Mass spectrometry from electron ionization

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. 

GC-coupled mass spectrometry with electron ionization (EI-MS) or with electron capture, negative ionization (ECNI-MS), and GC with electron capture detection (ECD) have been the major techniques for the analysis of aryl methyl sulfone fractions isolated from tissue (Sect. 5.1). GC/ECD detection and identification of MeS02-PCBs relies on a comparison of GC retention times relative to authentic standards, which is dependent on the absence of co-eluting interfe-rents. GC/MS techniques have provided important structural information for MeS02-PCBs, especially in the absence of authentic standards. 

Despite its inherent analytical difficulties, gas chromatography on capillary columns in combination with sensitive and specific mass spectrometry has been widely used for separation of these analytes. Typical examples of such applications are those interfacing gas chromatography with mass spectrometry via electron impact (470, 484, 480, 489), chemical ionization (481, 478, 483, 473), or both interfaces (474, 475, 487, 488). Apart from mass spectrometry, Fourier transform infrared spectrometry has also been suggested as an alternative very useful identification tool in the area of the -agonist analysis. Capillary gas chromatography with Fourier transform infrared spectrometry was successfully employed to monitor clenbuterol, mabuterol, and salbutamol residues in bovine liver and urine (471). 

We saw that numerous ionization techniques exist that yield radical cations or radical anions, protonated or deprotonated molecules, and various adducts. These ions yield fragments with an even number of electrons (closed shell) or with an odd number of electrons (open shell). Even though the radical cations derived from electron ionization sources retain a privileged status in common mass spectrometry, the other ionization methods become increasingly common. Electron ionization is not possible for many categories of molecules. Therefore, we will not limit the discussion to radical cations. 

Whereas the rotational and the gas temperature are particularly relevant to the evaporation processes in the plasma, the electron temperature, being a measure of the kinetic energy of the electrons, is relevant to the study of excitation and ionization by collisions with electrons. This is an important process for generation of the analyte signal both in optical atomic emission and in mass spectrometry. The electron temperature can be determined from the intensity of the recombination continuum or of the Bremsstrahlung , as described by Eq. (57). 

The application of mass spectrometry, including electron impact (El) studies, to the analysis of hydrocarbon fractions obtained from petroleum is as old as the earliest exploitation of the technique. Almost all of the ionization methods known have been applied to saturated hydrocarbons. The number of papers dealing with one or another aspect of the MS of alkanes and cycloalkanes is enormous. However, the computerized compilation of the mass spectra in libraries such as NBS makes the search easier ... 

Figure 19 Mass spectrum of 25-OH-D3 purified from Hep 3B cells (upper panel) compared to the mass spectrum of synthetic 25-OH-D3 (lower panel). Hep 3B cells were incubated with vitamin D3 (50 pM) for 48 h. Flasks were then extracted and the lipid extract dried under nitrogen and purified on HPLC (conditions Zorbax SIL [6.2 mm X 25 cm], solvent HIM 96/3/3, flow rate 2 mL/min). A metabolite peak possessing the vitamin D chromophore and comigrating with synthetic 25-OH-D3 was collected, purified further on a different HPLC system (conditions Zorbax CN [4.6 mm X 25 cm], solvent HIM 94/ 5/1, flow rate 1 mL/min), and then dried under nitrogen and subjected to direct probe mass spectrometry using electron impact EI(+) ionization. The putative 25-OH-D3 gave the expected molecular ion with m/z 400 the other ions observed were consistent with the molecule being 25-hydroxylated (see inset fragmentation pattern). (From Ref. 207.)...

The species produced through ionization of an electron from a ir-orbital (such as from a C-H or a C-C bond of an alkane in mass spectrometry) cannot be represented at all by a connection table, yet the RAMSES notation can account for it as shown in Figure 2-59. 

