Investigations of Sulfur Chemical Status with Synchrotron Micro Focused X-ray fluorescence and X-ray Absorption Spectroscopy

Sulfur (S) is an essential macronutrient for all living organisms. A variety of organic and inorganic S species with oxidation states ranging from -2 to +6 exist. Today few spectroscopic and biochemical methods are used to investigate sulfur oxidation state and reactivity in biological samples. X-ray absorption near edge spectroscopy (XANES) is a very well suited spectroscopic technique to probe the oxidation state and the surrounding chemical environment of sulfur. Microspectroscopy beamlines, operating at almost all synchrotron facilities, allow the combination of XANES with X-ray fluorescence mapping (XRF). Using this approach distribution maps of S in complex biological samples (intact parts of tissue, or individual cells) can be obtained using XRF and its oxidation state can be probed in-situ (XANES). Moreover, XRF mapping at specific energies enables for chemical contrast of S at different oxidation states without the need of staining chemicals. This review introduces the basic concepts of synchrotron XRF and XANES and discusses the most recent applications in life science. Important methodological and technical issues will be discussed and results obtained in different complex biological samples will be presented. Keywords: Chemical imaging, core level spectroscopy, cysteine, cystine, elemental mapping, oxidation state, radiation damage.