342
10.
PARTICULATE SPECIMENS
No standard rules can be given, and each new problem has to be approached with ingenuity, but with the knowledge that "tricks" may help.
Some replica films will be big enough and strong enough to grab with
forceps and pull. But more often a more delicate method of stripping is
required. Adhesive cellulose tape commonly is used. This is rubbed over
the replica to assure good contact. After stripping, the replica is separated
from the tape with a suitable organic solvent that does not affect the
replica (i.e. acetone if the replica is of Formvar).
If single-stage replication is being attempted with a plastic replica film
that must be stripped, it is usually desirable to transfer it directly to
specimen grids. This often can be done by laying grids upon the replica,
and protecting their upper exposed surfaces from the adhesive tape with
interposed bits of lint-free paper (cigarette paper serves well). When the
grids subsequently are pulled away from the adhesive their centers will
be clean. This method of transfer of course is hazardous, for too delicate
a replica film will tear in the vicinity of the grid and not strip there. The
bonding strength of replica and specimen often will be so great that there
is no possibility of transferring a thin enough replica to serve as a microscope specimen.
Films suitable for single-stage replication sometimes can be reinforced
for stripping by coating them with other substances of different solubility.
Gelatin is easy to use, and thick films can be built up from heat concentrated solutions upon Formvar or Parlodion replicas. The dry-stripped
replica finally is cleaned for mounting by soaking it in hot water.
Polyvinyl alcohol in water and carboxymethyl cellulose in water can be
used similarly. The very different solubilities of Formvar and Parlodion
allow one to be used to reinforce the other. Flexible resins such as Zapon
can be used. Various waxes have been employed. The English commonly
use the acrylic plastic "Bedacryl 122X" which is soluble in benzene,
chloroform, or acetone.
10.10. Materials for Replication, Deposited by Vacuum
Evaporation
It was early realized that replicating films might be built up by the
vacuum evaporation of light elements. Silica (ordinarily produced by the
evaporation of silicon monoxide which vaporizes at a lower temperature
than the dioxide) yielded good results because of its chemical inertness
10.
PARTICULATE SPECIMENS
No standard rules can be given, and each new problem has to be approached with ingenuity, but with the knowledge that "tricks" may help.
Some replica films will be big enough and strong enough to grab with
forceps and pull. But more often a more delicate method of stripping is
required. Adhesive cellulose tape commonly is used. This is rubbed over
the replica to assure good contact. After stripping, the replica is separated
from the tape with a suitable organic solvent that does not affect the
replica (i.e. acetone if the replica is of Formvar).
If single-stage replication is being attempted with a plastic replica film
that must be stripped, it is usually desirable to transfer it directly to
specimen grids. This often can be done by laying grids upon the replica,
and protecting their upper exposed surfaces from the adhesive tape with
interposed bits of lint-free paper (cigarette paper serves well). When the
grids subsequently are pulled away from the adhesive their centers will
be clean. This method of transfer of course is hazardous, for too delicate
a replica film will tear in the vicinity of the grid and not strip there. The
bonding strength of replica and specimen often will be so great that there
is no possibility of transferring a thin enough replica to serve as a microscope specimen.
Films suitable for single-stage replication sometimes can be reinforced
for stripping by coating them with other substances of different solubility.
Gelatin is easy to use, and thick films can be built up from heat concentrated solutions upon Formvar or Parlodion replicas. The dry-stripped
replica finally is cleaned for mounting by soaking it in hot water.
Polyvinyl alcohol in water and carboxymethyl cellulose in water can be
used similarly. The very different solubilities of Formvar and Parlodion
allow one to be used to reinforce the other. Flexible resins such as Zapon
can be used. Various waxes have been employed. The English commonly
use the acrylic plastic "Bedacryl 122X" which is soluble in benzene,
chloroform, or acetone.
10.10. Materials for Replication, Deposited by Vacuum
Evaporation
It was early realized that replicating films might be built up by the
vacuum evaporation of light elements. Silica (ordinarily produced by the
evaporation of silicon monoxide which vaporizes at a lower temperature
than the dioxide) yielded good results because of its chemical inertness