Mass spectrometry molecular ion

Poriferasterol (4-B) synthesized in the presence of [CDsJmethionine exhibited on mass spectrometry molecular ions with m e = -f-1, -1-2, and +4 no ions with m e = -1-3 or +5 were noted, as they would have been if the unsaturated methylene and ethylidene intermediates had not been involved (Smith eta/., 1966 Lederer, 1969). 

Grdgoire, D.C., Sturgeon, R.E. (1993) Background spectral features in electrothermal vaporization inductively coupled plasma mass spectrometry molecular ions result-... 

Examples of mass spectra of derivatives of 3,6-, 3,7- and 3,12-dihydroxy-cholanoates are shown in Figs. 4 and 5. At the temperatures used in gas chromatography-mass spectrometry, molecular ions are seen only with the free hydroxy compounds and the trimethylsilyl ether derivatives (intensity usually less than one percent of base peak). Molecular ions with a relative intensity of 5-10% are obtained from the methyl esters or acids when a direct probe is used (14, 15). Loss of 2 and 4 mass units from the molecular ions of compounds with free hydroxyl groups is most probably thermal and is not seen at low temperatures. 

Fragmentation pattern (Section 13 22) In mass spectrometry the ions produced by dissociation of the molecular ion... 

Bushnell and co-workers [117] employed extensive molecular modelling to understand the nature of cis and trans isomerism in tetrahedral p-phenylene vinylene oligomers, and to aid the interpretation of time of flight mass spectrometry and ion mobility studies. Molecules such as T4R, shown in Figure 18, with four equivalent arms can be used to control the crystallinity in thin films. The authors reported the observation of a species in the mass spectrum resulting from the loss of an arm from the central carbon. This species will be referred to as P4R. 

Most nitrile oxides are unstable, some of them are explosive. This fact hinders the study of their physical properties. Nevertheless, there are a number of publications concerning not only stable but also unstable nitrile oxides. In particular, mass spectral data for nitrile oxides among other unstable compounds containing an N+-X bond are summarized in a review (9). In such studies, the molecular ions must be generated using indirect procedures, including dissociative electron ionization, online flash-vacuum pyrolysis mass spectrometry, or ion-molecular reactions. Their characterization is mainly based on collisional activation and ion-molecular reactions. 

Figure 2.8. Schematic of a sputtering event in secondary ion mass spectrometry (SIMS). Mainly neutral species are ejected, but also some positively charged and negatively charged ions. For samples containing analyte with relatively low masses, intact molecular ions can be desorbed. The greater portion of ejected compounds is, however, fragments.

Mass analysis is a relatively simple technique, with the number of ions detected being directly proportional to the number of ions introduced into the mass spectrometer from the ion source. In atomic mass spectrometry the ion source produces atomic ions (rather than the molecular ions formed for qualitative organic analysis) which are proportional to the concentration of the element in the original sample. It was Gray who first recognized that the inductively coupled plasma would make an ideal ion source for atomic mass spectrometry and, in parallel with Fassel and Honk, and Douglas and French developed the ion sampling interface necessary to couple an atmospheric pressure plasma with a mass spectrometer under vacuum. 

Most analytical studies using FT-ICR mass spectrometry, where ions have been produced inside (or just outside) the analyzer cell, have used lasers as ionization sources. Other than some very limited Cs secondary ion mass spectrometry (SIMS) studies [77], most research utilized direct laser desorption to form various organic [78] and inorganic [79] ions, including metal [80] and semiconductor [81] (including carbon) clusters. More recently matrix assisted laser desorption ionization (MALDI) has been used to form ions of high molecular weight from polymers [82] and many classes of biomolecules [83]. 

Often soft ionization techniqnes such as fast-atom bombardment (FAB) and matrix-assisted laser desorption (MALDI) greatly increase the utility of mass spectrometry in the analysis of fluorinated compounds. Using a commercial matrix-assisted laser desorption time-of-flight mass spectrometer, molecular ions of a polyelectrolyte,... 

The molecular formula of calystegine A, (la,2p,3a-trihydroxynortropane) was first established as CtHhNOj, 139.0900, by high resolution El mass spectrometry. Ammonia ion CIMS... 

Fig. 51. A RE capacitively-coupled diode with a variety of plasma diagnostics, including Langmuir probe, optical emission spectroscopy, molecular beam mass spectrometry, and ion flux/energy analysis systems. After [35).

Organophosphorus Chemistry series regularly lists new mass spectra in the Physical Methods chapter . With this in mind, an approach which considers fundamental aspects of organophosphorus ions (i.e. structure and reactivity) in the gas phase has been adopted. The gas-phase structure and reactivity of ions can be probed via several different techniques, including thermochemical measurements, kinetic energy release of metastable ions, collisional activation mass spectrometry, neutralization reionization mass spectrometry and ion-molecule reactions. An example is the molecule HCP (Table 1) its ionization potentiaP, proton affinity and the IR and rotational spectroscopy of the HCP ion " have all been determined in the gas phase. Another important tool for understanding the structure and reactivity of gas phase ions is ab initio molecular orbital theory. With advances in computational hardware and software, it is now possible to carry out high-level ab initio calculations on smaller systems. Indeed, the interplay between experiment and theory has fuelled many studies ... 

A nondestructive detection technique is often desirable. One such technique is based on the excitation of motional resonances of trapped species. The information about the species is encoded in the frequency of the resonance. This technique has been used in the past for mass spectrometry of ion clouds in the gas or fluid state [29,69]. For a two-ion crystal containing a single molecular ion sympathetically cooled by a single atomic ion, a high mass resolution has been demonstrated [31]. 

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