Sources Mossbauer spectroscopy

The spectroscopic techniques that have been most frequently used to investigate biomolecular dynamics are those that are commonly available in laboratories, such as nuclear magnetic resonance (NMR), fluorescence, and Mossbauer spectroscopy. In a later chapter the use of NMR, a powerful probe of local motions in macromolecules, is described. Here we examine scattering of X-ray and neutron radiation. Neutrons and X-rays share the property of being found in expensive sources not commonly available in the laboratory. Neutrons are produced by a nuclear reactor or spallation source. X-ray experiments are routinely performed using intense synclirotron radiation, although in favorable cases laboratory sources may also be used. 

Fig. 3.19 Schematic illustration of the measurement geometry for Mossbauer spectrometers. In transmission geometry, the absorber (sample) is between the nuclear source of 14.4 keV y-rays (normally Co/Rh) and the detector. The peaks are negative features and the absorber should be thin with respect to absorption of the y-rays to minimize nonlinear effects. In emission (backscatter) Mossbauer spectroscopy, the radiation source and detector are on the same side of the sample. The peaks are positive features, corresponding to recoilless emission of 14.4 keV y-rays and conversion X-rays and electrons. For both measurement geometries Mossbauer spectra are counts per channel as a function of the Doppler velocity (normally in units of mm s relative to the mid-point of the spectrum of a-Fe in the case of Fe Mossbauer spectroscopy). MIMOS II operates in backscattering geometry circle), but the internal reference channel works in transmission mode...

For a comparison of experimental Mossbauer isomer shifts, the values have to be referenced to a common standard. According to (4.23), the results of a measurement depend on the type of source material, for example, Co diffused into rhodium, palladium, platinum, or other metals. For Fe Mossbauer spectroscopy, the spectrometer is usually calibrated by using the known absorption spectrum of metallic iron (a-phase). Therefore, Fe isomer shifts are commonly reported relative to the centroid of the magnetically split spectrum of a-iron (Sect. 3.1.3). Conversion factors for sodium nitroprusside dihydrate, Na2[Fe(CN)5N0]-2H20, or sodium ferrocyanide, Na4[Fe(CN)]6, which have also been used as reference materials, are found in Table 3.1. Reference materials for other isotopes are given in Table 1.3 of [18] in Chap. 1. 

The source preparation for tungsten Mossbauer spectroscopy is in general cumbersome, apart from the production of a, the parent nuclide of The first... 

Review article Excellent source of information about Mossbauer spectroscopy of iridium compounds and alloys... 

The most direct information on the state of cobalt has come from Mossbauer spectroscopy, applied in the emission mode. As explained in Chapter 5, such experiments are done with catalysts that contain the radioactive isotope 57Co as the source and a moving single-line absorber. Great advantages of this method are that the Co-Mo catalyst can be investigated under in situ conditions and the spectrum of cobalt can be correlated to the activity of the catalyst. One needs to be careful, however, because the Mossbauer spectrum one obtains is strictly speaking not that of cobalt, but that of its decay product, iron. The safest way to go is therefore to compare the spectra of the Co-Mo catalysts with those of model compounds for which the state of cobalt is known. This was the approach taken... 

In absorption Mossbauer spectroscopy, a source nuclide in a standard form (usually in a metallic matrix) is coupled with a sample to be investigated. This method requires at least 100 pg of Fe or Sn in the usual experimental setup even if a Mossbauer sensitive enriched stable isotope Fe-57 or Sn-119 is employed. In emission Mossbauer spectroscopy, however, 1 mCi of Co-57 or Sb-119, which corresponds nominally to 120 ng of Co-57 or 1.4 ng of Sb-119, is sufficient to permit measurement. This technique enables study of very dilute systems, especially those with ions directly bound to the substrate. 

The present method is still in its early stage of application. Both ex situ and in situ type measurements are applicable to a variety of mineral/aqueous solution interfaces. For example, the mechanism of selective adsorption of cobaltous ions on manganese minerals can be studied by this method. In addition to the two Mossbauer source nuclides described in the present article, there are a number of other nuclides which can be studied. We have recently started a series of experiments using Gd-151 which is a source nuclide of Eu-151 Mossbauer spectroscopy. Development of theory on surface magnetism, especially one including relaxation is desirable. Such a theory would facilitate the interpretation of the experimental results. 

