Gelsomina DeStasio, Instituto de Struttura della Materia del CNR, has begun the
application of spectromicroscopy to biological specimens at SRC. Her work uses the imaging
XSEM (X-ray secondary electron-emission microscope) which Brian Tonner designed and
developed. The microscope was designed for material science applications, and until five
years ago, was not applied to biological experiments.
Biological samples pose some difficulties for study by traditional surface science
techniques. These specimens by necessity must be prepared in complex liquid environments,
and transferred into the microscope through air transfer. This results in a residue of a
"contamination layer," which cannot be penetrated by a surface sensitive probe
such as photoemission spectroscopy.
The advantage of XSEM for biological applications was evident since De Stasios
first tests in 1991. With this microscope, the detected portion of the specimen is over a
depth of 20 - 100 angstroms, well beyond the level of surface contamination. An additional
capability of XSEM is the rapid production of element distribution maps. These maps may be
obtained by collecting and digitally subtracting images of a sample area using photon
energies above and below a specific absorption threshold of a given element.
Spectromicroscopy is applied to biological specimens in experiments by DeStasio using
cultured neurons from rat cerebellum (prepared in Rome, in collaboration with Delio and
Teresa Mercanti of the Institute of Neurobiology-CNR). These neuron cells are cultured on
specially designed sample holders. An important consideration is that the network of cells
be grown in a single layer, so each cell may be viewed from the top.
Neuron cultures are exposed to toxic elements (Fe, Co, Ni, Al, etc.) for a short time.
These elements may be actively incorporated by cells, in specific areas of their
structure. The cells are then carefully washed to remove of all contaminants which are not
absorbed, dried and placed in the microscope. X-rays strike the sample, causing electrons
to be emitted in an amount which corresponds to the number of absorbed rays and the
Among the many results of these experiments, the localization of Aluminum was the most
significant. When neuron cultures were exposed to an Al ion solution, Al was strictly
localized on a particular kind of neuron: Purkinje cells. Cross-checking this result on
several other neuron cell-cultures has confirmed that Al is absorbed only by Purkinje
The relevance of this result lies in Aluminums involvement in pathologies such as
Alzheimers and Parkinsons diseases and Guam Complex. Many authors have
reported the observation of aluminum in brain tissue samples taken (after death) from
patients who had these diseases. Now, localization and observation of Aluminums
selective uptake in Purkinje cell cultures signifies greater understanding of its
suspected toxic role in the etiology of these diseases.
In a recent development, human brain tissue sections were observed with the XSEM. These
samples were obtained from Alzheimer and Guam patients, and prepared in collaboration with
Daniel P. Perl of the Mount Sinai School of Medicine. The preliminary tests were
encouraging. Al spectra were successfully obtained, proving that XSEM is sensitive enough
to detect trace concentrations of Al in natural cases. Nevertheless, technical problems
are still being faced in the production of clear images of brain tissue sections. In the
future it is hoped that XSEM may be established as a technique for human tissue analysis.
The chemical sensitivity and lateral resolution of XSEM are projected to be at the same
level of other techniques such as PIXE (proton induced x-ray emission), SIMS (secondary
ion mass spectroscopy) and LAMMA (laser-assisted mass microanalysis) which are now used
for micro-chemical analysis of human brains. Based on the results from the past few years,
an optimized design for XSEM with synchrotron radiation has been developed by Tonner,
which if constructed, should reach spatial resolutions of below 50 nanometers.
The XSEM concept has begun to appear at a number of other synchrotron radation
laboratories around the world, with projects in place or planned at ALS, Trieste, Taiwan