5.23.
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183
solution itself and the containers in which it is stored. In general, once
an oil film has been deposited on the edge of the knife it becomes worthless, and one can only start over.
Also, plastic itself can be a "contaminant." If the knife edge is allowed
to become dry and then thin sections are cut, a section or part of a section
may get stuck to the glass and adhere so vigorously that it cannot be
floated free. Subsequently the acetone will not properly wet that portion
of the edge. In these circumstances, it is usually possible to move the knife
so that a new portion of the edge is used.
Simply drying an edge that has once been wet with acetone is apt to
destroy its usefulness. Such dried edges usually become hydrophobic
indicating that the acetone left an organic deposit behind as it retreated.
Thus it is essential that once an acetone meniscus is brought to the knife
edge, it be maintained there. If the operator leaves a wet knife to answer
a telephone, it may be too late when he gets back. If a wet knife must be
left briefly, it is wise to build up a high meniscus with surplus fluid in the
trough that later can be removed as necessary.
Considerable areas of hydrophobic glass or oily films may be seen easily
if the investigator is on the lookout. Often, however, the knife edge can
be so locally contaminated that nothing can be seen with the usual lowpower viewing microscope. This creates a problem for the beginner, but
usually not for the experienced investigator. The latter will be confident
that everything but the knife edge must be in order, and that he should
be getting good sections if it were not for contamination. Instead, he may
be getting "sections" that are crumpled, scraped, scratched, split, or
shredded. Perhaps these difficulties started after an interruption, so he
knows the knife was good to begin with. In any event, he expects a new
knife to take care of the situation. The novice can have no such confidence for he is uncertain about so many other aspects of his technique,
but he should always think of contamination first, and be prepared to
break lots of glass!
It seems fair to say that a knife edge deteriorates almost entirely by
contamination. If no grit is present to actually chip the edge, a knife can
cut soft tissue almost indefinitely. On one occasion, as an experiment
using a microtome with a motor drive, the present writer cut sections
continuously for over an hour without any discernible deterioration,
although several thousand slices had been made. Thus, when sectioning
is going well, there is no advantage in shifting to a new portion of the
CONTAMINATION
183
solution itself and the containers in which it is stored. In general, once
an oil film has been deposited on the edge of the knife it becomes worthless, and one can only start over.
Also, plastic itself can be a "contaminant." If the knife edge is allowed
to become dry and then thin sections are cut, a section or part of a section
may get stuck to the glass and adhere so vigorously that it cannot be
floated free. Subsequently the acetone will not properly wet that portion
of the edge. In these circumstances, it is usually possible to move the knife
so that a new portion of the edge is used.
Simply drying an edge that has once been wet with acetone is apt to
destroy its usefulness. Such dried edges usually become hydrophobic
indicating that the acetone left an organic deposit behind as it retreated.
Thus it is essential that once an acetone meniscus is brought to the knife
edge, it be maintained there. If the operator leaves a wet knife to answer
a telephone, it may be too late when he gets back. If a wet knife must be
left briefly, it is wise to build up a high meniscus with surplus fluid in the
trough that later can be removed as necessary.
Considerable areas of hydrophobic glass or oily films may be seen easily
if the investigator is on the lookout. Often, however, the knife edge can
be so locally contaminated that nothing can be seen with the usual lowpower viewing microscope. This creates a problem for the beginner, but
usually not for the experienced investigator. The latter will be confident
that everything but the knife edge must be in order, and that he should
be getting good sections if it were not for contamination. Instead, he may
be getting "sections" that are crumpled, scraped, scratched, split, or
shredded. Perhaps these difficulties started after an interruption, so he
knows the knife was good to begin with. In any event, he expects a new
knife to take care of the situation. The novice can have no such confidence for he is uncertain about so many other aspects of his technique,
but he should always think of contamination first, and be prepared to
break lots of glass!
It seems fair to say that a knife edge deteriorates almost entirely by
contamination. If no grit is present to actually chip the edge, a knife can
cut soft tissue almost indefinitely. On one occasion, as an experiment
using a microtome with a motor drive, the present writer cut sections
continuously for over an hour without any discernible deterioration,
although several thousand slices had been made. Thus, when sectioning
is going well, there is no advantage in shifting to a new portion of the