Mass base peak

Information including mass spectra, chemical structure, chemical name, molecular formula, molecular weight (nominal mass), base peak, reference and measurement condition. 

The collection edited by Rolf Kiiehnle contains 2200 mass spectra of drugs, pharmaceuticals and metabolites. The inclusion of the silylated derivatives as used for GC-MS analyses is of special value in this database. Additional information included are chemical structure, chemical name, molecular formula, molecular weight (nominal mass), base peak and the reference (Kiiehnle, R. (2006)). 

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... 

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. 

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. 

The alkaloid Nigellicine proved to be the pyridazino[l,2-u]indazolium-l 1-carboxylate (234) and forms yellow crystals (Scheme 77). It was isolated from the widely distributed herbaceous plant Nigella saliva L., which is used as a spice and for the treatment of various diseases (85TL2759). The structure was determined by an X-ray crystal structure analysis. The carboxylate bond distances are essentially equal (123.3 and 125.6 pm). An intramolecular hydrogen bond was found between the carboxylate oxygen atom and the hydroxy group. In mass spectrometry, the molecular peak was found at mjz —246 (20) and the base peak at mjz —202 which corresponds... 

The mass spectral fragmentations of 9,10-dimethoxy-2,3,4,6,7,ll/)-hexa-hydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140 and -2,4-diones 141, under electron ionization (at 70 eV) were examined by metastable ion analysis, a collosion-induced dissociation technique and exact mass measurement (97RCM1879). Methyl substituent on N(3) in 140 (R = Me) had a larger effect on both the fragmentation and on the peak intensities, than a methyl substituent on C(6) (R = Me). The ionized molecules of 140 (R = H) were rather stable, whereas 4-phenyl substitution on C(4) of 140 (R = Ph) promoted the fragmentations of the molecular ions. The hexahydro-1//-pyrimido[6,l-n]isoquinoline-2,4-diones 141 were more stable, than the hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140, and the molecular ions formed base peaks. 

The mass spectrum of a compound is typically presented as a bar graph with masses (m/z values) on the x axis and intensity, or relative abundance of ions of a given m/z striking the detector, on the y axis. The tallest peak, assigned an intensity of 100%, is called the base peak, and the peak that corresponds to the unfragmented cation radical is called the parent peak or the molecular ion (M+). Figure 12.2 shows the mass spectrum of propane. 

Mass spectral fragmentation patterns are usually complex, and the molecular ion is often not the base peak. The mass spectrum of propane in Figure 12.2, for instance, shows a molecular ion at m/z = 44 that is only about 30% as high as the base peak at m/z = 29. In addition, many other fragment ions are present. 

Figure 12.4 Mass spectrum of hexane (C6H14 MW = 86). The base peak is at m/z = 57, and numerous other ions are present.

Methylpentane (C.6H]4) has the mass spectrum shown. WTiich peak represents M+ Which is the base peak Propose structures for fragment ions of m z = 71, 57, 43, and 29. Why does the base peak have the mass it does ... 

Base peak (Section 12.1) The most intense peak in a mass spectrum. 

Besides a parent ion, the mass spectra of benzo- and dibenzothiepins show the corresponding naphthalene or phenanthrene radical cations as the base peak.2-16 The mass spectra of 1-benzo-thiepin 1-oxides and 1,1-dioxides show the same naphthalene radical cation, formed by loss of sulfur monoxide or sulfur dioxide, respectively.14 In contrast, in the mass spectrum of 2,7-di-terf-butylthiepin peaks resulting from the loss of sulfur are not found.17... 

Mass spectral data on l-(arylsulfonyl)-l//-azepines have been amassed,73 and the fragmentation patterns of several 1-acyl-1//-azepines elucidated.61 For the latter systems, the base peaks correspond to the azatropylium cation (m/z 92). Loss of hydrogen cyanide to yield the cyclo-pentadienyl cation (m/z 65) has also been noted. 

The mass spectra of l-acyl-l//-l-benzazepines have been recorded.23 The mass spectrum of 3-mesyl-3/7-3-benzazepine shows an intense base peak at m/e = 142duetothebcnzazepinylium ion and a peak (51 %) at m/e — 115 (-HCN) which is attributed to the indenium cation.26 Fragmentation patterns for 1H- and 5/7-2-benzazepines40 and for 5//-dibenz[c,e]azepine5 are available. The electron-impact induced fragmentation pattern of 5//-dibenz[6,/]azepine displays an intense molecular ion as the base peak, and a moderately intense (M + 1) peak.5 ... 

Fragmentation The TMS derivatives of amino sugars have their base peak at mass 131.0765. 

Molecular ion The presence of sulfur can be detected by the 34S isotope (4.4%) and the large mass defect of sulfur in accurate mass measurements. In primary aliphatic thiols, the molecular ion intensities range from 5-100% of the base peak. 

Base peak The peak in the mass spectrum with the greatest intensity. 

Compilations of Reference Spectra There are several compilations of reference mass spectra available of which the oldest is the American Petroleum Institute (Ref 82) collection of spectra obtained mostiy on the older type instruments. Recent collections index spectra variously, eg, under reference number (Ref 19). molecular weight (Refs 12 19), molecular formula (Ref 19), fragment ion values (Ref 19), and base peak (Refs 12 19). A quarterly journal, Archives of Mass Spectral Data ... 

In the mass spectrum of 10-propyl-3-amino-7,9-dichlorophenothiazine-5-sulfone (67) the molecular ion is quite intense, but the base peak is formed by the loss of Et from the propyl side-chain16. The other primary and secondary fragmentations are summarized in equation (31). The loss of two OH radicals (H2Oz) can occur favorably from the extra configuration of 67 (cf. Section II.D). 

The field desorption mass spectra of 154-158 always showed the ions [M+ 1]+, [M + 2] + and [M + 3] + in addition to the molecular ions which were the base peaks. No fragment peaks were seen in the latter spectra. 

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