Electron Ionization El

Electron ionization (previously called electron impact or electron bombardment) has a long history in MS, as it was the hrst widely nsed ionization method. El sonrces, located inside the instrument s vacnnm chamber, consist of a box (stainless steel, 1 ml, also called the ion volume), with a series of openings that allow the introduction of both the sample and the ionizing electrons and the ejection of the resulting ions into the analyzer (Eignre 2.9). El sonrces are held at 200-250 °C to maintain the analyte(s) in the vapor phase and to prevent their deposition on the walls. The vapor pressure of the samples in the sonrce mnst be in the lO -lO Torr range. A heated tungsten or rhenium hlament is used to produce an electron beam thermionic 

Electrons from the filament are collimated by a magnetic field and traverse the source in a helical path (to increase efficiency) to the electron trap. 

The electrons react with the analyte molecules to produce radical cations. 

FIGURE 2.10 Comparison of El spectra for (a) an aromatic and (b) an aliphatic compound. 

Positive ions dissociative ionization and dissociative rearrangements Negative ions electron capture and dissociative electron capture Both polarities ion-pair formation. 

In electron ionization (El) mass spectrometry, ions are produced by bombarding gaseous molecules with high-energy electrons. It is a hard technique and causes fragmentation of the parent molecule. 

In addition to fragmentation patterns, the appearance of a mass spectrum depends upon the naturally occurring isotopes of the elements (see Appendix 5). The presence of several isotopes of an element leads to the observation of peak envelopes. 

Assign the peaks in the El mass spectrum of POCI3 shown below. 

Note that, because Cl has two isotopes both of which are present in significant abundance, the mass spectrum exhibits groups of peaks peak envelopes). 

Determine the mjz value for the peak that you expect for the parent molecular ion, [M] , using the most abundant isotopes  

The energy required to remove an electron from an atom or molecule is its ionization potential or ionization energy. Most organic compounds have ionization potentials ranging between 8 and 15 electron volts (eV). However, a beam of electrons does not create ions with high efficiency until it strikes the stream of molecules with a potential of 50 to 70 eV. To acquire reproducible spectral features, including fragmentation patterns, that can be readily compared with electronic databases, a standard 70-eV electron beam is used. 

EI-MS has distinct advantages for routine mass spectrometry of small organic molecules. Electron ionization hardware is inexpensive and robust. The excess kinetic energy imparted to the sample during the El process leads to significant fragmentation of the molecular ion (Chapter 4). 

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The fragmentation pattern of a compound is reproducible, and many libraries of EI-MS data are available. This allows one to compare the mass spectrum of a sample compound against thousands of data sets in a spectral library in a few seconds using a PC, thus simplifying the process of determining or confirming a compound s identity. 

The fragmentation of the molecular ion under El conditions may also be considered a distinct disadvantage. Some compounds fragment so easily that the lifetime of the molecular ion is too short to be detected by the mass analyzer. Thus, one cannot determine a compound s molecular mass (Section 3.6) in such cases. Another drawback to EI-MS is that the sample must be relatively volatile so it can come into contact with the electron beam in the ionization chamber. This fact coupled with the fragmentation problem make it difficult to analyze high molecular weight (MW) compounds and most biomolecules using EI-MS. 

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A connnon feature of all mass spectrometers is the need to generate ions. Over the years a variety of ion sources have been developed. The physical chemistry and chemical physics communities have generally worked on gaseous and/or relatively volatile samples and thus have relied extensively on the two traditional ionization methods, electron ionization (El) and photoionization (PI). Other ionization sources, developed principally for analytical work, have recently started to be used in physical chemistry research. These include fast-atom bombardment (FAB), matrix-assisted laser desorption ionization (MALDI) and electrospray ionization (ES). 

This chapter should be read in conjunction with Chapter 3, Electron Ionization. In electron ionization (El), a high vacuum (low pressure), typically 10 mbar, is maintained in the ion source so that any molecular ions (M +) formed initially from the interaction of an electron beam and molecules (M) do not collide with any other molecules before being expelled from the ion source into the mass spectrometer analyzer (see Chapters 24 through 27, which deal with ion optics). 

Much of the energy deposited in a sample by a laser pulse or beam ablates as neutral material and not ions. Ordinarily, the neutral substances are simply pumped away, and the ions are analyzed by the mass spectrometer. To increase the number of ions formed, there is often a second ion source to produce ions from the neutral materials, thereby enhancing the total ion yield. This secondary or additional mode of ionization can be effected by electrons (electron ionization, El), reagent gases (chemical ionization. Cl), a plasma torch, or even a second laser pulse. The additional ionization is often organized as a pulse (electrons, reagent gas, or laser) that follows very shortly after the... 

Note that many of the terms mentioned in this chapter are discussed in detail elsewhere in this book. For example, the theory and practical uses of electron ionization (El) are fully discussed in Chapter 3. 

In a high vacuum (low pressure 10 mbar), molecules and electrons interact to form ions (electron ionization, El). These ions are usually injected into the mass spectrometer analyzer section. 

