Electronic ionizing techniques

In either case the material is passed into a chamber where an ionizing element, often 63Ni, a radioactive isotope that produces /3 particles (electrons), converts the molecules in the chamber to ions, the same technique used in many household smoke detectors. Newer designs sometimes use 241 Am, which decays in a particles and y rays. To avoid the regulatory inconvenience of radioactive material, several electronic ionizing techniques have also been proposed. 

Perusal of the data presented in the following sections would show that extremely accurate (better than 0.01 kcal mol ) ionization energies can be obtained with state-of-the-art optical spectroscopy and photoionization and electron ionization techniques. 

In GC-MS coupling, the electron ionization technique removes an electron from the eluted molecules of the chromatograph. Generally speaking, the electrons that are the easiest to remove are n electrons, free electron pairs of heteroatoms (N, O, S, P, halogens, etc), and n electrons of double and triple bonds. The sigma (a) electrons constituting simple bonds are the most difficult to remove. That is why hydrocarbons composed of exclusively a electrons are not ionized as efficiently as other molecules. 

One of the first successful techniques for selectively removing solvent from a solution without losing the dissolved solute was to add the solution dropwise to a moving continuous belt. The drops of solution on the belt were heated sufficiently to evaporate the solvent, and the residual solute on the belt was carried into a normal El (electron ionization) or Cl (chemical ionization) ion source, where it was heated more strongly so that it in turn volatilized and could be ionized. However, the moving-belt system had some mechanical problems and could be temperamental. The more recent, less-mechanical inlets such as electrospray have displaced it. The electrospray inlet should be compared with the atmospheric-pressure chemical ionization (APCI) inlet, which is described in Chapter 9. 

Alternative ( soft ) ionization techniques are not usually required for aromatic isothiazoles because of the stability of the molecular ions under electron impact. This is not the case for the fully saturated ring systems, which fragment readily. The sultam (25) has no significant molecular ion under electron impact conditions, but using field desorption techniques the M + lY ion. is the base peak (73X3861) and enables the molecular weight to be confirmed. 

In other articles in this section, a method of analysis is described called Secondary Ion Mass Spectrometry (SIMS), in which material is sputtered from a surface using an ion beam and the minor components that are ejected as positive or negative ions are analyzed by a mass spectrometer. Over the past few years, methods that post-ion-ize the major neutral components ejected from surfaces under ion-beam or laser bombardment have been introduced because of the improved quantitative aspects obtainable by analyzing the major ejected channel. These techniques include SALI, Sputter-Initiated Resonance Ionization Spectroscopy (SIRIS), and Sputtered Neutral Mass Spectrometry (SNMS) or electron-gas post-ionization. Post-ionization techniques for surface analysis have received widespread interest because of their increased sensitivity, compared to more traditional surface analysis techniques, such as X-Ray Photoelectron Spectroscopy (XPS) and Auger Electron Spectroscopy (AES), and their more reliable quantitation, compared to SIMS. 

A comparison of the various post-ionization techniques electron-gas bombardment, resonant and nonresonant laser ionization, etc. While some of the numbers are outdated, the relative capabilities of these methods have remained the same. This is a well-written review article that reiterates the specific areas where post-ionization has advantages over SIMS. 

In both electron post-ionization techniques mass analysis is performed by means of a quadrupole mass analyzer (Sect. 3.1.2.2), and pulse counting by means of a dynode multiplier. In contrast with a magnetic sector field, a quadrupole enables swift switching between mass settings, thus enabling continuous data acquisition for many elements even at high sputter rates within thin layers. 

The mass spectral fragmentations of 9,10-dimethoxy-2,3,4,6,7,ll/)-hexa-hydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140 and -2,4-diones 141, under electron ionization (at 70 eV) were examined by metastable ion analysis, a collosion-induced dissociation technique and exact mass measurement (97RCM1879). Methyl substituent on N(3) in 140 (R = Me) had a larger effect on both the fragmentation and on the peak intensities, than a methyl substituent on C(6) (R = Me). The ionized molecules of 140 (R = H) were rather stable, whereas 4-phenyl substitution on C(4) of 140 (R = Ph) promoted the fragmentations of the molecular ions. The hexahydro-1//-pyrimido[6,l-n]isoquinoline-2,4-diones 141 were more stable, than the hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140, and the molecular ions formed base peaks. 

There are numerous ionization techniques available to the mass spectrome-trist, but for GC/MS almost all analyses are performed using either electron impact ionization or chemical ionization. 

For many years, electron ionization, then more usually known as electron impact, was the only ionization method used in analytical mass spectrometry and the spectra encountered showed exclusively the positively charged species produced during this process. Electron ionization also produces negatively charged ions although these are not usually of interest as they have almost no structural significance. Other ionization techniques, such as Cl, FAB, thermospray, electrospray and APCI, however, can be made to yield negative ions which are of analytical utility. 

Metastable atom bombardment (MAB) is a novel ionization method for mass spectrometry invented by Michel Bertrand s group at the University of Montreal, Quebec, Canada, and described by Faubert et al.38 For the identification of bacteria by MS, MAB has a number of significant advantages relative to more familiar ionization techniques. Electron ionization (El) imparts so much excess energy that labile biomolecules break into very small fragments, from which the diagnostic information content is limited since all... 

The extension of analytical mass spectrometry from electron ionization (El) to chemical ionization (Cl) and then to the ion desorption (probably more correctly ion desolvation ) techniques terminating with ES, represents not only an increase of analytical capabilities, but also a broadening of the chemical horizon for the analytical mass spectrometrist. While Cl introduced the necessity for understanding ion—molecule reactions, such as proton transfer and acidities and basicities, the desolvation techniques bring the mass spectrometrist in touch with ions in solution, ion-ligand complexes, and intermediate states of ion solvation in the gas phase. Gas-phase ion chemistry can play a key role in this new interdisciplinary integration. 

M.P. Colombini, F. Modugno, E. Ribechini, Direct exposure electron ionization mass spectra metry and gas chromatography/mass spectrometry techniques to study organic coatings on archaeological amphorae, Journal of Mass Spectrometry, 40, 675 687 (2005). 

Electron ionization Volatile, Gas Electrons (eV) Hard technique, odd... 

The most important gas phase ionization techniques are electron ionization and chemical ionization. The latter will be described in Section 2.6.2. 

El is probably the most used ionization technique in the study of cultural heritage. In El the ionization of a molecule is produced by electrons. Often this technique and its acronym El are referred as electron impact , but this terminology is inappropriate. The term impact implies a crash, a collision between two or more objects. In the case of El no such impact or crash occurs. 

The term desorption ionization indicates those ionization techniques in which the production of ions is based on a desorption process. This consists of the rapid addition of energy to a sample in a condensed phase (i.e. liquid or solid) with subsequent production and emission of stable ions in the gas phase. These are generally even electron species that fragment only to a limited extent. The development of desorption methods has amplified the impact and utility of MS in a lot of fields, such as biology, biochemistry and proteomics. 

On account of having the same number of electrons and protons, isotopes of a given element have identical chemical properties and reactivity. However, since they differ in the number of neutrons, they have different atomic masses. As MS measures and discriminates mass, isotopes are detected and they are present in every mass spectrum, independent of the ionization technique used, instrumentation, etc. 

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