Discerning water contamination during freeze—fracturing on stearic acid crystal sheets

Abstract

In high vacuum (HV) systems, as well as in ultra high vacuum (UHV) systems that have not been sufficiently baked out, the main component of the residual gas atmosphere is water. Adsorbed water is present on all surfaces of the vacuum chamber. Whether or not ice introduced into such a vacuum system sublimes depends on its saturation pressure (at the surface, as a function of the specimen temperature )and on the surrounding partial pressure of water (pH2O). These facts must be considered when fracturing frozen biological material in vacuum systems. Thus the critical condensation temperature ( 162, 144.5, 130 K for 10−6, 10−8, 10-10 mbar, respectively) is an important factor in discussions of specimen fracture face contamination. Possibilities to reduce the rate of water condensation include the use of UHV (p < 10 —9 mbar) and/or surrounding the specimen with a cold, optically dense shroud system. When such a shroud is used, water molecules originating outside the shroud can only reach the specimen surface if they condense on and reevaporate from the shroud surface at least once (probability at T < 123 K is less than 10 4).