Biomedical Engineering Reference
In-Depth Information
10 Selection Criteria Related to Artifacts Induced
by the Preparation Technique
Each technique causes particular artifacts. It is necessary to choose the technique
whose induced artifacts present the least amount of problems for the analysis
planned. For example, to investigate the mechanical properties of a material, any
mechanical preparation must be avoided that produces extrinsic defects that would
be superimposed over the intrinsic defects. Therefore, it is very important to know
the artifacts induced by a specific technique (see Chapter 6).
There are four classes of artifacts corresponding to the four types of preparation
techniques: mechanical, chemical, ionic, and physical. These classes are, in addi-
tion to thermal defects, caused during thinning. For multiphase materials containing
phases with very different mechanical properties, ultramicrotomy preparation will
reveal these differences and extract the hard phases. If the material is hard, the tri-
pod polishing technique is suitable. If the material is of average hardness, an ionic
thinning technique (e.g., FIB) will be preferred since it keeps all of the phases, even
if they have mechanically different properties, and does not introduce dislocations.
In some cases, there is no alternative to this technique: e.g., biological materials pre-
pared primarily using ultramicrotomy. Specific defects must be recognized and the
observation must be performed on the structure that has not been deformed by the
knife during cutting. The major artifact resulting from dehydration must be avoided
in order to study the spatial conformation of proteins. Only cryo-techniques can be
used to observe them in the hydrated state.
In all cases it is necessary to select the technique inducing the fewest artifacts in
the material to be studied (see Chapter 6). The results of several types of preparation
techniques should be compared in order to ensure that the true nature of the structure
is observed.
11 Adaptation of the Technique Based on Problems Related
to Observation
Sometimes one technique alone is not enough to thin a sample down to elec-
tron transparency. In other cases, it forms artifacts that can be eliminated in
order to optimize the observation conditions by combining several thinning tech-
niques. Improvements to TEM observation can be made by reducing the thickness
of the thin slice or by eliminating the contamination or amorphization layers
using chemical or ionic etching. Poor contrast may be increased by adding heavy
elements.
11.1 Reducing Sample Thickness
During chemical, ionic, or tripod polish thinning, the same thickness cannot be
obtained on both sides of an interface when the material presents phases of differ-
ent compositions. This is due to the differential thinning rates in these techniques.
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