XANES

About: XANES is a research topic. Over the lifetime, 7737 publications have been published within this topic receiving 188032 citations.

X-ray absorption spectroscopy of biological samples. A tutorial

Abstract: X-ray absorption spectroscopy (XAS) is an element specific spectroscopy sensitive to the local chemical and structural order of the absorber element. XAS is nowadays increasingly used for the speciation analysis of chemical elements owing to the development of new synchrotron radiation facilities worldwide. XAS can be divided into X-ray absorption near edge structure (XANES), which provides information primarily about the geometry and oxidation state, and extended X-ray absorption fine structure (EXAFS), which provides information about metal site ligation. The main advantages of the XAS method are its subatomic (angstrom) resolution, the ability to analyze almost any type of samples including amorphous (non-crystalline) materials, the possibility to analyze such materials in situ requiring minor or no sample preparation. The main limitations of XAS are its sensitivity in the mM (or μg g−1) range, the difficulty to deconvolute the bulk data when the sample is composed of a mixture of structures of the absorber element, and the limited chemical selectivity of ligands to within one row of the periodic table. This tutorial will discuss the strengths and limitations of XAS and compare them to those of alternative or complementary methods such as X-ray diffraction and X-ray photoelectron spectroscopy. The tutorial will also present and discuss the specific needs in terms of sample preparation and preservation all along the process of storage and analysis, and discuss the importance of the use of cryogenic methods when XAS is applied to biological samples. Applications in life sciences are reviewed, not exhaustively, with a special emphasis on some characteristic examples. The article ends with some perspectives on future trends of XAS: micro- and nano-XAS, time-resolved XAS, and high energy resolution XAS.

Coordination and local structure of magnesium in silicate minerals and glasses; Mg K-edge XANES study

Abstract: We present Mg K-edge XANES spectra of selected Mg-bearing oxide and silicate minerals with different coordination states of Mg. The Mg K-edge peak shifts to higher energy with increasing coordination, from talMg in spinel, to tslMg in grandidierite, to t61Mg in diopside and many other silicates, and to I8lMg in pyrope. The correlation between the energy of the Mg K-edge and Mg-O bond distance of the model minerals is also established; it can be used to estimate the average Mg-O bond distance in disordered systems. The curve fitting of Mg K-edge XANES spectra may be used to distinguish the coordination of Mg, and to determine the relative proportion of different coordination sites, as demonstrated in yoderite, which contains both t5lMg and t6lMg. The structural role of Mg in CaMgSi2O6 @i) NaAlSi3Os (Ab) glasses was studied using Mg K-edge XANES spectra. The Mg-O bond distance in these glasses is estimated to be 2.00 + 0.04 A. Thus, Mg in these glasses may be five-coordinated with oxygen, or Mg may have multiple structural sites, t41Mg, t5lMg and t6lMg. Structutally, the Di-Ab glasses may possess a medium-range order, and have dramatically different multiple scattering (MS) paths from those of crystalline model minerals.

Towards structural dynamics in condensed chemical systems exploiting ultrafast time-resolved x-ray absorption spectroscopy.

Abstract: The authors present the case for exploiting time-resolved x-ray absorption to study structural dynamics in the liq. phase. With this aim in mind and considering the large differences between absorption coeffs. in the optical and the x-ray domains as well as the x-ray absorption cross sections due to unexcited species, the authors have estd. the anticipated signal-to-noise ratio (S/N) under realistic conditions with femtosecond laser pump pulses and synchrotron radiation x-ray probe pulses. As a model system, the authors examine I- photodetachment in H2O and detect the appearance of laser-generated neutral I atoms by their x-ray near-edge absorption structure (XANES) and by their extended x-ray absorption fine structure (EXAFS). While the S/N ratio critically depends on the photolysis yield, which itself is governed by the optical absorption cross section, the optimum sample concn. varies in a complex fashion as a function of pump laser intensity and optical absorption cross section. However, concns. yielding near total absorption of the pump laser deliver quite optimum S/N ratios. The calcns. presented here provide guidelines for the implementation of time-resolved x-ray absorption expts. in condensed phase chem. systems. [on SciFinder (R)]