Quasimolecular ion

In conclusion, SSIMS spectra provide not only evidence of all the elements present, but also detailed insight into molecular composition. Quasimolecular ions can be desorbed intact up to 15000 amu, depending on the particular molecule [3.17] and on whether an effective mechanism of ionization is present. Larger molecules can be identified from fragment peak patterns which are characteristic of the particular molecules. If the identity of the material being analyzed is completely unknown, spectral interpretation can be accomplished by comparing the major peaks in the spectrum with those in a library of standard spectra. 

Fig. 4.7. The molecular ion signal of Ceo at m/z 720 (a) and the quasimolecular ion signal of bovine insulin, m/z 5734.6, (b) as obtained on a TOF instrument in linear mode (left) and reflector mode (right). All other experimental parameters remained unchanged.

Note It is commonplace to denote [Mh-H]" and [M-H]" ions as quasimolecular ions because these ions comprise the otherwise intact analyte molecule and are detected instead of a molecular ion when Cl or other soft ionization methods are employed. Usually, the term is also applied to [M-i-alkali] ions created by other soft ionization methods. 

Fig. 7.7. Comparison of (a) 70 eV El and (b) isobutane-d spectrum of glycerol. Instead of a molecular ion, an [M+H] quasimolecular ion is observed in El mode, too. In addition to [M+H], the Cl spectrum shows few fragment ions and a weak [2M+H] cluster ion signal.

Sometimes, FI mass spectra show signals due to reactions of the analyte with the emitter surface or between molecules adsorbed to that surface. In case of acetone for example, it was demonstrated that [M+H]" quasimolecular ions are produced mainly by a field-induced proton-transfer reaction in the physically adsorbed layer. [59] The mechanism of this field-induced reaction depends on the existence of tautomeric structures of the neutral molecule. Besides the [M+H] quasimolecular ions, [M-H] radicals are formed ... 

Note Analytes possessing exchangeable hydrogens also tend to form [Mh-H]" ions in FI-MS. Occasionally, the quasimolecular ion occurs in favor of the molecular ion that can be of lower intensity or almost be absent (Fig. 8.13). Criteria to distinguish from [Mh-H]" ions have been published. [60]... 

Example ED from untreated wire emitters in the presence of intentionally added alkali metal salts was used to obtain mass spectra of tartaric acid, arginine, pentobarbital and other compounds. [78,80] Besides [Mh-H]" quasimolecular ions, m/z 175, the FD mass spectrum of arginine exhibits [M+Na], m/z 197, and [Mh-K]", m/z 213, ions due to alkali metal cationization as well as [2M-i-H], m/z 349, cluster ions (Fig. 8.14). [37]... 

In FD mode, the sensitivity (Chap. 5.2.4) of actual magnetic sector instruments is about 4 X 10 C pg for the quasimolecular ion of cholesterol, m/z 387, at R = 1000. This is 10" times less than specified for those instruments in El mode and 10 times less than for Cl mode. 

The FAB plasma provides conditions that allow to ionize molecules by either loss or addition of an electron to form positive molecular ions, M" , [52,89] or negative molecular ions, M, respectively. Alternatively, protonation or deprotonation may result in [Mh-H]" or [M-H] quasimolecular ions. Their occurrence is determined by the respective basicity or acidity of analyte and matrix. Cationization, preferably with alkali metal ions, is also frequently observed. Often, [Mh-H]" ions are accompanied by [MH-Na]" and [Mh-K]" ions as already noted with FD-MS (Chap. 8.5.7). Furthermore, it is not unusual to observe and [Mh-H]" ions in the same FAB spectmm. [52] In case of simple aromatic amines, for example, the peak intensity ratio M 7[Mh-H] increases as the ionization energy of the substrate decreases, whereas 4-substituted benzophenones show preferential formation of [Mh-H]" ions, regardless of the nature of the substituents. [90] It can be assumed that protonation is initiated when the benzophenone carbonyl groups form hydrogen bonds with the matrix. 

Example The FAB-CID-MS/MS spectrum of thymosin-Tl [M+H] quasimolecular ions, m/z 1427.7, as obtained from a magnetic four-sector instrument [144] shows numerous fragment ions due to A-terminal, C-terminal, and internal fragmentations (Fig. 9.18). [145]... 

Californium plasma desorption ( Cf-PD) dates back to 1973 [4-6,22,154-156] and was the first method to yield quasimolecular ions of bovine insulin. [157] Practically, Cf-PD served for protein characterization, a field of application which is now almost fully transferred to MALDI or ESI (Chaps. 10,11). [158]... 

Example The Cf-PD mass spectrum of bovine insulin exhibits the [M+H]" quasimolecular ion as well as the doubly charged [M+2H] and triply charged [M+3H] ion (Fig. 9.20). [156] Fragment ions corresponding to the A and B chain as well as some a-type peptide fragments ions are observed in addition. 

MALDI is the method of choice for the analysis of synthetic polymers because it usually provides solely intact and singly charged [62] quasimolecular ions over an essentially unlimited mass range. [22,23] While polar polymers such as poly(methylmethacrylate) (PMMA), [83,120] polyethylene glycol (PEG), [120,121] and others [79,122,123] readily form [M+H] or [M+alkali] ions, nonpolar polymers like polystyrene (PS) [99,100,105,106] or non-functionalized polymers like polyethylene (PE) [102,103] can only be cationized by transition metal ions in their l-t oxidation state. [99,100] The formation of evenly spaced oligomer ion series can also be employed to establish an internal mass calibration of a spectrum. [122]... 

