XANES as a tool for iron oxidation state determination in tissues

Abstract

The X-ray absorption near edge structure (XANES) technique was applied in fluorescence mode for the determination of the iron oxidation state in different structures of the kidney. The experiment was performed on 20-μm thick tissue sections at the National Synchrotron Light Source in Brookhaven National Laboratory, USA with the use of a monochromatized beam around 7.2 keV at the X26A beam line. The beam size was defined by four tantalum slits. The samples were taken from patients who were operated due to kidney cancer. First, two-dimensional scans on the selected areas of the kidney tissue sections were done in order to present distribution of iron concentration. The scanned areas were visible on analyzed sections after the measurements allowing the precise matching of tissue structure with analytical data. Others successfully used similar matching procedures with the particle beams methods but synchrotron radiation is less damaging to biological materials. Then, the XANES spectra were taken on selected points that contained high concentration levels of iron. The XANES measurements were done to determine the iron oxidation state, as this is regarded as one of the oxygen indicators in tissue, and free radicals generation. In order to do so the spectra had to be calibrated and normalized. Therefore, three standards were used. Following previous work [Geochim. Cosmochim. Acta 58 (1994) 5209] a synthetic fayalite [Fe 2SiO 4: standard for Fe(II)], pure magnetite [FeO·Fe 2O 3: standard for a mixture of Fe(II) and Fe(III) with the ratio 1:2], and pure hematite [Fe 2O 3: standard for Fe(III)] as standards for iron pre-edge peak position in the XANES spectra were used. It has been found that the iron concentration level is much higher in kidney glomeruli than in nephron regions and due to XANES spectra analyses in the glomerular regions of the tissue sections iron occurs in different oxidation states.