Base peaks

The main features are the molecular ions as the base peak and the M-t-1 ions arising from another species. For 2-aminothiazole the m/e 73 ion (M-HCN) is shifted to m/e 75 in the spectrum of the dideuteroamino derivative and, therefore, largely arises via rupture of 2-3 and 4-5 bonds (Scheme 18). This fragmentation process could involve the kind of intermediates postulated in photochemical rearrangements (see Chapter III, Section IX.3.B). The other fragments fit well the general pattern of fragmentation proposed by Clarke (136). 

The base peak in the mass spectrum of the LM free metal-ligand ion and the fragmentation patterns of this parent ion are of particuliar significance since they illustrate the effect of coordination upon the properties of the thiazole ligand. The free thiazole fragments upon electron impact by two major routes (Scheme 86 also cf. Section II. 6). 

The mass spectrum of benzene is relatively simple and illustrates some of the mfor matron that mass spectrometry provides The most intense peak m the mass spectrum is called the base peak and is assigned a relative intensity of 100 Ion abundances are pro portional to peak intensities and are reported as intensities relative to the base peak The base peak m the mass spectrum of benzene corresponds to the molecular ion (M" ) at miz = 78... 

Some classes of compounds are so prone to fragmentation that the molecular ion peak IS very weak The base peak m most unbranched alkanes for example is m/z 43 which IS followed by peaks of decreasing intensity at m/z values of 57 71 85 and so on These peaks correspond to cleavage of each possible carbon-carbon bond m the mol ecule This pattern is evident m the mass spectrum of decane depicted m Figure 13 42 The points of cleavage are indicated m the following diagram... 

Base peak (Section 13 22) The most intense peak in a mass spectrum The base peak is assigned a relative intensity of 100 and the intensities of all other peaks are cited as a per centage of the base peak... 

The mass spectrum is a fingerprint for each compound because no two molecules are fragmented and ionized in exactly the same manner on electron-impact ionization. In reporting mass spectra the data are normalized by assigning the most intense peak (denoted as base peak) a value of 100. Other peaks are reported as percentages of the base peak. 

Included in the table are all compounds for which information was available through the C, compounds. The mass number for the five most important peaks for each compound are listed, followed in each case by the relative intensity in parentheses. The intensities in all cases are normalized to the w-butane 43 peak taken as 100. Another method for expressing relative intensities is to assign the base peak a value of 100 and express the relative intensities of the other peaks as a ratio to the base peak. Taking ethyl nitrate as an example, the tabulated values would be... 

Molecular formula Name Parent peak Base peak Three next most intense peaks ... 

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. 

The ion having greatest abundance is said to form the base peak in the spectrum. The base peak may or may not be the same as the molecular ion peak. 

Base peak. The peak in a mass spectrum corresponding to the m/z value that has the greatest intensity. This term can be applied to the spectra of a pure substance or mixtures. 

Intensity relative to base peak. The ratio of intensity of a particular peak in a mass spectrum to the intensity of the mass peak of the greatest intensity. This ratio is generally equated to the normalized ratio of the heights of the respective peaks in the mass spectrum, with the height of the base peak being taken as 100. 

Infrared Data Com-mittee of Japan ir 19,000 PC-based peak wavenumbers and intensities... 

Substituent effects are in some instances predictable and/or easily rationalized, but are more subtle in other cases. iV-Oxides generally show an abundant M—16 peak which is sometimes the base peak, and di-iV-oxides show successive elimination of two atoms of... 

The dominant fragmentation mode of pyrimidine is loss of HCN twice, to give ionized acetylene, m/e 26, as base peak whether C-2 or C-4 is involved in the initial loss of HCN... 

A feature eommon to the pyrazine, quinoxaline and phenazine ring systems is their remarkable stability in the mass speetrometer and in all eases with the parent heterocyeles the moleeular ion is the base peak. In the ease of pyrazine, two major fragments are observed at mje 53 and 26, and these fragments are eonsistent with the fragmentation pattern shown in Seheme 1. 

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