Note Carbohydrates possess a high affinity towards alkali metal ions, and thus in MALDI spectra [M-tNa] and/or [M+K] are normally observed instead of or in addition to [Mh-H]" ions of very low abundance. Radical ions are not observed. It basically depends on the relative amount of alkali ion impurities or dopant which quasimolecular ion will be dominant. 

These Cl [M + H]+ ions (quasimolecular ions) are often prominent. Chemical ionization spectra sometimes have prominent [M — H]+ ions because of hydride ion abstraction from the M,+ ion by CH5+. Since the [M + H]+ ions are chemically produced, they do not have the great excess of energy associated with ionization by electron impact, and they undergo less fragmentation. For example, the El spectrum of 3,4-dimethox-yacetophenone shows, in addition to the molecular ion at m/z 180, 49 fragment peaks in the range of mJz 40-167 these include the base peak at m/z 165 and prominent peaks at m/z 137 and m/z 77. The CH4 induced Cl spectrum shows the quasimolecular ion (M + H+, m/z 181) as the base peak (100%), and virtually the only other peaks, each of just a few percent intensity, are the... 

One general statement may be made If a molecule MX (X is a functional group) is protonated by the reagent ion to give the quasimolecular ion MXH+, fragmentation usually produces the neutral protonated functional group XH and the fragment ion M+. 

Field Desorption (FD) Stable molecular ions are obtained from a sample of low volatility, which is placed on the anode of a pair of electrodes, between which there is an intense electric field. Desorption occurs, and molecular and quasimolecular ions are produced with insufficient internal energy for extensive fragmentation. Usually the major peak represents the [M + H]+ ion. 

It should be kept in mind that the fragmentation rules above apply to El mass spectrometry. Since other ionizing (Cl, etc.) techniques often produce molecular ions with much lower energy or quasimolecular ions with very different fragmentation patterns, different rules govern the fragmentation of these molecular ions. 

The El spectra of amino acids (Fig. 2.17a) or their esters give weak or nonexistent molecular ion peaks, but Cl and FD (Fig. 2.11b and c) give either molecular or quasimolecular ion peaks. The weak molecular ions in the El spectra arise since amino acids easily lose their carboxyl group and the esters easily lose their carboal-koxyl group upon electron impact. 

Since both ESP and ISP produce quasimolecular ions, more sophisticated techniques, such as LC-MS-MS are required to obtain diagnostic fragment ions and, thus, analyte structure elucidation (117, 118). Identification can often be achieved by using daughter ion MS-MS scans and collisionally induced dissociation (CID), most commonly on a triple quadrupole MS in this way, dissociation of the quasimolecular ion occurs and diagnostic structural information can be obtained (119). 

C6H5COOH)+, with m/z 232, occurs with high intensity. Elimination of the second benzoyl group creates33 an ion with m/z 127. The fragmentation behavior of isosorbide 5-nitrate (11) is well established, as characteristic fragments34 occur (e.i. method see Scheme 3). The c.i. mass spectrum of this compound also contains34 a quasimolecular ion (M + 1) with m/z 192. The... 

Extensive studies involving 57 aryl-substituted 4-hydroxycoumarins and using electrospray ionization (ESI) mass spectrometry have allowed the effect of the substituents in the aromatic ring on the fragmentation patterns to be determined. Deuterated compounds were used to prove some of the proposed fragmentation pathways, and the effect of tautomerism on the formation of quasimolecular ions and subsequent fragmentation was explained <2004EJS523>. 

Ion of the molecular species ion originating from the molecule by the abstraction of a proton [M - H] or by a hydride abstraction [M - H]+, or by interaction with a proton or a cation to form a protonated molecule [M + H]+ or a cationized molecule [M + Cat]+. These ions allow the molecular weight to be deduced. The ambiguous and obsolete terms quasimolecular ion or pseudomolecular ion should be avoided. 

Ion of the molecular species ion resulting from the ionization of a molecule by the addition or removal of a proton or an hydride, or by the formation of an adduct with another ion, such as Na+, Cl-, acetate, and so on. They allow the molecular mass to be deduced. In the past the terms pseudomolecular ion or quasimolecular ion have been used but should be avoided. 

Sample ionization. Requirements for sample ionization are much more severe in MS/MS than in GC/MS. For MS/MS, the ionization method should create one ion for each component, and the structure of the ion should be the same as that of the neutral surrogate. Electron ionization usually does not fulfill these requirements, since the ions formed often include those from rearrangement reactions, and the degree of fragmentation is excessive. Chemical ionization provides the requisite single ion for each component of the matrix in the form of the quasimolecular ion (MFH)+. 

In addition, the terms pseudomolecular ion and quasimolecular ion are frequently applied to indicated cationized or anionized molecules. Although the second one is actually recommended by the lUPAC [19], it will not be used in this text. Further information on nomenclature issues can be foimd elsewhere [19-21]. 

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