What is the expected spatial resolution with freeze-dried or negatively stained specimens?
Generally, freeze-dried biological specimens at -160C lose roughly 20A of resolution for every 10el/A2 of dose, becoming stable at a width equal to their initial height. This is seen most clearly on the ends of TMV particles, one of which is concave with very sharp corners. In a dose-response series, the TMV end gradually becomes completely rounded. Negative stain gives higher resolution but very limited possibility for mass mapping. The stain forms a shell and penetrates into crevices, preserving some of the shape even when the protein is vaporized. Nanovan (methylamine vanadate) gives the best performance in STEM, but may disrupt some complexes. A layer of vanadate thicker than the particles of interest permits determination of baseline between particles and rudimentary mass mapping. It is particularly useful for heavy atom cluster labeling, since both the clusters and the fine structure of the protein are visible at <5A resolution. Uranyl acetate gives higher contrast and seems gent
