X-ray microscopy and spectromicroscopy - tools for environmental studies

Full text: X-ray microscopy achieves a much higher resolution than light microscopy. This is due to the much shorter wavelength of X-rays compared to visible light. The smallest structures that can be seen in an X-ray microscope at present are about 20 nm in size. X-ray microscopy is also capable of imaging specimens directly in aqueous media. By choosing the wavelength of the X-radiation appropriately, it is possible to perform spectromicroscopy studies. Comprising, it is a tool very well suited to study colloidal structures in the environment. X-ray microscopy can be performed with two types of instruments. An X-ray microscope quickly takes high-resolution images of objects to be studied, whereas a scanning X-ray microscope is an analytical instrument for spectromicroscopy. Here, an object can be imaged using energies above and below the absorption edge of an element. Dividing both images gives rise to a map of the distribution of this element in the sample. Using near-edge resonances it is possible to conduct NEXAFS studies. As the X-ray energy is raised to match the absorption edge resonances are found, that reflect the chemical bonding state of the element. Therefore, it is possible with X-ray microscopy to combine high spatial resolution with high spectral resolution for studies of colloidal structures. The Institute of X-ray physics builds up an X-ray microscopy beamline at the electron storage ring BESSY II with both, an X-ray microscope and a scanning X-ray microscope. The status of this beamline will be presented in this talk. Colloidal structures play an important role in the environment. Due to their surface activity they are involved in various processes. Substances can be bound and immobilized or transported, colloids can attach to microorganisms building up microhabitats, and organic substances as humics can flocculate due the interaction with metals. A great variety of colloidal structures from the environment have been studied using X-ray microscopy, especially structures from soils, sediments and groundwater aquifers. Several examples will be presented: Dispersions extracted from these systems have been imaged with an X-ray microscope to obtain first of all a visual impression of the appearance of the colloids. The effect of changing chemical conditions in the aqueous dispersion media has been studied, too. The change in the appearance of the colloidal structures has been imaged and evaluated using fractal geometry. Clay dispersions and microhabitats have been imaged tomographically. Tilt series of images have been obtained with an X-ray microscope; the specimen was then reconstructed from these images. The resulting reconstruction conveys a detailed three-dimensional impression of the specimen structure, as will be shown. Using spectromicroscopy, the distribution of organic substances on inorganic soil colloids has been studied. A major fraction of these are humic substances. Spectra have been taken from humic substances with and without iron as a coagulation agent. Different functional groups have been identified and changes due to the influence of iron have been mapped. Copyright (2002) Australian Society for Electron Microscopy Inc