Neutral Fragment Masses

For every positively charged fragment ion, at least one neutral fragment is also formed. These fragments are unaffected by the electric and magnetic fields of the mass spectrometer and therefore cannot be studied in the normal ways their structures and properties can only be ascertained by examining the ions with which they 

Fragmentation reactions are often promoted by the formation of certain stable neutral fragments or molecules. The most common of these stable species are shown in Table 2.5. They are discussed in greater detail in the next section. 

Nearly every mass spectral reaction can be placed into one of two major categories 

Simple cleavage is the fragmentation of a single covalent bond to give an ion and a neutral particle. 

Equation 2.18 shows six possible ionic products that can be formed by simple cleavage of bonds in the molecular ion of a tetra-atomic molecule. 

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Neutral-fragment mass spectra of Pb(CH3)4 have been obtained with a double-ionization-chamber ion source. The unimolecular decomposition of excited Pb(CH3)4 was studied and dissociative ionization and appearance energies of the neutral fragments Pb(CH3)n (n = 1, 2, 3) have been determined [42]. 

Figure Bl.7.7. Summary of the other collision based experiments possible with magnetic sector instruments (a) collision-mduced dissociation ionization (CIDI) records the CID mass spectrum of the neutral fragments accompanying imimolecular dissociation (b) charge stripping (CS) of the incident ion beam can be observed (c) charge reversal (CR) requires the ESA polarity to be opposite that of the magnet (d) neutiiralization-reionization (NR) probes the stability of transient neutrals fonned when ions are neutralized by collisions in the first collision cell. Neutrals surviving to be collisionally reionized in the second cell are recorded as recovery ions in the NR mass spectrum.

Characteristic Low-Mass Neutral Fragments from the Molecular Ion... 

Stable ion. An ion that is not sufficiently excited to dissociate into a daughter ion and associated neutral fragments, or to react further in the time frame of the mass spectrometric analysis under stated experimental conditions. 

The behaviour under electron impact of IV- and C-trimethylsilylpyrazoles (mono-, di-and tri-substituted) has been studied by Birkofer et al. (740MS 8)347). Loss of a methyl radical followed by loss of HCN is the most common fragmentation feature of these compounds. When more than one trimethylsilyl group is present, a neutral fragment CaHgSi is expelled. Mass spectrometry of pyrazolium salts has been studied by Larsen etal. (8i OMS377, 830MS52). 

The mass spectra of alcohols often completely lack a peak corresponding to the parent ion. This is due to extremely rapid loss of neutral fragments following initial ionization. For example, the mass spectrum of 2-methyl-2-butanol lacks a parent peak and contains strong peaks at M-15 (loss of CH3O and M-18 (loss of H2O). 

Examine fragment ions to determine the mass of the neutral fragments that were lost from the molecular ion, even though these high-mass peaks may be of low abundances. Compare the neutral loss from the molecular ion with the neutral losses tabulated in Part III to see if these losses agree with the suspected structural type. 

Neutral Loss Only a limited number of neutral fragments of low mass which are eliminated in decompositions of molecular ions. Examples are H, H2, CH3 and OH. Therefore, the presence of a major ion below the molecular ion at an improbable interval (eg, loss of 4 to 14, 21 to 25 amu) will indicate that the latter is not the molecular ion Postulation of Molecular Structures The. postulation of the structure of an unknown molecule is based on several major kinds of general structural information available in the mass spectmm. McLafferty (Ref 63) suggests the following systematic approach ... 

To integrate one must know ts, which of course is a function of tp, and the form of this function depends upon the mechanism assumed for Reaction P. At this point we restrict Reaction P to a hydrogen transfer reaction in which the transferred species may be either a proton, hydrogen atom, or hydride ion and for which the masses of the primary ion, the molecule, the secondary ion, and the neutral fragment are identical and large compared with the transferred hydrogen. Three situations must be considered where the type of collision is defined by the relationship between uP and vQ, the velocities of the primary and secondary ions ... 

The dissociation of gas-phase ions can be a guide to the structure of the ions. There are two ways to dissociate gas-phase ions, either by collision-induced dissociation (CID) sometimes termed collision-activation dissociation (CAD) or by photodissociation. In each method, a mass selected ion is dissociated and the fragment ion (often called a daughter ion) is measured the neutral fragment cannot be experimentally observed. 

Consider an ion mi" decomposing in transit through a field-free region. Its kinetic energy is then distributed among the product ion m.2 and the neutral fragment n according to their relative contribution to the mass of the precursor ion... 

Longevialle, P. Better, R. Electron Impact Mass Spectra of Bifunctional Steroids. The Interaction Between Ionic and Neutral Fragments Derived From die Same Parent Ion. Org. Mass Spectrom. 1983,18, 1-8. 

Longevialle, P. Bolter, R. The Interaction Between Ionic and Neutral Fragments From the Same Par t Ion in the Mass Spectrometer. Int. J. Mass Spectrom. Ion Phys. 1983, 47,179-182. 

Orlov (8a) has compared the dissociative ionization and thermal decomposition of a series of Group IV compounds and discusses the use of the mass spectrometer to study the unstable or nonexistent compounds that are presumably thermolysis intermediates. These include bivalent compounds of Group IV such as silicon(II) derivatives and compounds with multiple bonds. Not only are ions observed important in such correlations, but the mass of the neutral fragment, as confirmed by the decomposition of metastable ions, is of equal importance. 

The dithiazolidine (44) <77CCC2672> and dioxazolidine (45) <69JOC2269> are ring systems in which the fragmentation pattern, has been used to affirm the structures. In the latter case the absence of CO2 primary loss allows differentiation from the isomeric oxadiazolidone (46) co-product. A new technique, e.g. the neutralization-reionization mass spectrometry (NRMS) in combination with... 

The major ions in the spectrum are due to the loss of the neutral fragment water, as in the case of n-butanol 1,4 elimination is probably involved.The base peak is formed via homolytic cleavage next to the OH group followed by proton transfer (Fig. 9.13). The base peak of the mass spectrum is formed as shown in Figure 9.13. 

Constant neutral loss. In this mode, both mass analysers are scanned simultaneously. However, the mass scale of the two analysers is offset by a given mass, which constitutes a neutral fragment (e.g. CO or C2H2). Thus, all ions present in the ion source leading to the loss of the same neutral mass are detected. 

Five peaks are important in the mass spectra of the dialkyl dithioacetals of the deoxy sugars 6 (as well as of the common pentoses and hexoses). These are fragments A, B, and C, the dithioacetal portion [CH(SR)2] resulting from C-l, and the remaining portion of the molecule [M — (CH(SR)j) ]. They result from carbon-carbon bond-cleavage on electron impact, with the production of charged and neutral fragments. 

The major differences in the mass spectra of the deoxy sugar dithioacetals can best be understood in terms of the apparent reluctance of their molecular ions to produce primary radicals (neutral fragments) when the molecules undergo bond cleavage. All of the deoxy sugars studied afford molecular ion peaks, thus allowing the direct determination of the molecular weight, M, from the spectra. 

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