Sublimation rates of ice in a cryo-ultramicrotome

Sublimation rates of ice in the nitrogen-filled cryochamber of an ultramicrotome have been measured. The values are four to five orders of magnitude smaller than vacuum sublimation rates at the same temperature. This is a consequence of the much shorter mean free path length of water molecules in nitrogen at atmospheric pressure, as compared to freeze drying under vacuum. The partial pressure at the phase boundary and gas diffusion and convection determine the rate of freeze drying. This has implications for section handling and the design of transfer equipment. Generally, it means that the danger of even partial dehydration of semithick (0.5-1 micrometer) sections used mainly for X-ray microanalysis is less serious than commonly assumed.