10.3.
SHADOW-CASTING
331
in a manner described later.) Two methods have been advanced to circumvent these forces. The first of these is a simplified method of "freezedrying" advocated by Williams (1953).
Williams used an atomizer to spray the suspension on a supporting
film precooled with liquid nitrogen. This was done at atmospheric pressure, but within the apparatus that subsequently was to be used for the
drying. Special precautions had to be taken to assure good thermal contact between the supporting film and the chamber walls.
After spraying, the chamber was evacuated and warmed to approximately —50°C. A cold finger near the specimen, however, was filled with
liquid nitrogen. Relatively rapid distillation occurred from the frozen
droplets of suspension to the cold finger so that drying was complete in
about 15 minutes.
The drying chamber was opened only after it had been warmed above
room temperature. The specimen then could be transferred to shadowcasting equipment. After shadowing, the supporting film was removed
from the copper pedestal to which it was lightly attached by a "drystripping" technique and mounted on a grid. In this procedure the dried
particulate matter falls down upon the supporting film where forces of
adhesion ordinarily are sufficient to attach the particles. Very delicate
structures may be preserved in three dimensions in this way.
Anderson (1951, 1953) developed the "critical point" method for
eliminating the artifacts of surface tension. This takes advantage of the
fact that the interface between a fluid in equilibrium with its own
vapor disappears at temperatures above the so-called critical point.
Moderate pressure liquefies carbon dioxide. Its critical point is at 31°C.
Material can be suspended in and infiltrated by liquid carbon dioxide at
room temperature if it has previously been infiltrated with amyl acetate
following an alcoholic dehydration. Finally the bomb is warmed to about
45°C, and the carbon dioxide allowed to escape slowly. A dry threedimensional specimen is left behind.
10,3. Shadow-Casting
To an electron microscopist shadow-casting means evaporating atoms
of a dense material, usually a metal, from a point source at an oblique
angle to the specimen. If the specimen is contoured, metal piles up on
surfaces which face the source, but surfaces facing away from the source
SHADOW-CASTING
331
in a manner described later.) Two methods have been advanced to circumvent these forces. The first of these is a simplified method of "freezedrying" advocated by Williams (1953).
Williams used an atomizer to spray the suspension on a supporting
film precooled with liquid nitrogen. This was done at atmospheric pressure, but within the apparatus that subsequently was to be used for the
drying. Special precautions had to be taken to assure good thermal contact between the supporting film and the chamber walls.
After spraying, the chamber was evacuated and warmed to approximately —50°C. A cold finger near the specimen, however, was filled with
liquid nitrogen. Relatively rapid distillation occurred from the frozen
droplets of suspension to the cold finger so that drying was complete in
about 15 minutes.
The drying chamber was opened only after it had been warmed above
room temperature. The specimen then could be transferred to shadowcasting equipment. After shadowing, the supporting film was removed
from the copper pedestal to which it was lightly attached by a "drystripping" technique and mounted on a grid. In this procedure the dried
particulate matter falls down upon the supporting film where forces of
adhesion ordinarily are sufficient to attach the particles. Very delicate
structures may be preserved in three dimensions in this way.
Anderson (1951, 1953) developed the "critical point" method for
eliminating the artifacts of surface tension. This takes advantage of the
fact that the interface between a fluid in equilibrium with its own
vapor disappears at temperatures above the so-called critical point.
Moderate pressure liquefies carbon dioxide. Its critical point is at 31°C.
Material can be suspended in and infiltrated by liquid carbon dioxide at
room temperature if it has previously been infiltrated with amyl acetate
following an alcoholic dehydration. Finally the bomb is warmed to about
45°C, and the carbon dioxide allowed to escape slowly. A dry threedimensional specimen is left behind.
10,3. Shadow-Casting
To an electron microscopist shadow-casting means evaporating atoms
of a dense material, usually a metal, from a point source at an oblique
angle to the specimen. If the specimen is contoured, metal piles up on
surfaces which face the source, but surfaces facing away from the source