Base peak, in mass spectrometry

Fig. 12.5 Mass numbers and abundances of CCI4, the base peak in mass spectrometry...

BP base peak (in mass spectrometry) benzo[a]pyrene... 

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

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... 

Figure 5.55 MS" spectra from rntz 329, the (M + H)+ ion of an hydroxy metabolite of Praziquantel, and the base peaks in subsequent product-ion spectra. Reprinted from J. Chromatogr., B, 708, Lerch, C. and Blaschke, G., Investigation of the stereoselective metabolism of Praziquantel after incubation with rat liver microsomes by capillary electrophoresis and liquid chromatography-mass spectrometry , 267-275, Copyright (1998), with permission from Elsevier Science.

Mass spectrometry is based upon the separation of charged ionic species by their mass-to-charge ratio, m/z. Within the general chemical context however, we are not used to taking into concern the isotopes of the elemental species involved in a reaction. The molecular mass of tribromomethane, CHBrs, would therefore be calculated to 252.73 g mol using the relative atomic masses of the elements as listed in most periodic tables. In mass spectrometry we have to leave this custom behind. Because the mass spectrometer does not separate by elements but by isotopic mass, there is no signal at m/z 252.73 in the mass spectmm of tribromomethane. Instead, major peaks are present at m/z 250, 252, 254 and 256 accompanied by some minor others. 

Presently, FAB-MS spectra are routinely used to characterize synthetic tyrosine O-sulfate peptides.152,57,63-671 Since partial hydrolysis of the sulfate ester occurs in the gas phase, quantification of the tyrosine O-sulfate residue by mass spectrometry is not possible, but combined with one-peak assignment in HPLC, FAB-MS represents a powerful analytical tool. On the other hand, partial hydrolysis in the gas phase excludes the presence of sul-fonated species which should be perfectly stable. In early studies the presence of such species were excluded by quantitative recovery of tyrosine upon acid hydrolysis or upon hydrolysis with arylsulfatase.1361 Recently, even MALDI-TOF-MS spectra of CCK-peptides1441 and of conotoxins a-PnIA and a-PnlB 138 were reported which show that in the positive-ion mode the [M + H-S03]+ ions represent the base peaks, while in the negative-ion mode, [M-H]-ions consistently correspond to the base peaks. In the CCK peptides intramolecular salt bridging of the sulfate hemi-ester with proximal positive charges of arginine or lysine side chains was found to reduce the extent of hydrolysis in the gas phase significantly.144,1491... 

The metabolism of ticlopidine, 45, has been investigated by mass spectral analysis. The principal metabolic pathways can be proposed based on supporting mass spectrometry (MS) fragmentation patterns. Chlorotropylium ion 46 (mlz= 125) is the base peak in the product ion mass spectrum of ticlopidine <2004MI49>. 

Mass spectrometry has also been used to characterize organostannoxanes of the type Me2Sn(Cl)0C(0)CF3 and their products of hydrolysis85. The base peak in all cases has been tentatively assigned to the CF30Sn+ ion but the mechanism leading to this rearrangement has not been explored in detail. 

Scaling to a base peak. In some forms of measurement, such as mass spectrometry (e.g. LC-MS or GC-MS), it is possible to select a base peak and scale to this for... 

The atomic masses used in common chemical calculations are based on averages resulting from mixtures of isotopes. In mass spectrometry, the calculation is based on the mass of the predominant isotope of each element. As the isotopes are separated in the spectrometer, we always face several peaks with different masses, and with intensity ratios defined as described earlier. Thus, for example, dichloromethane has a classical molecular mass equal to 12.01 + 2 x 1.00 + 2 x 35.45 = 84.91 Da. The molecular mass in mass spectrometry is (if mass defects are neglected) 12 + 2 + 2 x 35 = 84 u. Several isotopic peaks are observed in the spectrum, the second most important being observed at m/z 86 with an intensity equal to 64.8 % of that of the m/z 84 peak. 

Liquid chromatography-mass spectrometry (LC-MS) ideally combines advantages of the gentle separation of HPLC with high sensitivity and selectivity of mass spectrometry. There are two methods of ionization. The electrospray LC-MS ionization process can effectively transform the heterocychc amines from solution to protonated ions in the gas phase. The thermospray LC-MS ionization process produces abundant pseudo-molecular ions for this class of compounds. As a result, in both ionization processes the base peaks in the mass spectra are... 

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