6. Section Mounting
6.1. Introductory Remarks
A technology for making supporting films had been developed before
ultrathin sectioning became practical, for such films were and are used
to mount particulate specimens, in biology particularly by bacteriologists
to support bacteria and viruses (Chapter 10). It was realized immediately
after sectioning got underway that methacrylate sections required support of this sort for this plastic is softened and decomposed in the electron
beam to the point where it cannot sustain itself except across an extraordinarily fine mesh (Chapter 4.9).
During the "methacrylate era" supporting films were regarded as
necessary evils, for insofar as a film possesses mass, it scatters electrons
and undesirably reduces contrast. One can imagine the situation where
a specimen has the same density as the film. If it were embedded in the
film it then would be quite invisible. Unstained material fixed in an
aldehyde approaches this condition. Thus, from the very beginning,
efforts have been directed toward making the thinnest possible supporting films of materials consisting of light atoms, which therefore would
have the least possible adverse affect upon contrast.
It was natural that investigators turned to plastics in searching for good
supporting films. The requirements were for materials which could be
dissolved simply, and which would form tough films when the solvent
was allowed to evaporate. Parlodion and Formvar were used early for
such films and remain the most satisfactory plastics today, although
superior films of pure carbon supplant these old standbys to a considerable extent. Carbon films are formed by the deposition of vaporized
carbon in vacuo, a technique introduced by Bradley (1954). Pure carbon
films (or carbon stabilized plastic films) are so generally used now that
a high vacuum evaporating apparatus has to be regarded as an essential
item of equipment in an electron microscope laboratory that uses films
for any purpose.
Although simple Parlodion and Formvar films should not be used
anymore for quality work, they are often used in combination with a
carbon deposit which stabilizes them, and they are used as a possible
192
6.1. Introductory Remarks
A technology for making supporting films had been developed before
ultrathin sectioning became practical, for such films were and are used
to mount particulate specimens, in biology particularly by bacteriologists
to support bacteria and viruses (Chapter 10). It was realized immediately
after sectioning got underway that methacrylate sections required support of this sort for this plastic is softened and decomposed in the electron
beam to the point where it cannot sustain itself except across an extraordinarily fine mesh (Chapter 4.9).
During the "methacrylate era" supporting films were regarded as
necessary evils, for insofar as a film possesses mass, it scatters electrons
and undesirably reduces contrast. One can imagine the situation where
a specimen has the same density as the film. If it were embedded in the
film it then would be quite invisible. Unstained material fixed in an
aldehyde approaches this condition. Thus, from the very beginning,
efforts have been directed toward making the thinnest possible supporting films of materials consisting of light atoms, which therefore would
have the least possible adverse affect upon contrast.
It was natural that investigators turned to plastics in searching for good
supporting films. The requirements were for materials which could be
dissolved simply, and which would form tough films when the solvent
was allowed to evaporate. Parlodion and Formvar were used early for
such films and remain the most satisfactory plastics today, although
superior films of pure carbon supplant these old standbys to a considerable extent. Carbon films are formed by the deposition of vaporized
carbon in vacuo, a technique introduced by Bradley (1954). Pure carbon
films (or carbon stabilized plastic films) are so generally used now that
a high vacuum evaporating apparatus has to be regarded as an essential
item of equipment in an electron microscope laboratory that uses films
for any purpose.
Although simple Parlodion and Formvar films should not be used
anymore for quality work, they are often used in combination with a
carbon deposit which stabilizes them, and they are used as a possible
192