If a sample solution is introduced into the center of the plasma, the constituent molecules are bombarded by the energetic atoms, ions, electrons, and even photons from the plasma itself. Under these vigorous conditions, sample molecules are both ionized and fragmented repeatedly until only their constituent elemental atoms or ions survive. The ions are drawn off into a mass analyzer for measurement of abundances and mJz values. Plasma torches provide a powerful method for introducing and ionizing a wide range of sample types into a mass spectrometer (inductively coupled plasma mass spectrometry, ICP/MS). 

The previous discussion has centered on how to obtain as much molecular mass and chemical structure information as possible from a given sample. However, there are many uses of mass spectrometry where precise isotope ratios are needed and total molecular mass information is unimportant. For accurate measurement of isotope ratio, the sample can be vaporized and then directed into a plasma torch. The sample can be a gas or a solution that is vaporized to form an aerosol, or it can be a solid that is vaporized to an aerosol by laser ablation. Whatever method is used to vaporize the sample, it is then swept into the flame of a plasma torch. Operating at temperatures of about 5000 K and containing large numbers of gas ions and electrons, the plasma completely fragments all substances into ionized atoms within a few milliseconds. The ionized atoms are then passed into a mass analyzer for measurement of their atomic mass and abundance of isotopes. Even intractable substances such as glass, ceramics, rock, and bone can be examined directly by this technique. 

Diphenylthiirene 1-oxide and several thiirene 1,1-dioxides show very weak molecular ions by electron impact mass spectrometry, but the molecular ions are much more abundant in chemical ionization mass spectrometry (75JHC21). The major fragmentation pathway is loss of sulfur monoxide or sulfur dioxide to give the alkynic ion. High resolution mass measurements identified minor fragment ions from 2,3-diphenylthiirene 1-oxide at mje 105 and 121 as PhCO" and PhCS, which are probably derived via rearrangement of the thiirene sulfoxide to monothiobenzil (Scheme 2). 

Sputtered Neutral Mass Spectrometry (SNMS) is the mass spectrometric analysis of sputtered atoms ejected from a solid surface by energetic ion bombardment. The sputtered atoms are ionized for mass spectrometric analysis by a mechanism separate from the sputtering atomization. As such, SNMS is complementary to Secondary Ion Mass Spectrometry (SIMS), which is the mass spectrometric analysis of sputtered ions, as distinct from sputtered atoms. The forte of SNMS analysis, compared to SIMS, is the accurate measurement of concentration depth profiles through chemically complex thin-film structures, including interfaces, with excellent depth resolution and to trace concentration levels. Genetically both SALI and GDMS are specific examples of SNMS. In this article we concentrate on post ionization only by electron impact. 

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. 

Laser ionization mass spectrometry or laser microprobing (LIMS) is a microanalyt-ical technique used to rapidly characterize the elemental and, sometimes, molecular composition of materials. It is based on the ability of short high-power laser pulses (-10 ns) to produce ions from solids. The ions formed in these brief pulses are analyzed using a time-of-flight mass spectrometer. The quasi-simultaneous collection of all ion masses allows the survey analysis of unknown materials. The main applications of LIMS are in failure analysis, where chemical differences between a contaminated sample and a control need to be rapidly assessed. The ability to focus the laser beam to a diameter of approximately 1 mm permits the application of this technique to the characterization of small features, for example, in integrated circuits. The LIMS detection limits for many elements are close to 10 at/cm, which makes this technique considerably more sensitive than other survey microan-alytical techniques, such as Auger Electron Spectroscopy (AES) or Electron Probe Microanalysis (EPMA). Additionally, LIMS can be used to analyze insulating sam-... 

Analytical applications Mass spectrometry has been applied to a variety of analytical problems related to expls, some of which have already been mentioned. Identification of the principal constituents of expls has been attempted from electron impact cracking patterns (Refs 34, 50 58), as well as chemical ionization spectra (Refs 69,70 71). Such methods necessarily include vapor species analysis and are directed to detection of buried mines (Refs 50, 58, 61,... 

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