A Simple Apparatus for Freezing-Drying Histological Specimens

MANY of the conventional types of freezing-drying apparatus employ an oil or mercury diffusion-pump, backed by a rotary pump, to obtain the high vacua necessary to ensure that the water molecules which escape from the specimens have sufficient mean free path to enable them to reach the vapour trap. Clearly, the efficiency of the freezing-drying process might be greatly increased if the specimens could be placed inside a diffusion-pump designed to operate at the necessary low temperature. In such an apparatus the water molecules which escape from the specimens would be entrained in a stream of gas and carried out of the system. As considerations of mean free path would then become unimportant, the need for vapour traps and ultrahigh vacua would be eliminated. The first steps towards the design of an apparatus of this sort were taken independently by Treffenberg (1953) and Jensen (1954), both of whom effected dehydration of specimens by means of a stream of dry air. The devices designed by both these authors involve elaborate measures for drying the air and maintaining the necessary low temperature of the air and specimens during the drying process. The apparatus about to be described here is greatly simplified in that it uses solid carbon dioxide as the source of both dry gas and refrigeration. Any moisture which might be present in the carbon dioxide snow is effectively trapped, so that for practical purposes the gas evolved can be regarded as dry.