Ionization-fragmentation process

The decay of benzene from the S2 state under collision-free condition has also been studied. J. P. Reilly and co-worker studied the nanosecond UV laser induced multiphoton ionization/fragmentation processes. The rate equation model was used for the simulation and the lifetime of the second excited singlet state was estimated to be 20 ps.19 More recently the... 

Hsu, F.F. and Turk, J. (2001) Studies on phosphatidylglycerol with triple quadrupole tandem mass spectrometry with electrospray ionization fragmentation processes and structural characterization. J. Am. Soc. Mass Spectrom. 12,1036-1043. 

The possible mechanism of ionization, fragmentation of studied compound as well as their desoi ption by laser radiation is discussed. It is shown that the formation of analyte ions is a result of a multi stage complex process included surface activation by laser irradiation, the adsoi ption of neutral analyte and proton donor molecules, the chemical reaction on the surface with proton or electron transfer, production of charged complexes bonded with the surface and finally laser desoi ption of such preformed molecules. 

ToF-SIMS mass spectra are generally obtained for m/z ratio from 1 to 1000 or 2000. However, due to a high fragmentation process, ToF-SIMS mass spectra are generally very complex and the fragmentation/ionization processes are very different compared with other mass spectro-metric techniques. The main peaks are generally low mass fragments (m/z < 100). 

If the energy of the ionizing electrons is 13 eV (to minimize the secondary fragmentation processes), the intensities of the primary ions peaks with rn/z values 129, 115, and 101 will be 78%, 85%, and 100%, correspondingly. Theoretically calculated intensities should be (101/129) = 0.78 (78%) and (101/115) = 0.88 (88%). As one can see, the resulting values are fairly close to the theoretical ones. The estimation of the intensity of an ion peak resulting due to the loss of methyl radical has a value two to three times higher than the experimental one. 

The production of ions from neutral compounds and the examination of how these ions subsequently fragment is fundamental to mass spectrometry. Neutral sample molecules can be ionized by a variety of processes. The most important of these for the production of positively charged species is the removal of an electron or the addition of one or more protons to give either molecular ions (M+ ) or protonated molecular species (M+nH)"+. This initial stage of ionization is often followed by fragmentation to produce ionized fragments, fragment ions . 

Since the methods outlined permit determination of the origin of ions encountered it is clear that the technique should represent a powerful tool for the study of fragmentation processes associated with adsorption. By a felicitous accident it is just those relatively weak chemisorption processes which are most readily studied by field ionization that are of greatest catalytic interest. 

With increasing energy of the incident photons the photoionization process is accompanied by a rupture of valence bonds, leading to various ionized fragments, the identification of which requires complementary methods of analysis. During the last decade much progress has been achieved in the mass spectrometry of the photoionization products in various diatomic or polyatomic gases under vacuum u.v. irradiation. 

Before abandoning the idea, however, the work of Dejongh et al. was repeated. A lower ionization voltage was used to limit secondary fragmentation processes and hopefully to accentuate stereochemical differences. The 17 ev mass spectra of the pertrimethylsilylated derivatives of < - and / -D-glucopyranose are shown in Figure 6. The two spectra are very similar however, intensity differences do occur. In particular, attention has been focused on the ratio of the two peaks with m/e values of 435 and 393. The m/e 435 peak corresponds to the parent molecular ion (PMI) with loss of CH3 and TMSiOH, the 393 peak to the shard A minus CH3 (57). 

In the fragmentation of W(CO)6, competition can occur between different ionization and fragmentation processes, as shown by analysis of the electron-impact ionization efficiency curves (61). The ion W(CO)f can be formed by... 

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