Electron ionization mass spectra

A normal, routine electron ionization mass spectrum represents the m/z values and abundances of molecular and fragment ions derived from one or more substances. 

Figure 2.19 Comparison between the electron ionization mass spectrum obtained in my laboratory (a) and that reported in the NIST library 35] (b) of nonacosan 15 one...

Quite often a normal electron ionization mass spectrum appears insufficient for reliable analyte identification. In this case additional mass spectral possibilities may be engaged. For example, the absence of the molecular ion peak in the electron ionization spectrum may require recording another type of mass spectrum of this analyte by means of soft ionization (chemical ionization, field ionization). The problem of impurities interfering with the spectra recorded via a direct inlet system may be resolved using GC/MS techniques. The value of high resolution mass spectrometry is obvious as the information on the elemental composition of the molecular and fragment ions is of primary importance. 

Fig. 11.4. Electron ionization mass spectrum of nonanal. Unlike the previous example (toluene, Fig. 11.3), this 9-carbon alkyl aldehyde displays extensive fragmentation and a very low abundance molecular ion at mlz 142. The extensive degree of fragmentation exhibited by many compounds under El conditions makes manual interpretation complex and tedious. Consequently, computerized searches of spectral libraries find extensive use in compound identification.

Fig. 19.8. Electron ionization mass spectrum of toluene (top panel) from GC-MS analysis, and library search match (bottom panel) against the NIST library.

Fig. 1.2. Electron ionization mass spectrum of a hydrocarbon. Adapted with permission. National Institute of Standards and Technology, NIST, 2002.

Figure 22-8 Electron ionization mass spectrum (70 eV) of 1-bromobutane. [From A. lilies, R B. Shevlin, G. Childers, M. Peschke, and J. Tsai, Mass Spectrometry for Large Undergraduate Laboratory Sections," J. Chem. Ed. 1995, 72, 717. Referee from Maddy Harris.]...

TABLE 4 Accurate mass measurements of selected fragment ions in the electron-ionization mass spectrum of econazole nitrate. 

Figure 16.29 Electron ionization mass spectrum of butanone. The scheme explains the formation of the principal ion fragments. The molecular ion leads to the acetyl ion CH3CO] (m/z = 43), which is ten times more intense than ion 57, CH3CH2CO] +. ...

FI3. 20.10. Electron ionization mass spectrum of methanol. From NIST MassSpec Data Center. Reproduced by permission. 

Figure 7. Electron ionization mass spectrum of pregnenolone methoxime-TMS ether. The origins of the major fragment ions are indicated. Reproduced from reference [65] with permission.

Figure 8. Electron ionization mass spectrum of the Af-t-butylmethylsilyl-3-oxime-t-butyldimethylsilyl derivatives of (a) isatin and (b) 5-methylisatin. Reproduced from Reference 76 with permission.

Figure 12. Electron ionization mass spectrum of the picolinyl derivative of I4-methyloctadecanoic acid extracted from the gui nea pig Harderian gland, illustrating the determination of branching position. Reproduced from reference [I58J with permission.

There might be so little that its peak is small or absent in the mass spectrum. The electron ionization mass spectrum at the left side of Figure 21-12 does not exhibit an peak, which would be at miz 226. Instead, fragments appear at miz 197, 156, 141, 112, 98, 69, and 55. These peaks provide clues about the structure of the molecule. A computer search is commonly used to match the spectrum of an unknown to similar spectra in a library. 

The electron ionization mass spectrum of 1-bromobutane in Figure 21-15 has two peaks of almost equal intensity at miz 136 and 138. The peak at miz 136 is the molecular ion C4H9 Br. Because bromine has almost equal abundance of the isotopes Br and Br, the second peak of almost equal intensity is C4H9 Br. Any molecule or fragment containing just one Br will have pairs of peaks of nearly equal intensity in its mass spectrum. Other major peaks at m/z 107, 57, and 41 are explained by rupture of the bonds of 1-bromobutane in Figure 21-16. The peak at 107 has an equal-intensity partner at 109, so it must contain Br. Peaks at 57 and 41 do not have equal-intensity partners, so they cannot contain Br. 

Figure 22-12 a) Electron ionization mass spectrum of caffeine. [From NIST/ER /NIH Mass Spectral Database.] (b) Selected reaction monitoring gas chromatogram of water collected 5 m below the surface of the Mediterranean Sea and containing 4 ng of caffeine per liter. [From I. J. Buerge, T. Poiger,... 

Figure 8.50 Electron ionization mass spectrum of perfluorotributylamine (PFTBA). inserts show the peak profiles for m/z 69, 219, and 502.

Figure 8.53 Electron ionization mass spectrum of (a) bromobenzene and (b) dichloromethane.

Figure 8.56b Electron ionization mass spectrum of peak 2, and (c).

Experimentally, 3 was produced in the gas phase as detected by the 70-eV electron-ionization mass spectrum of Nenitezscu s hydrocarbon (25) and its benzo-analogues (eq 15). 8... 

MS term for ions that are highly characteristic for the compound measured, especially for ions whose formation reveals structural or compositional information. For instance, the phenyl cation in an electron ionization mass spectrum is a diagnostic ion for benzene and derivatives. Used typically, for extracted ion chromatograms, -> XIC. 

FIGURE 3.4 Electron ionization mass spectrum of venlafaxine (MW = 277). 

FIGURE 3.5 Electron ionization mass spectrum and chemical formula of flurazepam (MW = 387). 

The electron ionization mass spectrum of flurazepam represented in Figure 3.5 illustrates perfectly the limits of this ionization technique. We can indeed see that the spectrum is dominated by an ion at m/z 86 and that there are no ions of significant abundance at high m/z ratios whereas the molecular weight of the analyte is 387. The m/z 86 ion is not particularly characteristic of flurazepam and it will therefore be impossible to have a specific method of quantification for this compound in El. 

FIGURE 4.20 Mass spectra of simazine. Electron ionization mass spectrum (top) and methanol negative chemical ionization (bottom). 

FIGURE 8.1 Reduction of electron ionization mass spectrum of a-pinene (a) to its 16 most abundant ions (b). 

FIGURE 9.21 Electron ionization mass spectrum of n-ethyl,l-propanamine and possible a cleavages from the molecular ion. 

FIGURE 9.32 Electron ionization mass spectrum of methyl 2-methylpentanoate and McLafferty rearrangement occurring from the molecular ion leading to ion at m/z 88. 

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