Biomedical Engineering Reference
In-Depth Information
Chapter 9
Conclusion: What Is a Good Sample?
The act of preparing each sample, as well as observing it in the TEM, subjects
the material to various stresses. Each sample contains both intrinsic defects, giv-
ing the material its particular properties to be studied, and extrinsic defects, which
are associated with the different stages in its history (i.e., from its preparation to
its observation in the TEM). After passing under the microscope, each sample
will have undergone modifications that must be recognizable so as not to bias the
interpretation of the structural and chemical information obtained.
The important thing to be aware is that the sample being analyzed never corre-
sponds to the original material we are investigating. Artifacts, whether visible or not,
are undesirable defects and are always present in a sample. Their formation cannot
be prevented because they are inherent to a technique; they can only be minimized.
What is observed in the TEM is an infinitesimal representation of the sample, if the
surface area and the volume analyzed are taken into account. The process of turning
a material into a thin slice already gives the material a history: It will have undergone
a certain number of stresses (mechanical, chemical, ionic, physical) that may have
modified or transformed it. Another part of the material's history takes place during
its observation in the microscope under conditions that will also add stresses. In the
best-case scenario, microscope observation will not damage the material; however,
in other cases, serious damage will occur. Depending on the type of material, this
damage can result in the destruction of the sample under the beam. Despite all of
this, we are still able to observe and analyze the sample in the end.
Before returning to the microstructure of the initial material and the conse-
quences of its behavior, properties, structure, etc., it will be necessary to use a
combination of several preparation techniques to verify the validity of the struc-
tural, chemical, or spectroscopic information retained in order to describe the
sample.
In materials science, a good sample, i.e., a thin slice of the material, is not a
perfect sample. This means that it is not without defects. It is necessary to know and
recognize the intrinsic defects of a structure and, consequently, to be able to select
a preparation technique consistent with the type of analysis to be performed.
Artifacts are more numerous in biological materials than in materials science.
Chemical fixation, the step most often used for preparing any biological material,
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