Daughter ion fragmentation

The mass spectrometry portion of the analysis was carried out by coupling a Bruker Esquire ion-trap mass spectrometer to the LC-NMR system with a 20 1 splitter. The major portion of the flow was directed to the NMR system while the minor fraction went to the mass spectrometer. The system was plumbed such that the sample reached the mass spectrometer and the UV detector at the same time. In this configuration, it is possible to use the mass spectrometer as an intelligent detector, thus allowing stop-flow experiments to be initiated on the basis of observed molecular ions or daughter ion fragments. Data were acquired with electro-spray ionization (ESI) in the positive-ion mode. 

One way to carry out MS-MS is to link two or more mass analyzers in series to produce an instrument capable of selecting a single ion, and examining how that ion (either a parent or daughter ion) fragments. For instance, three quadrupoles can be linked (a so called triple quad ) to produce a tandem mass spectrometer. In this arrangement, the first quadrupole selects a specific ion for further analysis, the second... 

Fixed-product ion scans (sector instruments). High-voltage scan or linked scan at constant B /E. Both techniques give a spectrum of all precursor (parent) ions that fragment to yield a preselected product (daughter) ion. 

Constant neutral loss (or fixed neutral fragment) scans. The linked scan at constant B[1 -(E/Eg)] /E gives a spectrum of all product (daughter) ions that have been formed by loss of a preselected neutral fragment from any precursor (parent) ions. 

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. 

Parent ion/precursor ion An ion that undergoes fragmentation to produce a daughter ion and a neutral product. 

Product ion/daughter ion An ion related to a precursor or parent ion by a process such as fragmentation. For example, a parent ion fragments to produce a daughter ion. 

The question at this point was whether modifications could be made to the oxadiazole molecule to enhance metabolic stability and achieve comparable activity. This approach required knowledge of the site of metabolism and the nature of the metabolic products. This information was obtained from ion mass spectrometry. The identity of these products was determined by comparing the fragmentation pattern of metabolites A and B with the parent compound and the corresponding daughter ions (Fig. 25). 

Aromatic amines can be quantified by HPLC-MS either as their (M+H)+ ion or by further fragmentation into specific daughter ions (LC-MS-MS). Figure 39... 

Similar experiments on a large number of transition metal carbonyls have shown that this process favors dissociation to and detection of metal clusters or atoms. Since most metal-(CO)n photofragments are themselves subject to efficient dissociation, MPI experiments do not identify the primary photoproducts. This situation contrasts sharply with electron impact ionization where the parent ion is usually formed and daughter ions are seen as a result of parent ion fragmentation. Figure 4 shows the electron impact mass spectrum of Mn2(C0) Q (33). for comparison with the MPI mass spectrum of Figure 3. 

Product Ion An ion formed as the product of a reaction involving a particular precursor ion. The reaction can be unimolecular dissociation to form fragment ions, an ion/ molecule reaction, or simply involve a change in the number of charges. The terms fragment ion and daughter ion are deprecated (but still very much in use). 

Various instruments allow working in special regimes to detect only metastable ions (MI spectra). The conditions of experiments in this case are the same as for the MS/MS experiments, but without collision activation. Any sort of spectrum (daughter ions, parent ions, constant neutral losses) may be generated this way. These spectra are used to establish the pathways of fragmentation, to resolve structural problems. However, the abundance of the metastable signals and even their presence or absence in the spectrum depends on the energy of the parent ions. Therefore, in contrast to CID (see Chapter 3) spectra the difference in MI spectra of two parent ions does not confirm their different structures. 

It is known that in the vast majority of cases the activation energy E,. of the reverse reaction is very small or even negligible. Using Hammond s postulate [3], it is possible to assume that in the case of endothermic fragmentation the transition state will be much closer to the products than to the initial particle (Fig. 5.14). Thus, the stability of the products influences significantly the efficiency of fragmentation. It is important to consider stability of both products a neutral and a daughter ion. 

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. 

Fig. 2.6.7. APCI-FIA-MS-MS(+) daughter ion mass spectrum of [M + NH4]+ ion of octylcyclohexylethoxylate homologue at m/z 582 from reduced octylphenolethoxylate mixture (CgHi7-C6Hio-0-(CH2-CH2-0)m-H) (inset) fragmentation scheme under...

Daughter ion mass spectrum of the compound is shown in Figure 33. A dominant daughter ion was observed at m/z of 259.07757. Observation of this ion conformed to the likely fragmentation pattern expected for the Famotidine molecule. Scheme 1 illustrates the fragmentation pathway leading to the formation of this dominant daughter ion. 

The following examples illustrate the range of apphcations of LC/MS for trace analysis of explosives ESI-LC/MS/MS-CID fragmentation processes of a series of nitroaromatic, nitramine and nitrate ester explosives were studied in the negative-ion mode using daughter-ion, parent-ion and neutral loss scans [14]. Table 1 shows the CID daughter ions in ESI-MS/MS of TNT. 

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