High-Resolution stereo imaging of frozen-hydrated biological samples by Cryo-SEM
The introduction of cryo-techniques to SEM allowed the biologist to examine frozen-hydrated specimens at low temperatures on a cryo-stage in the SEM. The problems associated with conventional cryo-SEM included ice crystal formation caused by low cooling rate, poor resolution, limited magnification (<10 kX), specimen contamination, beam damage and charging. Recently, the effectiveness of this technique has been improved because a number of new cryo-preparation instruments have become commercially available. The high pressure freezer permits cryo-fixation of large biological specimens (diameter up to about 1mm) without ice crystal artifacts. Cryo-coating units permit fine-grain metal coating of fractured specimens. In addition, microscopes combining a short focal length lens with a fieldemission (FE) source permit high resolution imaging at low beam voltage (Vo). However, for high resolution cryo-SEM, especially for producing stereo pictures, the artifacts caused by electron irradiation remain a critical problem.The purpose of this work was to evaluate the extent to which beam damage varies with Vo, and to determine the most appropriate Vo for stereo imaging.