Design and operation of a differentially pumped environmental chamber for the HVEM.

A differentially pumped environmental chamber, or DIFPEC, has been developed for the AEI EM7 1.2 MV HVEM. The chamber is suitable for imaging and diffraction studies and is capable of operation at pressures approaching one atmosphere. Any mixture of gases can be used including water vapor supplied from an internal reservoir. Correct operation is demonstrated by measurements of temperature, pressure, and water consumption. High resolution (0.2 nm) diffraction patterns were recorded from unfixed, unstained, fully hydrated catalase crystals. This is good functional test of DIFPEC operation and specimen preparation because catalase disorders irreversibly of exposed to less than 95% relative humidity. High- and low-angle diffraction patterns were recorded from unfixed, unstained, fully hydrated rat hemoglobin crystals. All patterns were recorded using 10-4 C/cm2. Whole cell mounts prepared in different ways were imaged and show that 5.3 kPa of nitrogen gas has no detrimental effect on image contrast or resolution. The column vacuum and differentially pumped volume pressure were measured for a number of pumping configuration, as well as for several gases and pressures in the specimen volume. The pressure in the high vacuum portion of the objective lens gap is conductance-limited when the DIFPEC is in place.