Molecules can interact with energetic electrons to give ions (electron ionization, El), which are electrically charged entities. The interaction used to be called electron impact (also El), although no actual collision occurs. 

The beam of substrate molecules then passes straight into the ion source (electron ionization, El, or chemical ionization. Cl) for ionization before entry into the mass analyzer. 

The mass spectrometer should provide structural information that should be reproducible, interpretable and amenable to library matching. Ideally, an electron ionization (El) (see Chapter 3) spectrum should be generated. An interface that fulfils both this requirement and/or the production of molecular weight information, immediately lends itself to use as a more convenient alternative to the conventional solid-sample insertion probe of the mass spectrometer and some of the interfaces which have been developed have been used in this way. 

Ionization methods that may be utihzed in LC-MS include electron ionization (El), chemical ionization (Cl), fast-atom bombardment (FAB), thermospray (TSP), electrospray (ESI) and atmospheric-pressure chemical ionization (APCI). 

As discussed in Section 3.2.1, an electron ionization (El) spectrum arises from a number of competing and consecutive fragmentation reactions of the molecular... 

The method using GC/MS with selected ion monitoring (SIM) in the electron ionization (El) mode can determine concentrations of alachlor, acetochlor, and metolachlor and other major corn herbicides in raw and finished surface water and groundwater samples. This GC/MS method eliminates interferences and provides similar sensitivity and superior specificity compared with conventional methods such as GC/ECD or GC/NPD, eliminating the need for a confirmatory method by collection of data on numerous ions simultaneously. If there are interferences with the quantitation ion, a confirmation ion is substituted for quantitation purposes. Deuterated analogs of each analyte may be used as internal standards, which compensate for matrix effects and allow for the correction of losses that occur during the analytical procedure. A known amount of the deuterium-labeled compound, which is an ideal internal standard because its chemical and physical properties are essentially identical with those of the unlabeled compound, is carried through the analytical procedure. SPE is required to concentrate the water samples before analysis to determine concentrations reliably at or below 0.05 qg (ppb) and to recover/extract the various analytes from the water samples into a suitable solvent for GC analysis. 

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. 

The electron ionization (El) mass spectra of TMS ethers and esters are generally characterised by weak or absent molecular ions. The [M—15]+ ion formed by loss of a methyl radical is generally abundant and in the case of alcoholic functions, the loss of a trimethylsilanol molecule [M—90]+ is also diagnostic. The peak at mJz 73, corresponding to the TMS group, is important in nearly all the TMS-derivative mass spectra. Figure 8.2 shows the fragmentation of TMS esters and ethers in mass spectrometric analyses. 

Electron ionization (El) mass spectra of 1,3,4-oxadiazole itself and its 2-mono- and 2,5-disubstituted derivatives, including the proposed main fragmentation pathways have already been discussed in CHEC(1984) and CHEC-11(1996) <1984CHEC(6)427, 1996CHEC-II(4)268>. Molecular ions of the compounds are usually of high intensity and the most important fragmentation pathways of the molecular ions involve loss of respective HCN, RCN molecules, or RCO cations. Loss of HNCO is significant in the spectra of 2-amino derivatives. 

Electron ionization El Election induced ionization Volatile molecular ions Smaller molecules GC-MS Extensive libraries... 

Electron ionization (El) was introduced in 1921 by Dempster, who used it to measure lithium and magnesium isotopes [31]. Modern El sources are, however, based on the design by Bleakney [32] and Nier [33, 34], who both worked in Prof. J. T. Tate s laboratory. In El ions are produced by directing an electron beam into a low pressure vapor of analyte molecules. 

Chromatographic methods are used to separate the components in a mixture, but in a complex mixture, a single chromatographic method or step many not separate all components. In these cases, using simple retention time to identify the components will not suffice and the identification of components in the mixture will be incorrect. Thus, the addition of a method of identification such as mass spectrometry (MS) or Fourier transform infrared (FTIR) is essential. In some cases, it may even be necessary to confirm either an FTIR or MS identification by the same method applied in a different way. For example, FTIR may be followed by MS, or electron ionization (El) MS followed by chemical ionization (Cl) MS or by an entirely different method. 

A little recognized systematic error in the calculation of accurate masses of, for example, small radical cation molecular ions (as in electron ionization (El)) or protonated molecular ions (as seen in the soft ionization methods) is the fact that the electron has a small, but finite mass. The accurate masses of radical cations, in which a valence electron has been removed, of anions that have been created by capture of an electron, and of protonated species produced by soft ionization processes, should take into consideration this small mass difference [19]. For example, there is a small difference between the relative atomic mass of a neutral hydrogen atom and a proton. The accepted accurate mass of 1H° is 1.007825 Da. The accurate mass of 1H+ is 1.007276 Da. To be completely correct, expected accurate masses of protonated molecular ions, [M+H]+, produced by electrospray should be calculated using the mass of one H+, rather than all of neutral hydrogen atoms. Mamer and Lesimple do acknowledge, however, that, for large molecules, the error is of little consequence. 

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