Ion dissociation techniques

The mass spectral fragmentations of 9,10-dimethoxy-2,3,4,6,7,ll/)-hexa-hydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140 and -2,4-diones 141, under electron ionization (at 70 eV) were examined by metastable ion analysis, a collosion-induced dissociation technique and exact mass measurement (97RCM1879). Methyl substituent on N(3) in 140 (R = Me) had a larger effect on both the fragmentation and on the peak intensities, than a methyl substituent on C(6) (R = Me). The ionized molecules of 140 (R = H) were rather stable, whereas 4-phenyl substitution on C(4) of 140 (R = Ph) promoted the fragmentations of the molecular ions. The hexahydro-1//-pyrimido[6,l-n]isoquinoline-2,4-diones 141 were more stable, than the hexahydro-l//-pyrimido[6,l-n]isoquinolin-2-ones 140, and the molecular ions formed base peaks. 

The energy imparted to the ions depends on the energy of the rf pulse and the duration of the pulse. The energy does not have to be raised in one event but may be absorbed by the ion in small portions. A technique called sustained off-resonance excitation (SORT) (82) uses a low-amplitude rf pulse that is off-resonance to the ion cyclotron frequency. The difference of the cyclotron frequency and the excitation frequency (-500 Hz) causes the ion to experience in- and out-of-phase excitation that has the effect of a repeated expansion and shrinkage of the cyclotron orbit. In this process, the ion undergoes a large number of low-energy collisions and the Ecom slowly increases until the ion dissociates. 

During the last decade knowledge of the ion chemistry of nitro compounds in the gas phase has increased significantly, partly due to the more widespread use of specialized techniques. Thus various ionization methods, in particular electron impact ionization and chemical ionization, have been used extensively. In addition, structure investigations as well as studies on fragmentation pathways have involved metastable ion dissociations, collision activation and neutralization/reionization studies, supplementary to studies carried out in order to disclose the associated reaction energetics and reaction dynamics. In general, the application of stable isotopes plays a crucial role in the in-depth elucidation of the reaction mechanisms. 

Tandem mass spectrometry or MS/MS summarizes the numerous techniques where mass-selected ions (MSI) are subjected to a second mass spectrometric analysis (MS2). [53,54] Ion dissociations in transit through the mass analyzer may either occur spontaneously metastable. Chaps. 2.7.1, 2.8.2) or can result from intentionally supplied additional activation, i.e., typically from collisions with neutrals. The methods to activate or react otherwise stable ions in he field-free region (FFR) between MSI and MS2 are discussed in Chap. 2.12 and applications of MS/MS on TOF instruments are shown in Chaps. 10-12. 

Thermochemical information about neutral species can also be obtained from measurements of ions. Indeed, accurate bond dissociation energies for neutral molecules have been obtained from gas-phase ion chemistry techniques. In this section, we will summarize both the negative-ion and hydride-affinity cycles involving silicon hydrides (RsSiH) which are connected to electron affinity (EA) and ionization potential (IP) of silyl radicals, respectively [22-24]. 

At the higher pressures of other ion-molecule techniques, such as flowing afterglow or pulsed high-pressure mass spectrometry," both of which operate with a bath gas pressure of about 1 torr, collisions of such an excited intermediate with the bath gas occur on a nanosecond to microsecond time-scale, in competition with the unimolecular dissociation rate. For these techniques, ions that are the... 

K. Sannes-Lowery, R.H. Griffey, G.H. Krappa, J.P. Spier, S.A. T ofA.aA e.r, Multipole storage assisted dissociation, a novel in-source dissociation technique for ESI generated ions. Rapid Commun. Mass Spectrom., 12 (1998) 1957. 

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