Electron Impact El Ionization

Volatile molecules can be ionized in the gas phase by colliding them with a beam of high-energy (70 eV) electrons. Typically, lower molecular mass compounds are more volatile. El is suitable for the analysis of these compounds in the molecular mass range of 1-800 Da. (Note, however, that there are compounds, such as fluo-rinated hydrocarbons or some transition metal complexes, that have MWs higher than 1000 Da and, yet, they are still volatile enough for El analysis.) 

The excess internal energy can easily be provided by collisions with 70 eV electrons since the electron energy is significantly larger than the ionization energies of common organic molecules (8-10 eV). Thus, during the ionization not only the elimination of an electron from a molecule (M) occurs but also an excited molecular ion (M ) is obtained (Equation (1)). 

In Equation (2), Fy denotes the fragment ion formed in the ith competitive and jth consecutive fragmentation step. 

Three characteristic 70 eV El ionization spectra are shown in Figs. 1,6, and 7a (acetone, benzene, and tributyl amine, respectively). In the El spectrum of acetone (Fig. 1) the molecular ion is at m/z 58 and this is the nominal MW of the neutral 

Even though the 70 eV spectra of acetone (Fig. 1) and benzene (Fig. 6) show characteristic differences, there are important similarities as well. In both cases, the odd-electron molecular ions tend to lose neutral radicals to form even-electron cations. These reactions are driven by the fact that, generally speaking, even-electron ions are more stable than odd-electron ions. (As will be seen in the following text, this general rule has an important consequence for fragmentation of even-electron-protonated (or deprotonated) molecules that prefer to lose even-electron fragments, e.g., small neutral molecules.) 

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Verification of the molecular weight of thiirene dioxides by mass spectrometry, employing the conventional electron-impact (El) ionization method, has been unsuccessful due to the absence or insignificant intensity of molecular ion peaks in their mass spectra. The base peak is rather characteristic, however, and corresponds to the formation of the disubstituted acetylene ion by loss of sulfur dioxide91 (equation 3). 

Since 1960 mass spectrometry has always been an important tool to investigate the molecular composition of sulfur vapor, sulfur melts, and the solid dlotropes [201]. Mostly spectra obtained by electron impact (El) ionization have been reported, except for one study in which the main species present in sulfur vapor (S2-Ss) were studied by photoionization mass spectrometry [202]. The following ionization potentials were reported (in eV) [202] ... 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

The MM-1 is a linear quadrupole-based mass spectrometer with electron impact (El) ionization.4 An ion pump provides the required vacuum. By today s standards, its size (0.34m3/12.2ft3) and weight (177kg/3901bs) are... 

For amino acid analysis the labeled protein needs to be hydrolyzed and derivatized. Most commonly the hydrolysis is performed in 6 M HC1, and the amino acids are converted into their isopropyl ester and pentafluoropropanamide derivatives (Fig. 1.1) before GC/ MS analysis. The molecular ion is not always visible after standard electron impact (El) ionization, and the fragment after loss of the carboisopropoxy group is the highest observable peak. This leaves m/e=175 plus the mass of the amino acid side chain, from which the degree of labeling can be directly deduced. 

Electron impact (El) ionization is one of the most classic ionization techniques used in mass spectrometry. A glowing filament produces electrons, which are then accelerated to an energy of 70 eV. The sample is vaporized into the vacuum where gas phase molecules are bombarded with electrons. One or more electrons are removed from the molecules to form odd electron ions (M+ ) or multiply charged ions. Solids, liquids and gases can be analyzed by El, if they endure vaporization without decomposition. Therefore the range of compounds which can be analyzed by El is somewhat limited to thermally stable and volatile compounds. The coupling with gas chromatography has been well established for... 

Interfacing of solution-based separation techniques with mass spectrometry has historically been a challenge because of the incompatibility of the used solvent with the vacuum system. Standard electron impact (El) ionization with techniques such as particle beam require samples to be vaporized under high vacuum for ion formation to occur. 

This classic text describes fragmentation pathways and mechanisms for ions formed using electron impact (El) ionization. In addition, this edition contains additional information regarding desorption ionization and the corresponding related fragmentation mechanisms. 

Analytical pyrolysis is defined as the characterization of a material or a chemical process by the instrumental analysis of its pyrolysis products (Ericsson and Lattimer, 1989). The most important analytical pyrolysis methods widely applied to environmental samples are Curie-point (flash) pyrolysis combined with electron impact (El) ionization gas chromatography/mass spectrometry (Cp Py-GC/MS) and pyrolysis-field ionization mass spectrometry (Py-FIMS). In contrast to the fragmenting El ionization, soft ionization methods, such as field ionization (FI) and field desorption (FD) each in combination with MS, result in the formation of molecule ions either without, or with only very low, fragmentation (Lehmann and Schulten, 1976 Schulten, 1987 Schulten and Leinweber, 1996 Schulten et al., 1998). The molecule ions are potentially similar to the original sample, which makes these methods particularly suitable to the investigation of complex environmental samples of unknown composition. 

