El mass spectra

The formation of a simple El mass spectrum from a number (p) of molecules (M) interacting with electrons (ep. Peak 1 represents M , the molecular ion, the ion of greatest mass (abundance q). Peaks 2, 3 represent A+, B. two fragment ions (abundances r, s). Peak 2 is also the largest and, therefore, the base peak. 

S7O crystallizes in orange monochnic crystals which are yellow at —80 °C and melt with decomposition at 55 °C. However, even at 25 °C S7O decomposes quantitatively within 24 h in diffuse daylight, more slowly in the dark, to SO2, polymeric sulfur and httle Sg. Thermolysis in a high vacuum at 60 °C produces S2O and elemental sulfur. Therefore, the El mass spectrum exhibits only ions originating from these decomposition products (SO, 830, S ). In CH2CI2 solution S7O decomposes at 20 °C within 3 h to SO2, polymeric sulfur and Sio the latter can be isolated in 7% yield based on the starting material 87 [62] ... 

From CS2 solution S7O2 is obtained as intensely orange colored crystals which on heating spontaneously decompose at 60-62 °C with evolution of sulfur dioxide. S7O2 is far less soluble in CS2 (ca. 1 g at 0 °C) than S7O. The solution decomposes within 1 h to a mixture of sulfur homocycles and SO2. Solid S7O2 decomposes at 25 °C within minutes and quantitatively within 2 h, even in the dark. Heating in a high vacuum to 50-60 °C produces S2O and elemental sulfur. The El mass spectrum therefore exhibits peaks due to these decomposition products only [67]. 

On heating SioO decomposes with melting and SO2 evolution at 51 °C. At 20 °C the sohd oxide decomposes quantitatively within 2 h to SO2, elemental sulfur and a polymeric sulfuroxide (S 0)x with n>10. This polysulfuroxide also decomposes within 24 h at 20 °C to SO2, Ss, and polymeric sulfur. The solubility of SioO in CS2 is much higher than in CH2CI2 while the substance is practically insoluble in n-pentane. At -80 °C, SioO can be stored for longer periods of time without decomposition. The El mass spectrum shows large peaks for SO and S20 but no molecular ion [60]. 

Compound 37a showed the absence of an aldehydic proton and the singlet around 8.15 ppm was assigned to the ethylenic proton located p with respect to the electron-withdrawing cyano and ester groups. The benzofuranyl coumarins 38 exhibited the carbonyl-stretching band around 1690 cm in the IR spectra (Table 6). PMR data for 13 compounds are given in Table 2. The El mass spectrum of 36a showed a molecular ion peak at m/z 324 (41%). 

Enhanced molecular ion implies reduced matrix interference. An SMB-El mass spectrum usually provides information comparable to field ionisation, but fragmentation can be promoted through increase of the electron energy. For many compounds the sensitivity of HSI can be up to 100 times that of El. Aromatics are ionised with a much greater efficiency than saturated compounds. Supersonic molecular beams are used in mass spectrometry in conjunction with GC-MS [44], LC-MS [45] and laser-induced multiphoton ionisation followed by time-of-flight analysis [46]. 

A similar reaction takes place with sulfuric acid. It takes 2 to 3 months to achieve thermodynamic equilibrium between 5- and 6-membered heterocycles. However, the ratio of the products may be easily predicted in several minutes measuring the ratio of intensities of peaks of [M — CH3]+ and [M — C2H4]+ ions in an El mass spectrum of the initial substituted arylcyclopropane [23]. 

Figure 5.21. El mass spectrum of a compound with molecular mass 94 Da.

What is the difference between a product-ion MS-MS scan on a molecular ion and a conventional El mass spectrum ... 

Fragmentation trees similar to that shown here can be constructed from any El mass spectrum, providing ten thousands of examples for the sixth assumption of QET that fragment ions may again be subject to dissociation, provided their internal energy suffices. 

Example For the case of 4-methyl-1-pentene, the breakdown graph, the internal energy distribution from the photoelectron spectrum, and the 70 eV El mass spectrum are compared (Fig. 2.22). [99] From the fragmentation threshold to about... 

Example The NR mass spectrum of acetone closely resembles its 70 eV El mass spectrum (Chap. 6.2.1), thereby demonstrating that the molecular ion basically retains the structure of the neutral (Fig. 2.29). [144] However, the isomeric CsHeO" ions formed by McLafferty rearrangement of 2-hexanone molecular ion are expected to have enol structure (Chap. 6.7.1), and thus the corresponding NR mass spectrum is easily distinguished from that of acetone. 

Fig. 3.8. El mass spectrum of 1-butene. Adapted with permission. NIST, 2002.

Fig. 3.20. Partial 70 eV El mass spectrum of perfluorokerosene, PFK. The peaks are evenly spaced over a wide m/z range. In the low m/z range peaks from residual air do occur.

Example The El mass spectrum of Qo also shows an abundant doubly charged molecular ion, at m/z 360 with its isotopic peaks located at 0.5 u distance and a signal at m/z 240 of very low intensity (Fig. 3.28). [22] The isotopic pattern remains the same for all of them. As a consequence of the compressed m/z scale, the doubly charged fragment ion is detected at m/z 348. More examples of multiply charged ions can be found throughout the book. 

Fig. 4.37. El mass spectrum of perfluorotributylamine (mass calibrant FC43) to demonstrate unit resolution of a quadmpole analyzer. The expanded views a-c show peaks separated to almost identical degree.

Fig. 5.1. El mass spectrum of l,2,3,4,5,6-hexaliydrophenanthro[l,10,9,8-op<7ra]perylene. AH signals in the expanded inset correspond to doubly-charged ions. Adapted from Ref. [6] with permission. Elsevier Science, 2002.

In an El mass spectrum of ABXY, all ionic species formed as a result of numerous competing and consecutive reactions would be detected, but there is no simple rule which of them should give rise to intensive peaks and which of them would be almost overlooked. 

The most intensive peaks in the El mass spectrum of methane are the molecular ion, m/z 16 and the fragment ion at m/z 15 (Fig. 6.1). Explicitly writing the electrons helps to understand the subsequent dissociations of CH4 to yield CH3, m/z 15, by H loss (oi) or by CH3 loss (02), respectively. In general, it is more convenient to write the molecular ion in one of the equivalent forms. The charge and radical state are then attached to the brackets (often abbreviated as 1) enclosing the molecule. 

Fig. 6.1. El mass spectrum of methane. Used by permission of NIST. NIST 2002.

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