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Mikroskopie und Tomographie Poster: Mi., 14:00–16:30 M-P107
Spectromicroscopy studies of colloid systems using cluster analysis
Genoveva Mitrea 1 , Juergen Thieme 1 , Charlotte Gleber 1 , Eva Pereiro
Lopez 2
1 Friedrich Hund Platz 1, 37077 Goettingen Germany – 2 ALBA Synchrotron Light
Source, Apartado de Correos 68, 08193 Bellaterra, Barcelona, Spain
We report results of first spectromicroscopy experiments that have been performed
with the scanning transmission X-ray microscope (STXM) at the electron storage ring
BESSY II in Berlin. This instrument has been designed especially for the use with the
undulator U41, which supplies it with radiation in the range of 200-600 eV. This energy
range has been chosen as the K-absorption edges of carbon, nitrogen and oxygen, as well
as the L-absorption edges of potassium and calcium can be used for spectromicroscopy
experiments. It is well-known within the field of X-ray microscopy as the so-called
water window. The main goal of the STXM at BESSY II is to enable studies of
colloidal systems from the environment and from material sciences directly in aqueous
media. By finely tuning the energy of the used X-radiation around an absorption edge
it is possible to first of all determine the distribution of that element within the sample.
Secondly, just before the edge jump in absorption resonance, i.e. below the actual edge
energy, resonance features can be observed representing chemical binding states.
We have performed first spectromicroscopy experiments with colloidal systems from
environmental as well as from material sciences. A soil science wise well-characterized
sample from a Chernozem soil with 0.1 % humics has been used, because of its known
high organic content (approx. 4 %). Sequences of images, so called stacks are recorded
around the carbon edge in the fashion described above. This data set consists in N
transmission images in (x, y) taken at N energies EN with P number of pixels each. The
doses on each pixel applied when taking a stack is the same as when fixing the position
of the X-ray spot on the sample and just taking a spectrum by tuning only the energy.
The advantage is the availability of the chemical information within the whole image.
The same (x, y, E) data set would have been obtained by acquiring a series of spectra
of adjacent pixels.
The data sets of the stacks obtained during the experiments have been analyzed
using cluster analysis. This algorithm consists of a method of grouping pixels with
similar experimentally determined spectra and then analyze the data according to
these groups. A principal component analysis representation is priory applied to the
data set for noise filtering and orthogonalizing it. Two cluster analysis methods have
been involved, so that the pixels are grouped according to their Euclidian distance from
a given cluster center, and respectively to their angular distribution with respect to the
cluster center. We were able to localize the clay particles reach in potassium inside
the soil colloids, as well as the organic domains reach in carbon. Spatially distinctive
images of these domains are to be observed together with the corresponding spectra
where potassium, respectively carbon absorption edges are lined out.