Time using synchrotron sources

The HCA formation on sol-gel glasses is thought to follow the same mechanism as the melt-derived glasses. Real-time studies on 70S30C sol-gel glass in SBF using synchrotron source XRD showed that octacalcium phosphate (OCP)... 

An interesting feature of polarized IR spectroscopy is that rapid measurements can be performed while preserving molecular information (in contrast with birefringence) and without the need for a synchrotron source (X-ray diffraction). Time-resolved IRLD studies are almost exclusively realized in transmission because of its compatibility with various types of tensile testing devices. In the simplest implementation, p- and s-polarized spectra are sequentially acquired while the sample is deformed and/or relaxing. The time resolution is generally limited to several seconds per spectrum by the acquisition time of two spectra and by the speed at which the polarizer can be rotated. Siesler et al. have used such a rheo-optical technique to study the dynamics of multiple polymers and copolymers [40]. 

In terms of contrast, Pluronic micelles have a much more higher contrast using SANS (with D20 as a solvent) than with SAXS. It is illustrated in figure 5, where the SANS and SAXS curves in absolute units are compared for the same sample, micelles of P123 in pure D20 at 40°C and at a volume fraction of 2.6 %. The absolute intensity of the SANS curve is about 103 times greater than the SAXS ones. The maximal flux on the D22 experiment is about 10s neutrons/s. Then, in order to perform kinetics experiments with SAXS, with a temporal resolution equal or better than 30 s, the high flux of a synchrotron source (10nphotons/s or more) is needed. 

It took the short time of one year or so to solve the structure of rhinovirus which causes the common cold. This relied on two major advances in methods. The first was the use of synchrotron radiation in data collection. Nearly a million reflections were collected on the protein crystallography facility at the Cornell Synchrotron source in a matter of days. This conveyed a speed advantage over data collection on a conventional source and also ameliorated an otherwise impossible problem of radiation damage when long exposure times were used. The far greater rate of radiation damage in the X-ray beam in relation to plant viruses is symptomatic of an inherently less stable protein capsid and the absence of quasi-symmetry. The capsid consists of 60 copies each of four proteins and the virus with about 30 % RNA has a total molecular weight of about 8.5 million. 

From a practical standpoint, the use of the glancing angle X-ray method, while powerful, requires a synchrotron source and therefore, due to the constraints of beam time, is necessarily limited in the number of systems that can be studied in a given time period. Of the optical methods listed, the fluorescence and resonance Raman techniques directly measure spectra of an embedded... 

For EXAFS and particularly for XANES, data analysis is complex. The oscillation frequency/bond distance dependence means that extensive use is made of Fourier transform analysis. Most applications to date have been in the EXAFS region. In order to acquire sufficiently strong signals in a reasonable time, use has to be made of high-intensity photon fluxes, which are available at synchrotron facilities. These provide a broad-band tuneable source of high-intensity radiation, but the reduced number of facilities limits widespread dissemination of the technique. Reflection (fluorescent detection) mode is usually preferred to transmission. Experiments can be conducted in any phase, and the probing of electrode surfaces in situ is an important application. 

XRD in dedicated laboratory environments provides and will provide a wealth of useful information, because they allow operation with few time constraints and unsurpassed experimental flexibility. Synchrotron sources, on the other hand, provide unique opportunities to combine XRD with EXAFS spectroscopy, which together provide an enormous advantage as long as the conduct of the experiments is not constrained by beam time limitations. Bulk transformations under reaction conditions are typically slow, being characterized by time scales of hundreds of hours, and are therefore prohibitive for standard user operations at synchrotrons. It is also difficult to handle many catalytically relevant reactants safely at synchrotrons, so that XRD investigations are limited to a few reactants, in contrast to the situation in most catalyst characterization laboratories. 

X-rays are obtained by the conventional method where accelerated electrons are used to bombard a metal anode. X-rays of higher intensity can be obtained from synchrotrons where they are emitted by accelerating electrons or positrons. The higher X-ray intensity of the synchrotron source is helpful in obtaining larger numbers of diffraction spots or e flections in shorte r exposure times. 

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