They are still the workhorses of coupled mass spectrometric applications, as they are relatively simple to run and service, relatively inexpensive (for a mass spectrometer), and provide unit mass resolution and scanning speeds up to approximately 10,000 amu/s. This even allows for simultaneous scan/ selected ion monitoring (SIM) operation, in which one part of the data acquisition time is used to scan an entire spectrum, whereas the other part is used to record the intensities of selected ions, thus providing both qualitative information and sensitive quantitation. They are thus suitable for many GC-MS and liquid chromatography-mass spectrometry (LC-MS) applications. In contrast to GC-MS with electron impact (El) ionization, however, LC-MS provides only limited structural information as a consequence of the soft ionization techniques commonly used with LC-MS instruments [electrospray ionization (ESI) and atmospheric pressure chemical ionization (APCI)]. Because of this limitation, other types of mass spectrometers are increasingly gaining in importance for LC-MS. 

The electron impact (El) ionization mass spectra of all four 2-methyloxazolopyridine isomers has been reported <930MR(28)45l>, and are discussed in Section 7.08.3.3.4. 

The most common ionization technique is electron impact (El) ionization at 70 eV. As applied to lignin pyrolysis products, this mode of ionization produces molecular and typical fragment ions in sufficient abundance to permit structure assignments to be made. 

It is preferable to have a mass spectrum which shows the molecular ion of the compound and for this reason soft ionisation techniques such as chemical ionization (Cl), fast atom bombardment (FAB) and electrospray are now widely used instead of or as well as electron impact (El) ionization. 

Electron impact (El) ionization is a technique in which a neutral molecule, M, is ionized following bombardment by an accelerated electron beam creating a radical cation, M +, and an additional free electron [5—12] as follows ... 

Most of the N-nitrosamines analyses by GC-MS have been performed using capillary columns. The use of these columns avoids the necessity of intensive clean-up of the extracts prior to the analysis. The clean-up is needed for removing most of the many potentially interfering substances contained in environmental samples. On the other hand, the packed columns present the advantage of their greater sample capacity. The electron impact (El) ionization mode is the most frequently... 

The detection method most applied to capillary gas chromatography of PAHs in environmental samples is MS using electron-impact (El) ionization. " Fragmentation of PAHs is minimal under the 70 eV conditions generally used for El therefore single-ion monitoring (SIM) of the molecular ion provides maximum sensitivity although identification of individual isomers is virtually lqDs in the low to sub parts-per-billion level... 

SP-2401" and 3% SP-2250. ° Detectors used by EPA standards procedures, include photoionization (PID)," electron capture (ECD)," Eourier transform infrared spectrometry (PTIR), " and mass spectrometry detectors (MSD)." ° Method 8061 employs an ECD, so identification of the phthalate esters should be supported by al least one additional qualitative technique. This method also describes the use of an additional column (14% cyanopropyl phenyl polysiloxane) and dual ECD analysis, which fulfills the above mentioned requirement. Among MSDs, most of the procedures employ electron impact (El) ionization, but chemical ionization (CI) ° is also employed. In all MSD methods, except 1625, quantitative analysis is performed using internal standard techniques with a single characteristic m/z- Method 1625 is an isotope dilution procedure. The use of a FTIR detector (method 8410) allows the identification of specific isomers that are not differentiated using GC-MSD. 

In most cases, the molecular ions of lupin alkaloids are detectable in electron impact (El) ionization technique, and, therefore, useful for determination of molecular mass and composition by the combination with high resolution mass spectrometry. However, N-oxides usually exhibit very small molecular ion peaks in the El mode. Inbeam ionization technique provides relatively strong molecular ions of the alkaloid N-oxides. However, fast atom bombardment (FAB) mass spectrometry has been recently proved very useful for the determination of the molecular masses of N-oxides. 

Electron impact (El) ionization is useful for elements that are either volatile or form volatile compounds. Typical ionization efficiencies are in the range of 0.1 to 1 percent. Suitable elements include noble gases and light elements such as C, N, O. Other elements include those that form volatile compounds (e.g., uranium in the form of UFg). Except for specialized applications (e.g., noble gas analysis), El is no longer widely used for elemental and isotopic analysis. 

One shortcoming of GC-MS for isomer analysis is that the normal electron impact (El) ionization mechanism does not usually differentiate between isomers. 

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