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The choice of the appropriate microtome is
determined by the size of the specimen and by the hardness of
thematerial to be cut. Therefore, these basic parameters have to be
checked first.
From the variety of clamping systems / specimen
holders that are available, the very best combination has to befound
that gives the best results as far as stability is concerned.
Generally, not in all cases it is necessary to embed
the sample for sectioning. For hard foils or plastic components,the
foil clamping system, which does not require embedding, can be used
for instance. That means, if the specimen
is stable enough and a direct clamping device is available, it can
be mounted in the specimen holder without
additional embedding. Otherwise, the specimen / sample has to be
embedded to increase the overall stability of the
sample.
The choice of the embedding material depends on the
specimen type and the customer's sectioning requirements,
including sample treatments after sectioning, such as
staining. In most cases, samples of a maximum size of 1.5
cm have to be prepared using a microtome. In this case, one
of the rotary microtomes mentioned above will be the
instrument of choice.
When sectioning very hard or large samples, the
rotary microtomes may not be the right system to obtain high
quality sections. In this case, a heavy duty microtome for
large and hard sections can be used. The LEICA SM
2500 - series features very high cutting forces and special
knife holders designed to achieve maximum stability. As
the maximum cutting force of a rotary microtome is limited,
the hardness of MMA for instance is the upper limit that
allows the use of a rotary microtome for sectioning hard
materials.
The most suitable kind of microtome and specimen
clamping, knife holder and knife have to be chosen.
For sectioning soft materials, knives with a small wedge
angle are suitable. For hard materials, it is just the
opposite. Choosing the optimal knife angle depends directly
on the customer's experience.
From a practical point of view, it has to be
mentioned that the majority of plastic materials can be cut best at
knife angles between 20 and 45 degrees
celsius, The clearance angle to be selected on the knife holder
should be around 5 to 10 degrees celsius
and must be tested prior to routine serial sectioning.
In most cases, tungsten carbide knives or tungsten
carbide disposable blades (TC 65) are successfully used for
plastic sectioning. For very small samples, usually glass
knives are recommended, as they produce higher quality
sections when cutting below 1 microns. An even better quality
section could be achieved by using a diamond knife.
After having prepared everything according to the
above mentioned details and sectioned the material, one major
problem still remains unsolved: removing the sections from
the knife. Needless to say that section removal requires
a lot of skill and experience. Due to the fact that sections
tend to curl during sectioning, it might be helpful to guide
the section using a small brush or carefully with a needle or
forceps. In some cases, the tape method can be
extremely helpful to prevent sections from rolling. Tape
method means that a tape is applied directly onto the block
surface prior to sectioning. After sectioning, the sections
sticks on the tape. Please note that you should use a
special tape to avoid intensive colours of the tape itself in
polymerized light. |
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If the use of embedding
medium, e.g. xylene, on glass slides is planned, it has to be ensured that
the plastic material is not soluble in the
mounting medium, otherwise, the sample is damaged using this method. If
xylene cannot be used, different kinds of oils can
be suitable alternatives. To produce permanent slides, the section has to
be protected from oxygen with a cover slip.
The appropriate cutting temperature is a very important
factor in sectioning plastic materials. Especially low
temperature sectioning is very common. If it becomes necessary to cut
at low temperatures, the Liquid Nitrogen
Attachment Leica LN 21 can be used. The Liquid Nitrogen Attachment is of
particular importance to cut elastomers and
thermoplasts which cannot be sectioned satisfactorily at ambient temperature
but only below - 100 degrees celsius.
From a practical point
of view, it has to be mentioned that the majority of plastic materials can
be cut best at knife angles between 20 and 45
degrees celsius, The clearance angle to be selected on the knife holder
should be around 5 to 10 degrees celsius and must
be tested prior to routine serial sectioning.
In most cases, tungsten carbide knives or tungsten carbide
disposable blades (TC 65) are successfully used for
plastic sectioning. For very small samples, usually glass knives are
recommended, as they produce higher quality
sections when cutting below 1 microns. An even better quality section could
be achieved by using a diamond knife.
After having prepared everything according to the above
mentioned details and sectioned the material, one major
problem still remains unsolved: removing the sections from the knife.
Needless to say that section removal requires a
lot of skill and experience. Due to the fact that sections tend to curl
during sectioning, it might be helpful to guide
the section using a small brush or carefully with a needle or forceps. In
some cases, the tape method can be extremely
helpful to prevent sections from rolling. Tape method means that a tape is
applied directly onto the block surface prior to
sectioning. After sectioning, the sections sticks on the tape. Please note
that you should use a special tape to avoid
intensive colours of the tape itself in polymerized light.
If the use of embedding medium, e.g. xylene, on glass slides
is planned, it has to be ensured that the plastic
material is not soluble in the mounting medium, otherwise, the sample is
damaged using this method. If xylene cannot be
used, different kinds of oils can be suitable alternatives. To produce
permanent slides, the section has to be protected
from oxygen with a cover slip.
The appropriate cutting temperature is a very important
factor in sectioning plastic materials. Especially low
temperature sectioning is very common. If it becomes necessary to cut
at low temperatures, the Liquid Nitrogen
Attachment Leica LN 21 can be used. The Liquid Nitrogen Attachment is of
particular importance to cut elastomers and
thermoplasts which cannot be sectioned satisfactorily at ambient temperature
but only below - 100 degrees celsius.
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