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3D Micro X-ray absorption spectroscopy

left: 3D Micro-XAS setup with a stratified sample composed of a thin copper foil (2 µm) on top followed by layers of lacquer with different copper compounds (thickness about 50 µm)
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3D Micro X-ray fluorescence (3D Micro-XRF) spectroscopy enables three-dimensional resolved, nondestructive elemental analysis in the micrometer regime (3D Micro-XRF). A nondestructive investigation of the electronic structure of atoms (e.g. oxidation state) and their local geometric environment (e.g. distance to neighbors) can be performed by the use of X-ray absorption spectroscopy (XAFS). There are many questions, especially in connection to cultural heritage, making a nondestructive depth-resolved chemical speciation useful. Thus we aim to combine these two methods.

3D Micro-XAFS spectra are obtained by tuning the exciting energy in the region of an absorption edge while the probing volume is placed in one layer (figure above, left). The fluorescence radiation as a function of the exciting energy is measured.

For the interpretation of the obtained spectra one of the main challenges is due to self absorption in the sample. When measuring a homogeneous sample the excitation radiation is absorbed according to the probed depth and this absorption changes drastically in the region of the absorption edge. Also, the fluorescence radiation is attenuated on the way to the detector. The result is a disturbance of the measured spectrum. For layered samples the energy dependent absorption of the upper layers has to be taken into account, too.

Based on the quantification of the 3D Micro-XRF a reconstruction procedure of the original spectra for homogeneous samples is being developed. 3D Micro-XRF measurements yield the composition, the depth of the probing volume in the sample and the calibration factors typical for the alignment. Then the ionization cross section for the investigated edge (e.g.: K-edge) can be calculated from the 3D Micro-XANES spectrum directly.

For stratified samples it is possible to calculate the absorption in one layer and thus the reconstructed spectrum and the missing fundamental atomic parameters. The missing parameters are the ionization cross sections from the other edges, which are also excited (e.g.: for an investigation of the K-edge: L,M,N,...) and the scattering cross sections for the inelastic and elastic scattering. Knowing the absorption of upper layers, it is possible to normalize the spectrum of each layer to the incident intensity and treat it like derived from a homogeneous sample. An example for the reconstruction of a 3D Micro-XANES spectrum measured in an intermediate layer is shown in the figure above.

Measurements are performed at the µSpot beamline at BESSY II, which has a dedicated endstation for micro-analysis. By now investigations of stratified samples are possible with about a minimal thickness of 40 µm for each layer in the region of the iron and copper K-edge (~ 7 - 9 keV). The resolution is limited due to the exciting energy (element and edge), due to the used optics and the reconstruction procedure.

Recently quantitative confocal chemical mapping was demonstrated in order to visualize different chemical species of one element in a sample. Also, 3D Micro-XANES was used for the investigation of attic vases and their manufacturing process.

Contact: Lars Lühl

 

Relevant publications:

L. Lühl, I. Mantouvalou, W. Malzer, I. Schaumann, C. Vogt, O. Hahn and B. Kanngießer , Anal. Chem., 2012, 84, 1907−1914

Lars Lühl, Ioanna Mantouvalou, Ina Schaumann, Carla Vogt, and Birgit Kanngießer, Anal. Chem., 2013, 85 (7), pp 3682–3689

Lars Lühl, Bernhard Hesse, Ioanna Mantouvalou, Max Wilke, Sammia Mahlkow, Eleni Aloupi-Siotis, and Birgit Kanngiesser, Anal. Chem., 2014, 86 (14), pp 6924–6930

 

 

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