Electron Impact and Chemical Ionization

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 

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The chemical potentials and free energies of the 2-isoxazolines have also been studied and the electron impact and chemical ionization mass spectra determined (77MI41614). Fragmentation pathways and retrocycloadditions of various derivatives were discussed in these reports. 

The elimination of HX(X = SOCH3, SCHj) from the [M-STol]- and [MH-HSTol] cations (Tol = CHjCgH ) from 5-9 under electron impact and chemical ionization depends strongly on the spatial orientation of the vicinal substituents . The intensity of the [M — STol] ion is appreciably higher when X and Y are cis than when they are trans. The stereochemistry of the sulfoxide group is not important since 7 and 8 exhibit very similar spectra. The elimination of HSOCHj occurs more easily from 7 and 8... 

Identification of compounds in the river water extracts was based on the coincidence of gas chromatographic retention times and on the equivalence of electron impact and chemical ionization mass spectra with those of authentic compounds. Quantitation was based on standard curves generated for selected compounds. 

Electron Impact and Chemical Ionization Mass Spectral Data for Benzoic Acid... 

With many compounds there is little fragmentation on chemical ionization. An example of a comparison of the spectra resulting from electron impact and chemical ionization is given in Figure 27-13. The simplicity of the spectra... 

ELECTRON IMPACT AND CHEMICAL IONIZATION MASS SPECTROMETRY OF CAROTENOIDS... 

Lange, C., Basselier, J.-J., Bagneres, A.-G., Escoubas, P., Lemaire, M., Lenoir, A., Clement, J.-L., Bonavita-Cougourdan, A., Trabalon, M. and Campan, M. (1989). Strategy for the analysis of cuticular hydrocarbon waxes from insects using gas chromatography with electron impact and chemical ionization. Biomed. Environ. Mass. Spectrom., 18, 787-800. 

The repeatedly noted ring-opening of ionized cyclopropane derivatives (Section III.A) seems also to account for the randomization of methoxy groups in ionized dimethyl 2-methoxy-3,3-dimethylcyclopropane-l,l-dicarboxylate. Similarly, the electron impact and chemical ionization mass spectra of benzoylcyclopropanes can be interpreted by ring-opening followed by facile 1,2-hydrogen migration, which precede the main dissociations... 

Vermeulen, N. P. E Cauvet, J., Luijten, W. C. M. M., and van Bladeren, E J. (1980). Electron Impact and Chemical Ionization Mass Spectrometry of cis and trans-S-(2-Hydroxycyclohexyl)-N-acetyl-Ucysteinemethyl Esters, Biomed. Mass Spectnm., 7 413-417,... 

A mass spectrometer has three basic components something to volatilize and ionize the molecule into a beam of charged particles something to focus the beam so that particles of the same mass charge ratio are separated from all others and something to detect the particles. All spectrometers in common use operate in a high vacuum and usually use positive ions. Two methods are used to convert neutral molecules into cations electron impact and chemical ionization. 

In El, the electron beam is so energetic that many fragments are produced. These fragments, however, are very useful in identifying the molecular species entering the mass spectrometer. Only electron impact and chemical ionization are used with GC/MS. 

Gas chromatographic/mass spectrometry (GC/MS) results were obtained at the Ohio State University Chemical Instrumentation Center using a Finnigan 4021 GC/MS instrument. Both electron impact and chemical ionization were performed on samples following separation of compounds by a gas chromatograph equipped with capillary columns containing 3% 0V-17 or 5% carbowax 20M. 

Identified by Blank et al. (1992a) in a roasted arabica by comparison with a reference substance (MS/ electron impact and chemical ionization, retention index on several columns). 

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