Physical Principles of Electron Microscopy: An Introduction to TEM ...

An Introduction to Microscopy 15
Figure 1-12. TEM image of a stained specimen of mouse-liver tissue, corresponding
approximately to the small rectangular area within the light-microscope image on the left.
Courtesy of R. Bhatnagar, Biological Sciences Microscopy Unit, University of Alberta.
The TEM has been equally useful in the life sciences, for example for
examining plant and animal tissue, bacteria, and viruses. Figure 1-12 shows
images of mouse-liver tissue obtained using transmission light and electron
microscopes. Cell membranes and a few internal organelles are visible in the
light-microscope image, but the TEM image reveals much more structure in
the organelles, due to its higher spatial resolution.
Although most modern TEMs use an electron accelerating voltage
between 100 kV and 300 kV, a few high-voltage instruments (HVEMs) have
been constructed with accelerating voltages as high as 3 MV; see Fig. 1-13.
The main motivation was the fact that increasing the electron energy (and
therefore momentum) decreases the de Broglie wavelength of electrons and
therefore lowers the diffraction limit to spatial resolution. However,
technical problems of voltage stabilization have prevented HVEMs from
achieving their theoretical resolution. A few are still in regular use and are
advantageous for looking at thicker specimens, as electrons of very high
energy can penetrate further into a solid (1 �m or more) without excessive
scattering.
One of the original hopes for the HVEM was that it could be used to
study the behavior of living cells. By enclosing a specimen within an
environmental chamber, water vapor can be supplied to keep the cells