X-ray absorption spectroscopy of biological samples. A tutorial

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 mu g 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.