Mass spectrum base peak

Alkyl-2-methoxypyrazines exhibit a base peak at miz 124 in the mass spectrum. The peak corresponds to a molecular ion in 2-methoxy-3-methylpyrazine (24a) and to a fragment ion, resulting from a McLafferty rearrangement of an alkyl group, in 3-isopropyl-2-methoxypyrazine (24b) and 3-5cc-butyl-2-methoxy pyrazine (24d) (97). 

A quaternary base, C18H22ON+, chloride (mp 203° [ ]d -1-50.5°) was isolated from the roots of this plant and further characterized as the picrate (mp 152°) and the trifluoroacetate (mp 203°). Exhaustive spectral data indicated structure XC for this base in particular, the mass spectrum gave peaks corresponding to fragments XCI and XCII. Pyrolysis on a larger scale gave a mixture from which it was possible to isolate actinidine (XCI) 245), whose structure had already been established 246). 

Orotidine-TMS breaks down on the gas chromatographic columns, however, the fragments are quite characteristic and readily identifiable The mass spectrum of peak 133 of Fig 2 show the characteristic fragmentation of the sugar moiety a relatively weak peak for the fully trimethylsilylated sugar, and base peak corresponding to the loss of an OTMSi group (m/e 259). 

The mass spectrum of peak (B) is reported in Fig. 3. The molecular ion at m/e 208 (an increase of 16 mass units in comparison to the spectrum of the parent compound) and the subsequent loss of 29 mass units from the molecular ion to give the base peak at m/e 179, suggested the presence of an epoxide at the 10,11-position. The loss of 29 amu has been rationalized as shown in Scheme 2. 

The collection edited by Hugh L. J. Makin contains 2979 El mass spectra of androgens and estrogens and their trimethylsUyl-, O-methoxyoxime- and acetal derivatives. Each spectrum is accompanied by the structure and trivial name, molecular formula, molecular weight, nominal mass and base peak. All spectra of androgens and estrogens have been obtained on the same mass spectrometer under identical conditions (Makin, 2005). The database is available in the formats for many popular MS data system. 

The fragmentation of the base peak results from cleavage of the C—H bond rather than the C—O bond because the C—O bond is stronger. We can see this effect from the mass spectrum The peak at m/z = 15 is a methyl carbocation. It is only about 12% as intense as the base peak. The P+1 peak is the radical cation of the CH30H (the oxygen bears the positive charge, as in the parent peak). The peak at m/z 29 results from loss of two hydrogen atoms from the base peak, as shown below. 

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. 

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. 

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. 

In quadrupole-based SIMS instruments, mass separation is achieved by passing the secondary ions down a path surrounded by four rods excited with various AC and DC voltages. Different sets of AC and DC conditions are used to direct the flight path of the selected secondary ions into the detector. The primary advantage of this kind of spectrometer is the high speed at which they can switch from peak to peak and their ability to perform analysis of dielectric thin films and bulk insulators. The ability of the quadrupole to switch rapidly between mass peaks enables acquisition of depth profiles with more data points per depth, which improves depth resolution. Additionally, most quadrupole-based SIMS instruments are equipped with enhanced vacuum systems, reducing the detrimental contribution of residual atmospheric species to the mass spectrum. 

The structures shown for ions, such as in Equations 7 and 8, are entirely speculative since the mass spectrum gives only relative abundance of the various m/e-groups formed. Chemical reasoning, meta-stable-ion peaks, peak shifts in the deuterated analogs, and published mass spectra of similar compounds are the bases for the proposed modes of fragmentation and for the postulated structures.)... 

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