Cryofixation and ultra-low-temperature freeze-drying as a preparative technique for TEM

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Abstract
We have developed cryofixation and ultra-low-temperature molecular distillation drying as a method for preparing biological samples for electron microscopic analysis. To validate this approach, we have investigated the relationship between the drying characteristics and ice phases present within frozen samples. Two sample types were investigated. In the first, pure deuterium oxide (D2O), or heavy water, was vapour condensed under vacuum conditions onto a gold-coated copper sample holder held at −175 or −110°C. Additionally, D2O was slow-rate cooled from room temperature under an ultra-pure dry nitrogen gas atmosphere. The second sample type was rat liver biopsies from animals after 5 days of feeding with D2O loaded water and ultra-rapid cooling by metal-mirror cryofixation. Ice forms present in the latter samples, determined by electron diffraction of frozen-hydrated cryosections, were amorphous, cubic, and hexagonal. Drying of samples was achieved using a molecular distillation configuration with continuous, microprocessor-controlled sample heating. The vacuum contents of the drying column were monitored by residual gas analysis (RGA) throughout the drying cycle. D2O vapour in the vacuum chamber, as analysed by RGA, was found to increase in a phasic manner across a broad temperature range. These phases had characteristic onset temperatures and could be removed sequentially. For condensed D2O samples, these onset temperatures were — 160, — 148, — 125 and — 90°C. Rat liver samples also demonstrated phasic drying patterns which were more complex than those detected with pure D2O samples. Ultrastructural analysis of samples cryofixed and dried in this manner demonstrated a morphology consistent with the ice phases demonstrated in the frozen-hydrated cryosections. This, together with the RGA results, suggests the absence of devitrification or ice crystal growth during the drying procedure.