Beam techniques mass-spectrometric

The ability to cool (and eventually liquefy) gases by adiabatic expansion underlies industrial gas liquefaction processes. Adiabatic cooling of gaseous nozzle-jet expansions is also an important technique in modem molecular beam and mass spectrometric research. Thermodynamicist John Fenn, winner of the 2002 Nobel Prize in Chemistry, pioneered many of the techniques of adiabatic nozzle-beam cooling. 

Guided ion-beam tandem mass spectrometric techniques have been used to measure reaction threshold energies for proton transfer in the endoergic reactions... 

Table 7.83 lists the main characteristics of TLC-FAB-MS/LSIMS. A key difference between EI/CI and FAB/LSIMS/LD is the fact that sampling in FAB and LSIMS is from a specified location that corresponds to the impact footprint of the primary particle beam. The natural compatibility of FAB, LSIMS and LD with the direct mass-spectrometric analysis of TLC plates is readily apparent. Most mass-spectrometric measurements are destructive in nature, but FAB and LSIMS are surface-sensitive techniques in which the material actually consumed in the analysis is sputtered only from the top few microns of the sample spot. The underlying bulk is not affected, and can be used for further probing. The major limitation of TLC-FAB depends on the capability of the compounds to produce a good spectrum. 

Undoubtedly, the technique most suited to tackle polyatomic multichannel reactions is the crossed molecular beam (CMB) scattering technique with mass spectrometric detection and time-of-flight (TOF) analysis. This technique, based on universal electron-impact (El) ionization coupled with a quadrupole mass filter for mass selection, has been central in the investigation of the dynamics of bimolecular reactions during the past 35 years.1,9-11 El ionization affords, in principle, a universal detection method for all possible reaction products of even a complex reaction exhibiting multiple reaction pathways. Although the technique is not usually able to provide state-resolved information, especially on a polyatomic... 

The technique of neutralization/reionization mass spectrometry (NRMS), originally introduced by McLafferty, invokes the formation of fast neutrals from a preselected ion beam, any residual ions being deflected, followed by collision-induced reionization of the neutrals and a subsequent mass spectrometric analysis16-21. 

Different mass spectrometric techniques can be classified according to the evaporation and ionization methods applied. Evaporation of solid samples can be performed, for example, by thermal (e.g., on a hot tantalum filament or in a heated graphite furnace) or laser-induced evaporation, and by electron or ion bombardment. Electron ionizaton (El), ionization during the sputtering process with a primary ion beam, resonant or non-resonant laser ionization or thermal surface ionization... 

In 2006, Lobinski et al.1 reported on the imaging and speciation analysis of trace elements to study the element distribution, oxidation state, metal site and metal structure in biological environments using mass spectrometric techniques (LA-ICP-MS, SIMS, MALDI-MS) and non-mass-spectrometric techniques such as micro-PIXE (proton induced X-ray emission), XANES (X-ray absorption near edge structure) and EXAFS (extended X-ray absorption fine structure) -the latter two techniques are very sensitive due the use of a more intense synchrotron beam.1... 

Figure 1.15 shows the lateral and depth resolution achievable with the three mass spectrometric techniques described in this section. As can be seen, the depth resolution obtained with the GD techniques is similar to that with dynamic SIMS (with the additional advantage of less matrix effects in the GD sources). However, the lateral resolution obtained with SIMS is much better because the primary ion beam in SIMS is highly focused whereas in a GD the limitations in the source design make it necessary to sputter a sample area with a diameter of 14 mm. On the other hand, the depth resolution obtained with techniques based on lasers is not yet as good as with SIMS or GDs. 

In subsection 7.2.1, it was discussed that the neutral clusters generated from the beam source have a wide range of cluster sizes. Although there is no single, easily available technique which allows the size specific detection of neutral clusters, it is now well accepted that the cluster size distribution can be manipulated, to a certain extent, by careful control of the various experimental parameters. Mass spectrometric detection of clusters in general and tandem mass spectrometry in particular offer the inherent advantage of enabling size specific studies of the cluster ions. 

Mass spectrometric investigation of gas phase clusters has come to play a significant role in cluster research because it is presently the only technique that allows size-dependent analysis of cluster beams. We anticipate that, in the near future, CMS studies utilizing tandem mass spectrometric techniques will be invaluable in confirming the importance of metastable decompositions and in determining the factors responsible for the enhanced intensities of various ions in the CMS. Such an instrument is currently under construction in our own labs, and we should soon have the ability to perform collision-induced dissociation studies on large vdW clusters. 

Experimental. The surface reaction rate and the kinetic equation were determined by investigations in which the diffusion constraint was removed by use of molecular beam mass spectrometric techniques which have been described elsewhere. (2,3)... 

In this review results from two surface science methods are presented. Electron Spectroscopy for Chemical Analysis (ESCA or XPS) is a widely used method for the study of organic and polymeric surfaces, metal corrosion and passivation studies and metallization of polymers (la). However, one major accent of our work has been the development of complementary ion beam methods for polymer surface analysis. Of the techniques deriving from ion beam interactions, Secondary Ion Mass Spectrometry (SIMS), used as a surface analytical method, has many advantages over electron spectroscopies. Such benefits include superior elemental sensitivity with a ppm to ppb detection limit, the ability to detect molecular secondary ions which are directly related to the molecular structure, surface compositional sensitivity due in part to the matrix sensitivity of secondary emission, and mass spectrometric isotopic sensitivity. The major difficulties which limit routine analysis with SIMS include sample damage due to sputtering, a poor understanding of the relationship between matrix dependent secondary emission and molecular surface composition, and difficulty in obtaining reproducible, accurate quantitative molecular information. Thus, we have worked to overcome the limitations for quantitation, and the present work will report the results of these studies. 

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