) reactions to occur. 

The text describes mass spectra produced by electron ionization, discussing how the spectral peak pattern relates to molecular structure. It details the use of high-resolution and accurate mass measurement to determine elemental composition of ions in order to identify unknown substances. The book also introduces some of the recent techniques that can be employed to extend the usefulness of mass spectrometry to high molecular weight substances and more polar substances. It includes examples and problems representing a cross section of organic chemistry to help readers integrate the principles presented. 

There is a part of the truth in this idealized picture. For most cases, it is sufficient to limit oneself (by concentration on optical excitations and photo-electron ionization, obeying the principle of Franck and Condon or by resignation) to scrutinize the electronic density determined by fixed nuclear positions. The really great success of quantum mechanics was the application to monatomic entities (but some restraining comments are made in section 4.8) where the nucleus is firmly put at the origin (unless below 10 amu, in which case perfectionists start worrying about the center of gravity shifted a trifle by the electrons). 

Another important electronic structure principle is the maximum hardness principle " (MHP) which may be stated as, There seems to be a rule of nature that molecules arrange themselves to be as hard as possible . Numerical verification of this principle has been made in several physico-chemical problems such as molecular vibrations , internal rotations , chemical reactions" , isomer stability , pericyclic reactions and Woodward-Hoffmann rules , stability of magic clusters , stability of super atoms ", atomic shell structure" , aromaticity , electronic excitations , chaotic ionization, time-dependent problems like ion-atom collision and atom-field interaction " etc. 

Suppression of ionization efficiency is important when the total ionizing capability of the ionization technique is limited, so that there is a competition for ionization among compounds that are present in the ion source simultaneously. In principle such a saturation effect must be operative for all ionization techniques, but in practice it is most important for electrospray ionization (Section 5.3.6), slightly less important for atmospheric pressure chemical ionization (Section 5.3.4), atmospheric pressure photoionization (Section 5.3.5) and matrix assisted laser desorption ionization (Section 5.2.2) it does not appear to be problematic under commonly used conditions for electron ionization and chemical ionization (Section 5.2.1) or thermospray (Section 5.3.2). Enhancement of ionization efficiency for an analyte by a co-eluting compound is less commonly observed and is, in general, not well understood. 

In the early stages of ESR application to polymer research, many studies on the identification of free radicals produced by irradiation with ionizing radiation, x-ray, and ultraviolet light were made. Some of the irradiation effects in polymeric materials were considered to originate from the radical processes and, therefore, clear identification of the radicals trapped in irradiated polymers was one of the most important problems at that stage. In this meaning, ESR application was considered to be a very convenient technique for this purpose, because detection and identification of the free radicals bearing unpaired electrons in principle can be done easily by the ESR method without any chemical modification of the materials. 

This indicates that excited vibrational states are almost fully unoccupied at room tenperature and only the energetically much lower-lying internal rotations are effective under these conditions. Upon electron ionization, the situation changes quite dramatically as can be concluded fiom the Franck-Condon principle and therefore, energy storage in highly excited vibrational noodes becomes of key importance for the further fate of ions in a mass spectrometer. In case of an indene... 

Entities Mendeleev beheved could not exist were ones that contravened his realist attitude to chemical elements. Matter, according to Mendeleev, had three essential properties It was integral (atoms, should they exist, must be assumed to be integral and without substructme) it was immutable (transmutation between elements was impossible, an extension of the quasi-Platonic picture of chemical elements) and each element had a specified valency. Mendeleev s often-noted early resistance to inert gases stemmed from a contravention of the third principle. His opposition to electrons (first principle) and radioactivity (second principle) sustained itself until his death [Gordin, 2004, chapter 8 1998]. Similarly, ionized elements could not exist in the abstract, and thus could certainly not be present in any solutions. 

In certain mass spectrometers known as internal ionization ion traps the same space within a single device plays in turn the roles of source and analyzer. The principle of electron ionization rmains the same but we will see in Chapter 4 dedicated to analyzers, the specificities of internal ionization ion traps compared to systems using an exetemal source. 

The third chapter is dedicated to the ionization modes that can be operated in GC-MS electron ionization as well as positive and negative chemical ionization. We explain the principles of operation of internal and external sources and compare the performances achieved by each ionization mode. 

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