CRITICAL POINT DRYING WITH CO2
- Make sure that all 3 valves (gas cylinder, gas intake and gas outlet) are closed by turning clockwise.
- Check the seating of the CPD chamber lid gasket. Replace if loose or damaged.
- Fully open the shut-off valve on the CO2(l) cylinder (red collar). If new cylinder, first disconnect the black connection hose and blow out the glass filings and check to be sure the washer for the connection hose is in place.
- Make sure that there is no CO2(g) in the connection hose leading from the gas cylinder to the critical point drier (CPD) chamber by opening (clockwise) the intake valve and looking for CO2(l) in the specimen chamber.
- Check that both gas-intake and gas-out valves are closed by turning valves clockwise (arrows).
- Press the green master switch MAIN (light is on in the "on" position).
- Set the temperature to read 15 degrees.
- Place specimens into the immersion basket, which must be submerged in 100% acetone. Be sure to keep the sample submerged under absolute acetone.
- Record specimen identity by slot number (1-9) in immersion basket.
- Secure the top to the immersion basket.
- Fill CPD chamber from 1/2 to 2/3 full with absolute acetone (i.e., the minimum required to keep specimens fully immersed).
- Quickly transfer immersion basket into the CPD chamber.
- Slowly fill CPD chamber to top with CO2(l) by turning gas-intake valve (right side) counter-clockwise. Watch first for condensation on top sight glass. Then watch for bubble to disappear to know when chamber is full.
Pressure will reach ~ 500 bars.
- Close gas-intake valve.
- After 5 min, slowly open gas-out valve until the top of the liquid just reaches the top of the immersion basket. This step should take at least 15 min.
- Close the gas-out valve.
- Repeat steps 13 - 16 until 7 to 9 times (for a total of 6 - 10 changes).
- While draining the final volume of CO2:
- Close the shut-off valve on the CO2 cylinder.
- Start the degassing procedure for the Hummer Jr. Sputter Coater
(Briefly hold down top of chamber to assure tight seal).
- Double check to make sure that the CO2 level has been lowered. Set temperature on CPD to 38oC.
- Watch the pressure gauge to make certain the rising pressure does not exceed the upper limits of the gauge. You will need to let out CO2 if the pressure gets dangerously high.
- When the CO2(l) has reaches the critical point (i.e., changed to gas):
- Put the tube leading out of the gas-out valve into a beaker of water
- Very slowly open the gas-out valve to allow a slow but steady release of gas.
- When bubbles no longer escape from the tube (i.e., the chamber is at equilibrium with room pressure), open the top of the chamber and remove the immersion bucket.
- Press MAIN to shut down the CPD.
- Carefully place specimen onto adhesive on aluminum stub.
- Sputter coat with the Hummer Jr.
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