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A Pumping Station for Cryo-Transfer Specimen Holders

Cryo electron microscopy has become an important technique for studying the structure and composition of biological specimens in their frozen hydrated state. Many modern transmission electron microscopes can be modified for cryogenic analysis by incorporating liquid nitrogen (2) cooled anti-contaminator blades and a special 2 cooled cold stage specimen holder. These specimen holders are used routinely for the examination of frozen hydrated specimens, and many laboratories have several such specimen holders to permit efficient handling of multiple specimens. Typically, these devices are cooled conductively via intimate contact with a reservoir of 2 inside of a small-evacuated dewar. The periodic evacuation of these dewars is one of the primary purposes of the pumping station. In addition, a means of storing the cryo holders under low vacuum is an important design feature of the pumping station. In our experience the commercially available unit for pumping and storing the cryo holders proved unsatisfactory in two important aspects. The commercial unit made no provision for isolating the pump vibrations from the fragile cryo holders. Occasionally, this resulted in damage to the devices and subsequent costly repairs. Economically, the mechanical arrangement of the ports and associated valves were not as convenient to operate as they could have been. The purpose of this paper is to describe a custom pumping station that was designed and built with off-the-shelf components that overcomes the shortcomings of the commercial unit.