Proton probe analysis

The use of particles heavier than electrons, and especially proton-induced x-ray emission analysis, was developed under the supervison of Professor Sven Johansson at Lund University, Sweden, during the 1970s.9 Generally referred to as PIXE (particle- or proton-induced x-ray emission) analysis, it has proven to be a sensitive trace element technique. The initial response among medical researchers was a cautious one, most likely due to the fact that the problems of specimen preparation initially were 

FIGURE 5.3 Sphere of secondary radiation. The detector sees only a fraction of the total number of emitted x-ray quanta. (Adapted from Lindstrom, B., Acta Radiologica Suppl. 125, 206, 1955.) 

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A recent development of proton probe analysis involves multidimensional statistical analysis of all data extracted from mass and elemental maps. The SIMCA program, which essentially analyzes... 

Particle probe analysis and, in particular, proton probe analysis, which is sensitive to trace element levels in tissue sections have been demonstrated to reveal important details about cellular physiology in the differentiating epidermis of normal and pathological skin. Such a physiological approach will serve to complement data from other techniques. A future collective approach of this kind will make it possible to understand how a dry and eczematous skin develops and also what the mechanisms of subsequent healing are. 

In particle probe analysis systems, x-rays are generated from the elements due to an excitation caused by the impinging particles, whether they are electrons or protons, and these secondary x-rays are emitted in all directions. However, the detector can only cover a small part of the sphere of secondary radiation (Figure 5.3), even if the geometry of the experimental setup allows the detector to come very close to the object, which will increase the spatial angle from which the detector sees the volume of analysis. Here we see a factor which influences markedly the sensitivity of the analysis method. 

TRACE ELEMENT ANALYSIS IS POSSIBLE WITH THE PROTON PROBES... 

Werner-Linde, Y., Pallon, J., and Forslind, B., Physiologically important trace elements of paralesional psoriatic skin. Quantitative analysis of distributions using scanning proton probe technique, Scanning Microsc., 12, 599, 1998. 

All the techniques discussed here involve the atomic nucleus. Three use neutrons, generated either in nuclear reactors or very high energy proton ajccelerators (spallation sources), as the probe beam. They are Neutron Diffraction, Neutron Reflectivity, NR, and Neutron Activation Analysis, NAA. The fourth. Nuclear Reaction Analysis, NRA, uses charged particles from an ion accelerator to produce nuclear reactions. The nature and energy of the resulting products identify the atoms present. Since NRA is performed in RBS apparatus, it could have been included in Chapter 9. We include it here instead because nuclear reactions are involved. 

The electrochemical reduction of 4//-thiopyrans bearing four electron-withdrawing substituents leads to 5,6-dihydro-2//-thiopyrans. Four diastereoisomers are produced, their relative proportions depending on the electrolytic conditions. Their conformations have been established using the vinylic proton as an nmr probe and confirmed in some instances by X-ray analysis <96JCS(P2)2623>. 

Spectroscopy. In the methods discussed so far, the information obtained is essentially limited to the analysis of mass balances. In that re.spect they are blind methods, since they only yield macroscopic averaged information. It is also possible to study the spectrum of a suitable probe molecule adsorbed on a catalyst surface and to derive information on the type and nature of the surface sites from it. A good illustration is that of pyridine adsorbed on a zeolite containing both Lewis (L) and Brbnsted (B) acid sites. Figure 3.53 shows a typical IR ab.sorption spectrum of adsorbed pyridine. The spectrum exhibits four bands that can be assigned to adsorbed pyridine and pyridinium ions. Pyridine adsorbed on a Bronsted site forms a (protonated) pyridium ion whereas adsorption on a Lewis site only leads to the formation of a co-ordination complex. 

A remarkable application of phosphines by Grey and coworkers for acid site characterization is the use of diphosphines with alkyl chain spacers of different length between the phosphine moieties. Based on careful NMR analysis and appropriate loading levels with diphosphines, the Al distribution can be determined [223, 224], The idea behind this tool is that the phosphine groups will be proto-nated, when they are close to an acid site in the zeolite structure. Protonation of both phosphine groups in one probe molecule will only occur, when the distance between the two acid sites is compatible with the molecular dimension of the diphosphine. 

Giant dipole resonance. Isovector giant resonances contain information about the SE through the restoring force. In particular the excitation of the isovector giant dipole resonance (GDR) with isoscalar probes has been used to extract A R/R [32], In the distorted wave Bom approximation optical model analysis of the cross section the neutron and proton transition densities are needed as an input. For example, in the Goldhaber-Teller picture these are... 

Thus, cosolvents can be used as perturbing tools and as probes for conformation change after careful, comparative analysis of their effects compared to those induced by various physiological agents, such as protons, monovalent and divalent cations, and other ligands which act to regulate equilibria and rate processes. 

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