The second problem in relation to Mossbauer spectroscopy, which is trivial but irksome, is agreeing on a reference point with respect to isomer shifts. Each particular nuclide has several possible chemical environments for the source and therefore several possible reference... 

Let us now discuss some recent work by Sano and myself on completely characterizing barium stannate, a material first proposed by Plotnikova, Mitrofanov, and Shpinel (21), as a source for tin Mossbauer spectroscopy. It is easily prepared, is a stoichiometric compound and has all the properties one desires in a Mossbauer matrix. The recoil-free fraction at room temperature is about 0.55 with about a 10% error. The line width extrapolated to zero absorber thickness is about 6% larger than natural—i.e., the line width observed is ca. 0.318 mm./sec. at zero ab-... 

Mossbauer spectroscopy The Mossbauer effect is resonance absorption of 7 radiation of a precisely defined energy, by specific nuclei. It is the basis of a form of spectroscopy used for studying coordinated metal ions. The principal application in bioinorganic chemistry is Fe. The source for the 7 rays is Co, and the frequency is shifted by the Doppler effect, moving it at defined velocities (in mm/s) relative to the sample. The parameters derived from the Mossbauer spectrum (isomer shift, quadrupole splitting, and the hyperfine coupling) provide information about the oxidation, spin and coordination state of the iron. 

The rapid progress in the understanding of the active site of aconitase in the 1980 s has primarily originated from the work of H. Beinert and his collaborators. Three essential factors contributed to the success of this work 1) a ready and consistent source of enzyme (gram quantities), 2) a solid chemical and biochemical understanding of aconitase, and 3) close interactions with outstanding collaborators (most notably E. Munck s group for Mossbauer spectroscopy and B. M. 

Further identification of the particles is made with 57Fe Mossbauer spectroscopy. Mossbauer spectra were recorded with a conventional constant acceleration spectrometer with 57Co in Rh matrix as a y-ray source. Velocity calibration was made using a 5-pm a-Fe foil at 293 K. Figure 1.6.10 shows the Mossbauer spectra of the sample recorded at 293 K and 4.2 K. Spectra were fitted with theoretical... 

Mossbauer spectroscopy, also called recoil-free nuclear resonance absorption, depends upon resonant absorption of y-rays emitted by a radioactive source by atomic nuclei.120 The phenomenon was initially difficult to observe, but the German physicist Mossbauer devised a way in which to record the absorption of a quantum of energy equal to the difference in two energy states of the atomic nucleus. The method depends upon a Doppler effect observed when the sample or source moves. Consequently, Mossbauer spectra, such as that in Fig. 16-18, are plots of absorp-... 

Due to effects caused by the nuclear decay in the sample, these so-called source experiments may be difficult to perform and interpret. Several papers dealing with these effects can be found (23). In principle, however, the applicability of Mossbauer spectroscopy to catalytic studies can be extended to include both the Mossbauer isotopes and the corresponding parent nuclides. We therefore list below the Mossbauer isotopes and corresponding parent nuclides that may be of greatest use in catalytic studies, as deduced from their nuclear properties. 

Most often the transmission mode is found to be the most convenient in Mossbauer spectroscopy, i.e., the y radiation passes from the source through the absorber, and the attenuation of the primary beam is measured at the various Doppler velocities. However, there are a number of cases where a "scattering geometry may be advantageous (SO). The basis for this geometry lies in those processes that take place after resonant absorption of y radiation by the Mossbauer isotope. Specifically, after excitation the Mossbauer isotope may reemit the y ray, or it may decay by emission of internal conversion electrons and X rays [with the probability of internal conversion equal to a/(l + a)]. 

Archaeologists have long sought methods for obsidian characterization that were rapid, reliable, non-destructive, and low-cost. Among the various methods investigated were visual techniques (79), density measurements (20), magnetic properties (27), thermoluminescence (22), fission-track analysis (25), Mossbauer spectroscopy (24), and natural radioactivity (25). Although some of the methods occasionally identified differences between sources, the overlap between sources was such that their overall reliability was unsatisfactory. The most successful method of characterization for obsidian provenance research has been compositional analysis (26). 

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