Mass spectrometry daughter ions

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). 

Instrnments combining several analyzers in sequential order are very common. This combination allows mass spectrometry and mass spectrometry experiments (MS/MS) to be carried out. Modern MS/MS includes many different experiments designed to generate substructural information or to qnantitate componnds at trace levels. A triple quadru-pole mass spectrometer allows one to obtain a daughter ion mass spec-trnm resnlting from the decomposition of a parent ion selected in the first qnadrnpole. The MS/MS experiments using an FTICR or ion trap, however, are carried ont in a time-resolved manner rather than by spatial resolntion. 

The extension of an LC-NMR system to include mass spectrometry has been in application for several years [35,36]. By directly coupling a mass spectrometer to an LC-NMR system it is possible to obtain valuable mass spectral data. Configuring the system to have the sample reach the mass spectrometer before it reaches the NMR flow-cell enables the mass spectrometer to be employed as an experimental control device for analysing complex mixtures. Mass spectrometry is an ideal detector, provided that the molecules of interest are ionizable. It provides data rapidly and can thus yield valuable information on parent or daughter ion masses prior to initiating time-consuming NMR experiments. This synergy is not possible when the instruments are not directly coupled. 

Tandem mass spectrometry (i.e., MS-MS) is another technique that has recently become popular for the direct analysis of individual molecular markers in complex organic mixtures [87,505,509,578 - 583]. This technique provides a rapid method for the direct analysis of specific classes of molecular markers in whole sample extracts. In this approach the system is set up to monitor the parent ions responsible for a specific daughter ion as described above and the distribution of parent ions obtained under these conditions should provide the same information as previously obtained by GC-MS [505, 582]. Even greater specificity can be achieved by a combination of GC-MS-MS [516,584]. In view of the complexity of COM samples and the need to detect the presence of individual organic compounds or classes of compounds, it would seem that MS-MS, especially coupled with GC, would be extremely valuable in future environmental organic geochemistry studies. 

As with the Sm- Nd and Re- Os systems, careful chemistry is required to cleanly separate the parent and daughter elements because mass spectrometry cannot resolve 176Lu from 176Hf. The ion-exchange chemistry is similar to that for samarium-neodymium. In fact, fractions of samarium, neodymium, lutetium and hafnium are often produced in a single procedure. Mass spectrometry is done by ICPMS because this is the only method that effectively ionizes hafnium. 

A critical review of the inherent limitations of modern PFA analytical methods is available in Martin et al. [18], but some examples are discussed here. For quantitative determination, the HPLC system is often interfaced to a Micromass or Sciex tandem mass spectrometer operated in the negative ion electrospray mode. Instrumental parameters are optimized to transmit the [M - H] ion for all analytes (Table 5). When possible, multiple daughter ions are monitored, but quantitation is generally based on a single product ion (Table 5, Fig. 4). In the electrospray tandem mass spectrometry (ES MS/MS) system, the 499 Da -> 80 Da transition can provide a stronger signal than... 

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. 

Tandem mass spectrometry (MS/MS) is a new analytical technique applied to problems in food and flavor analyses. Rapidity of analysis, a high discrimination against chemical noise, and the ability to analyze mixtures for functional groups are attributes of MS/MS that make it attractive for such problems. Sanple collection and pretreatment differ frcm methods used in GC/MS. Correct choice of an ionization method is paramount. Daughter ion MS/MS spectra are used for conpound identification via comparison with those of authentic compounds, and parent and neutral loss spectra are useful in functional group analysis. Applications to direct analysis of volatiles emitted from fruits and to spice analyses are considered. 

MS/MS is an empirical method of analysis. As is evident from the exanples presented, the interpretation of a daughter ion MS/MS spectrum is often based on the same correlation principles derived frcm electron and chemical ionization mass spectrometry. More often, the comparison of the spectrum obtained to that of the authentic capound is used for identification. This is a fundamentally unsatisfying procedure. While electron and chemical ionization spectra can be compared to a spectral library which has been compiled over the past thirty years, no comparable library of MS/MS spectra exists. Data systems may be used within individual... 

Tandem mass spectrometry (MS/MS) Involves three main stages (i) selection of parent (precursor) ion (ii) fragmentation of parent ion hy so-called collision-induced dissociation (CID) and (iii) analysis of fragmented daughter (product) ions. MS/MS uses mass analysers in series, hence the name tandem mass spectrometry. 

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