344
10. PARTICULATE SPECIMENS
Larger specimens which cannot be handled on specimen grids, as well
as those requiring strong acids for solution, can be dissolved in small
containers after depositing the carbon film. Even concentrated sulfuric
acid with permanganate and dichromate added as strong oxidizing agents
can be used to clean such carbon replicas. (If permanganate is used,
manganese oxides will be formed and must be removed by an additional
treatment with concentrated hydrochloric acid). Obviously this treatment
also would remove any preshadowed metal films other than those of the
noble metals. However, since gold supporting grids are available, even
these rigorous agents can be employed with carbon replicas so mounted.
Evaporated carbon today is almost the universal choice as the final
replica in two-stage replication. If the primary replica consists of a thin
film of Formvar or Parlodion it is not difficult to dissolve away leaving
the intact carbon replica behind. One must perform this operation rather
gently, however, and it is wise to begin with solvents that are not too
active (start with ethylene dichloride rather than chloroform if the replica
is of Formvar, or amyl acetate rather than acetone if of Parlodion). Before
beginning, the replica should be cut into squares or pieces less than ^ in.
wide. They then may be laid across grid screens, plastic side down, and
flushed with solvent from below. This can be done on a standard 1x3
in. microscope slide using, alternately, a pipette to add solvent, and filter
paper to remove it. If the grids are allowed to dry once or twice during
the procedure, the carbon films will settle down flatly upon them and
finally adhere to them. Then powerful solvents can be used for the final
cleaning.
An alternative method involves placing a grid with its replica upon it
on a metal or wooden peg. This is then continually flushed for as long as
necessary with drops of solvent released from a burette.
Difficulties begin when primary replicas are thick. Plastics are prone to
swell as they soften, and before going into solution. This is apt to tear
apart a delicate carbon replica. Both Formvar and Parlodion are bad in
this respect. If a thick primary replica of one of these substances must be
used, it usually is necessary to put a supporting film on the carbon until
the primary replica is dissolved. Thus, taking advantage of their different
solvents, a thin layer of Parlodion can be used to protect the carbon film
while a Formvar replica is being removed. In this circumstance, no
attempt is made to mount the replica on a grid screen until finally the
supporting film is to be dissolved. Waxes can be used similarly.
In this writer's limited experience, resins do not swell badly before
10. PARTICULATE SPECIMENS
Larger specimens which cannot be handled on specimen grids, as well
as those requiring strong acids for solution, can be dissolved in small
containers after depositing the carbon film. Even concentrated sulfuric
acid with permanganate and dichromate added as strong oxidizing agents
can be used to clean such carbon replicas. (If permanganate is used,
manganese oxides will be formed and must be removed by an additional
treatment with concentrated hydrochloric acid). Obviously this treatment
also would remove any preshadowed metal films other than those of the
noble metals. However, since gold supporting grids are available, even
these rigorous agents can be employed with carbon replicas so mounted.
Evaporated carbon today is almost the universal choice as the final
replica in two-stage replication. If the primary replica consists of a thin
film of Formvar or Parlodion it is not difficult to dissolve away leaving
the intact carbon replica behind. One must perform this operation rather
gently, however, and it is wise to begin with solvents that are not too
active (start with ethylene dichloride rather than chloroform if the replica
is of Formvar, or amyl acetate rather than acetone if of Parlodion). Before
beginning, the replica should be cut into squares or pieces less than ^ in.
wide. They then may be laid across grid screens, plastic side down, and
flushed with solvent from below. This can be done on a standard 1x3
in. microscope slide using, alternately, a pipette to add solvent, and filter
paper to remove it. If the grids are allowed to dry once or twice during
the procedure, the carbon films will settle down flatly upon them and
finally adhere to them. Then powerful solvents can be used for the final
cleaning.
An alternative method involves placing a grid with its replica upon it
on a metal or wooden peg. This is then continually flushed for as long as
necessary with drops of solvent released from a burette.
Difficulties begin when primary replicas are thick. Plastics are prone to
swell as they soften, and before going into solution. This is apt to tear
apart a delicate carbon replica. Both Formvar and Parlodion are bad in
this respect. If a thick primary replica of one of these substances must be
used, it usually is necessary to put a supporting film on the carbon until
the primary replica is dissolved. Thus, taking advantage of their different
solvents, a thin layer of Parlodion can be used to protect the carbon film
while a Formvar replica is being removed. In this circumstance, no
attempt is made to mount the replica on a grid screen until finally the
supporting film is to be dissolved. Waxes can be used similarly.
In this writer's limited experience, resins do not swell badly before