In-beam desorption

Figure 8. Total ion chromatogram, mass chromatograms aud selected scans obtained for In-beam desorption CH4-CI of fucoxanthln.

Note Although desorption chemical ionization being the correct term, [92] DCI is sometimes called direct Cl, direct exposure Cl, in-beam Cl, or even surface ionization in the literature. 

Because extensive fragmentation is typical of El and Cl mass spectra, molecular ions or protonated molecules might not be observed. In order to confirm the molecular weight of a carotenoid, desorption El or desorption Cl (also known as in-beam El and Cl) can be utilized to increase the abundance of the molecular ion species. If the molecular weight of the carotenoid remains uncertain, then softer ionization techniques should be investigated, such as FAB-MS, ESI, MALDI, or APCI. 

In addition to thermal desorption, gas desorption has been found to result from electron, ion and photon bombardment of surfaces. Therefore, simultaneous particle and photon bombardments can be expected to alter desorption rates, as well as the nature and charge distribution of the desorbed species. Furthermore, simultaneous bombardment of a surface by neutrons and ions could affect diffusion processes, e.g., by radiation-induced segregation. In turn, desorption processes can be influenced by altering the diffusion of species from the bulk to the surface. The type, energy, and angular distribution of particles expected to strike neutral beam injector dump areas (such areas can represent 1/9 of total first wall area) can cause synergistic effects on gas desorption which can be quite different from those expected from the interaction of plasma radiations with the first wall. 

Finally, the desorption chemical ionization (DCI) mass spectra of several synthetic nucleosides, mostly in the pyrimidine series, have been recorded. The potential application of protonated or cationized (NH4 ) molecular ions and bases for the detection of these nucleosides has been demonstrated. One of the important features of these spectra is the presence of [ B + H ] and/or [B -f- NHJ ions, which confirms the previously described general fragmentation scheme. In desorption chemical ionization studies (which describe in-beam experiments in a Cl source), the desorption of intact molecules, followed by ionization, is often confused with the desorption of preformed ions. Slow hydrolysis within a Cl source leads to intense ions (e.g., [BH]" ) such as those observed for nucleosides, as well as for intact DNA. ... 

Amoxicillin trihydrate did not give a useful mass spectrum by the conventional electron impact ionisation technique, even with a specialised in beam procedure which gave spectra from several other penicillins [26], Laser desorption [27] and desorption chemical ionisation [28] both gave pseudomolecular ions and the latter technique also gave significant fragmentation. 

The second application is to the direct measurement of adsorption-desorption processes using the Auger peak height of the particular element as a monitor. The principal limitation is the possible influence of the electron beam on the adsorbate, which can result in beam-induced desorption, adsorption or dissociation. The basis of electron-stimulated desorption (ESD) was established some time ago independently be Menzel and Gomer [38] and Redhead [39]. Electron impact causes Franck—Condon transitions of bound electrons in the adsorbed species into excited states. There is, therefore, a probability of dissociation with subsequent desorption of the particular species involved. As an example of these effects on semiconductor surfaces, Joyce and Neave [40] have reported results on silicon, while Ranke and Jacobi [41] have discussed the electron-stimulated oxidation of GaAs. 

In view of the ionization of samples of low volatility, desorption chemical ionization (DCI) is one of the most promising new techniques. The sample is placed on a heatable probe tip or on a field desorption emitter probe and introduced directly into the plasma of a chemical ion source. It was shown that by such an exposure of the sample to the ion plasma, mass spectra can be obtained at much lower temperatures than usually required . This extraordinarily simple sample handling and the apparently more intense and better reproducible ion beam than in field desorption ionization have obviously induced a rapidly growing field of applications of DCI. This development is also supported by a growing number of DCI probe devices available from different manufacturers at relatively reasonable prices. Although many applications, for instance to underivatized peptides , to cyclic adenosine monophosphate, guanosine, and thiophosphoric acid pesticides , as well to creatinine and arginine have been reported, the analytical potential of DCI is not yet exhausted and will obviously result in many new possibilities in analytical research (for recent publications see ). 

Besides probe pyrolysis, a filament pyrolysis technique, usually called Desorption Chemical Ionization (DCI) is also used. The polymeric material is deposited on a filament, which is rapidly heated to the degradation temperature. The pyrolysis occms very close to the electron beam, and therefore EXZI is often referred to as an "in-beam" pyrolysis. ... 

Negative ion in-beam c.i.-m.s. of several oligosaccharides has been shown to correspond to flash desorption c.i.-m.s. It gave not only the quasl-molecular ion of stachyose but also cluster... 

Though these contributions to the surface bond will not be considered in this review, one should note that these terms may become important when interactions are weak. They may be of relevance for so-called precursor states that are shortlived and are observed in surface molecular beam experiments [7] or postulated in thermal desorption experiments [8]. 

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